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Is Hydrogen gas present in biogas, and does it spontaneously ignite?

Is Hydrogen gas present in biogas, and does it spontaneously ignite?


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Is Hydrogen gas present in biogas? As it is highly inflammable, may it ignite the other gases like methane?


The constitution of the biogas depends greatly on its source and the fermenting microbe. But in general, it does contain hydrogen about 0-1%. Even if it did contain large amounts, the mixture would be inflammable, but it would not burst into flames without any external igniter, which could be any small fluctuation of heat if the mixture is too unstable. But in general, an inflammable gas doesn't start burning on its own.


The auto ignition temperature of a combustible mixture of hydrogen in air is 500 degrees C source - Wikipedia. It will not spontaneously ignite, some part of it must first reach that temperature through a spark, flame, or other heat source. Once it ignites, the methane of course will burn too.


Biogas: introduction

Biogas is mostly methane (around 60%) with carbon dioxide (around 40%) and a little hydrogen and hydrogen sulphide. It’s made by anaerobic bacteria breaking down organic matter in the absence of oxygen (when the organic matter is waterlogged – i.e. a slurry). The process also occurs in landfill sites, and in the digestive system of humans and other animals (yes, farts are biogas).

Biogas is generated naturally in the mud at the bottom of marshes – it’s called marsh gas, and can cause little ‘will-o-the-wisp’ flames over the water, due to bacterially-produced gases igniting spontaneously and lighting the methane.

Small experimental biogas digester. Waste material is put into the oil drum the neoprene cover rises when full of gas the gas is tapped into a container (upside-down plastic drum with water seal) which rises as more gas enters. When full, gas can be tapped off and used via a little gas ring.

But we can make it ourselves from plant and animal wastes, and even human waste soft material is better than twigs / woody material. Biogas can be burnt to drive a generator, or on a smaller scale, for cooking or lighting gas lamps. Also, biogas engines have been developed for transport.

The equipment in which the organic matter breaks down anaerobically is called a digester, and there’s also some sort of storage container for the gas produced. Raw biogas can be ‘scrubbed’ by passing it through slaked lime, which removes most of the CO 2 and increases its calorific value.

Mini-biogas experiment, working on exactly the same principle as the digester above.

The two main types of digesters are the continuous and the batch. Continuous digesters have a constant throughput of material, and batch digesters extract the gas from a contained batch of material, which is then emptied and a new batch added.

Biogas digesters are already widely used in developing countries, especially India and China, as firewood for cooking becomes scarce. There are millions of small family plants in India and China. In the West, digesters tend to be larger-scale, taking animal slurries and human sewage. But they can be domestic-scale too, for individuals looking to reduce their dependency on fossil fuels.

Teeny-weeny biogas digester, run on animal manure and kitchen waste, that produces gas for cooking and compost runoff for the garden, and uses a car inner tube for storing the gas!

What are the benefits of biogas?

Reduces CO 2 emissions

Because it’s a substitute for natural gas. Because CO 2 from biogas is from recently-alive plant matter (even if it was fed to animals), it’s part of a cycle – i.e. CO 2 given off by burning biogas is absorbed by plants that will provide future biogas, and so on.

Reduces methane emissions

Animal agriculture is responsible for around 40% of methane released into the atmosphere by human activity. When methane is burnt it releases CO 2 , but methane is around 30 times more potent as a greenhouse gas than CO 2 , so it’s a good idea to burn it rather than release it. However, it’s better for organic waste to be separated and put into an anaerobic digester instead of collecting methane from landfill sites and it would save more energy if all organic waste, including paper, was recycled instead of landfilled – plus it would prevent leaching of contaminants into groundwater and soil.

Simplified cross-section of a type of digester used for animal and human waste all over China and the far east. A toilet can be incorporated into this system. Image: Tkarcher, CC BY-SA

Reduces resource use

Biogas doesn’t need millions of miles of pipes to deliver it, and doesn’t need to be liquefied and shipped across the world, with all the resources and energy that these things entail. Plus it saves trees (for firewood). Natural gas is finite, so won’t last forever – and there will probably be wars for it as it runs out.

Slightly larger digester, filled with animal manure collected from local smallholdings, with a steel wool filter to remove hydrogen sulphide, and gas storage tanks with a water seal plus a demonstration of cooking with the gas.

Creates two renewable resources

Sewage sludge and animal slurries usually end up as fertiliser anyway, so it’s better to obtain fuel from it first, and prevent runoff and methane emissions at the same time – and you still get fertiliser at the end of the process. It’s the missing link for those wanting to switch from fossil fuels – many people heat their homes with wood and their water with solar, and get their electricity from wind and solar – but cooking is a problem it’s too expensive with electricity, and agas are expensive, take a long time to fire up and will make your space too hot in the summer. Gas is best for cooking, but with biogas it can be done without gas bills.

NB: as with other biofuels, we think that the feedstock (raw materials) should be waste material. We don’t think it’s a good idea to set aside large areas of land for growing fuels when much of the world doesn’t have enough food (although the waste from food crops is fine). See Biofuel Watch. Also, large-scale digesters need to be fed by large operations like factory farms or sewage plants. These bring their own problems, such as hormones, animal cruelty, and energy-intensive transport and chemicals. We think that the best solution is usually the smallest scale possible – in this case the farm or domestic scale.

Adding food waste to a small digester.

What can I do?

Setting up

Batch digesters based on some kind of drum / container are feasible on the domestic scale. Continuous digesters are popular in Asia – an inlet and outlet pit with a concrete or steel gas container. You can build your own – read a book, see our links page or attend a course.

How to build a domestic / farm scale biogas digester.

Sizing

In India, for a family of 8 with a few animals (say 8-10 cows), a 10m³ digester is recommended, with 2m³ gas storage. But a typical small family digester will be around one cubic metre. For cooking and lighting, you don’t need much. Every kg of biodegradable material will yield around 0.4 m³ (400l) of gas, and gas lights need around 100l per hour. 2 gas rings for a couple of hours a day will use between 1-2 m³, so if you have some livestock, plus kitchen and human waste, you can do this easily. When it comes to driving any kind of engine (e.g. a generator or a pump) it’s a different matter, and is way beyond the domestic-scale. How long you leave the material in a batch digester depends on temperature (2 weeks at 50°C up to 2 months at 15°C). The average is around 1 month, so gauge how much material you will add each day, and multiply it by 30 to calculate the size of the digester.

Biogas digester on a family farm in India there’s no reason that they can’t be used successfully in the West too.

What we’re not supporting is the building of gigantic digesters to take huge amounts of maize, grown specially to feed the digesters. See here and here for more on this. This doesn’t mean that biogas is a bad idea, it’s just a ‘small is beautiful‘ issue, best used to take waste to produce energy and compost on the farm scale. It’s the scale that’s the problem not the technology. From the second of those two articles:

“The first and most obvious problem is that it means taking land out of food production. A biogas plant with a capacity of one megawatt ‘requires 20,000-25,000 tonnes [of maize] a year, accounting for 450-500 hectares of land’. Consider, when you read that, that the average capacity of an offshore wind turbine is four megawatts. Four hundred and fifty hectares of land or one concrete pillar in the seabed – can there be any doubt about which the better option is?”

Boiling water with biogas.

Use

The waste input must be a slurry – so add water if it’s too solid. Try to keep the temperature as high as possible it generates a little heat, but in colder countries the digester will need insulation and even a little extra heat in the winter (which could be provided by some of the biogas). A greenhouse is a good place for it.

Safety

Methane is explosive – see Adelaide University’s website for safety considerations.

Whilst you’re here, why not take a look at the other 25+ utilities topics available? And don’t forget to visit our main topics page to explore over 200 aspects of low-impact living and our homepage to learn more about why we do what we do.

We'd love to hear your comments, tips and advice on this topic, and if you post a query, we'll try to get a specialist in our network to answer it for you.


Biohydrogen

23.2.1 Biohydrogen by gasification

Besides production of biohydrogen from biogas, it can also be produced through gasification of biomass, similar to the production of bio-SNG. A gasification method has to be used that produces a gas with higher hydrogen content. Otherwise, additional steam reforming is needed to convert methane into hydrogen. Water gas shift is used to increase the hydrogen yield. Then the remaining CO 2 is removed by pressure swing adsorption or ceramic membrane separation, which leaves biohydrogen, which is to be used as an automotive fuel. In order to use it in this way, it has to be compressed or liquefied or stored in metal hydrides. Hydrogen can be used in either internal combustion engines or fuel cells. Since fuel cell vehicles are not commercially available yet and a distribution infrastructure for hydrogen cannot be realised in the short term, biohydrogen is considered a longer-term option for the transport sector. The main challenges for further development of biohydrogen are similar to those of other gasification-derived biofuels (except SNG).

Supercritical gasification, an option for the production of SNG, is also a useful technology for production of biohydrogen. However, in that case, steam reforming is necessary to convert the formed methane into hydrogen, which makes the process more expensive. Another option to produce hydrogen from wet biomass, which is also still at lab scale, is a technology called dark and photo fermentation. Hydrogen can be produced directly by anaerobic digestion (biogas). Dark fermentation is a similar process however, it is manipulated in such a way that the desired end-product hydrogen is produced directly without the forming of methane, whereas hydrogen is normally an intermediate product in anaerobic digestion. During dark fermentation, besides hydrogen, organic acids are produced, which can be converted to hydrogen by a process called photo fermentation.


Is Hydrogen gas present in biogas, and does it spontaneously ignite? - Biology

Methane fermentation is a versatile biotechnology capable of converting almost all types of polymeric materials to methane and carbon dioxide under anaerobic conditions. This is achieved as a result of the consecutive biochemical breakdown of polymers to methane and carbon dioxide in an environment in which a variety of microorganisms which include fermentative microbes (acidogens) hydrogen-producing, acetate-forming microbes (acetogens) and methane-producing microbes (methanogens) harmoniously grow and produce reduced end-products. Anaerobes play important roles in establishing a stable environment at various stages of methane fermentation.

Methane fermentation offers an effective means of pollution reduction, superior to that achieved via conventional aerobic processes. Although practiced for decades, interest in anaerobic fermentation has only recently focused on its use in the economic recovery of fuel gas from industrial and agricultural surpluses.

The biochemistry and microbiology of the anaerobic breakdown of polymeric materials to methane and the roles of the various microorganisms involved, are discussed here. Recent progress in the molecular biology of methanogens is reviewed, new digesters are described and improvements in the operation of various types of bioreactors are also discussed.

Methane fermentation is the consequence of a series of metabolic interactions among various groups of microorganisms. A description of microorganisms involved in methane fermentation, based on an analysis of bacteria isolated from sewage sludge digesters and from the rumen of some animals, is summarized in Fig. 4-1. The first group of microorganisms secrete enzymes which hydrolyze polymeric materials to monomers such as glucose and amino acids, which are subsequently converted to higher volatile fatty acids, H 2 and acetic acid (Fig. 4-1 stage 1). In the second stage, hydrogen-producing acetogenic bacteria convert the higher volatile fatty acids e.g., propionic and butyric acids, produced, to H 2 , CO 2 , and acetic acid. Finally, the third group, methanogenic bacteria convert H 2 , CO 2 , and acetate, to CH 4 and CO 2 .

Polymeric materials such as lipids, proteins, and carbohydrates are primarily hydrolyzed by extracellular, hydrolases, excreted by microbes present in Stage 1 (Fig. 4-1). Hydrolytic enzymes, (lipases, proteases, cellulases, amylases, etc.) hydrolyze their respective polymers into smaller molecules, primarily monomeric units, which are then consumed by microbes. In methane fermentation of waste waters containing high concentrations of organic polymers, the hydrolytic activity relevant to each polymer is of paramount significance, in that polymer hydrolysis may become a rate-limiting step for the production of simpler bacterial substrates to be used in subsequent degradation steps.

Lipases convert lipids to long-chain fatty acids. A population density of 10 4 - 10 5 lipolytic bacteria per ml of digester fluid has been reported. Clostridia and the micrococci appear to be responsible for most of the extracellular lipase producers. The long-chain fatty acids produced are further degraded by p-oxidation to produce acetyl CoA.

Proteins are generally hydrolyzed to amino acids by proteases, secreted by Bacteroides, Butyrivibrio, Clostridium, Fusobacterium, Selenomonas, and Streptococcus. The amino acids produced are then degraded to fatty acids such as acetate, propionate, and butyrate, and to ammonia as found in Clostridium, Peptococcus, Selenomonas, Campylobacter, and Bacteroides.

Polysaccharides such as cellulose, starch, and pectin are hydrolyzed by cellulases, amylases, and pectinases. The majority of microbial cellulases are composed of three species: (a) endo-(3-l,4-glucanases (b) exo-p-l,4-glucanases (c) cellobiase or p-glucosidase. These three enzymes act synergistically on cellulose effectively hydrolyzing its crystal structure, to produce glucose. Microbial hydrolysis of raw starch to glucose requires amylolytic activity, which consist of 5 amylase species: (a) a-amylases that endocleave a ±1-4 bonds (b) p-amylases that exocleave a ±1-4 bonds (c) amyloglucosidases that exocleave a ±l-4 and a ±l-6 bonds (d) debranching enzymes that act on a ±l-6 bonds (e) maltase that acts on maltose liberating glucose. Pectins are degraded by pectinases, including pectinesterases and depolymerases. Xylans are degraded with a ²-endo-xylanase and a ²-xylosidase to produce xylose.

Hexoses and pentoses are generally converted to C 2 and C 3 intermediates and to reduced electron carriers (e.g., NADH) via common pathways. Most anaerobic bacteria undergo hexose metabolism via the Emden-Meyerhof-Parnas pathway (EMP) which produces pyruvate as an intermediate along with NADH. The pyruvate and NADH thus generated, are transformed into fermentation endo-products such as lactate, propionate, acetate, and ethanol by other enzymatic activities which vary tremendously with microbial species.

Thus, in hydrolysis and acidogenesis (Fig. 4-1 Stage 1), sugars, amino acids, and fatty acids produced by microbial degradation of biopolymers are successively metabolised by fermentation endo-products such as lactate, propionate, acetate, and ethanol by other enzymatic activities which vary tremendously with microbial species.

Thus, in hydrolysis and acidogenesis (Fig. 4-1 Stage 1), sugars, ammo acids, and fatty acids produced by microbial degradation of biopolymers are successively metabolised by groups of bacteria and are primarily fermented to acetate, propionate, butyrate, lactate, ethanol, carbon dioxide, and hydrogen (2).

Although some acetate (20%) and H 2 (4%) are directly produced by acidogenic fermentation of sugars, and amino acids, both products are primarily derived from the acetogenesis and dehydrogenation of higher volatile fatty acids (Fig. 4-1 Stage 2).

Obligate H 2 -producing acetogenic bacteria are capable of producing acetate and H 2 from higher fatty acids. Only Syntrophobacter wolinii, a propionate decomposer (3) and Sytrophomonos wolfei, a butyrate decomposer (4) have thus far been isolated due to technical difficulties involved in the isolation of pure strains, since H 2 produced, severely inhibits the growth of these strains. The use of co-culture techniques incorporating H 2 consumers such as methanogens and sulfate-reducing bacteria may therefore facilitate elucidation of the biochemical breakdown of fatty acids.

Overall breakdown reactions for long-chain fatty acids are presented in Tables 4-1 and 4-2. H 2 production by acetogens is generally energetically unfavorable due to high free energy requirements ( a ”G o, > 0 Table 4-1 and 4-2). However, with a combination of H 2 -consuming bacteria (Table 4-2, 4-3), co-culture systems provide favorable conditions for the decomposition of fatty acids to acetate and CH 4 or H 2 S ( a ”G o, < 0). In addition to the decomposition of long-chain fatty acids, ethanol and lactate are also converted to acetate and H 2 by an acetogen and Clostridium formicoaceticum, respectively.

The effect of the partial pressure of H 2 on the free energy associated with the conversion of ethanol, propionate, acetate, and H 2 /CO 2 during methane fermentation is shown in Fig. 4-2. An extremely low partial pressure of H 2 (10 -5 atm) appears to be a significant factor in propionate degradation to CH 4 . Such a low partial pressure can be achieved in a co-culture with H 2 -consuming bacteria as previously described (Table 4-2,4-3).

Methanogens are physiologically united as methane producers in anaerobic digestion (Fig. 4-1 Stage 3). Although acetate and H 2 /CO 2 are the main substrates available in the natural environment, formate, methanol, methylamines, and CO are also converted to CH 4 (Table 4-3).

Table 4-1 Proposed Reactions Involved in Fatty Acid Catabolism by Syntrophomonas wolfei

+ 2 H 2 O 2 CH 3 COO - + 2H 2 + H +

CH 3 CH 2 CH 2 CH 2 CH 2 COO -

+ 4 H 2 O 3 CH 3 COO - + 4H 2 + 2H +

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 COO -

+ 6 H 2 O 4 CH 3 COO - + 6H 2 + 3H +

+1 H 2 O CH 3 CH 2 COO - + CH 3 COO - +2 H 2 + H +

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 COO -

+ 4 H 2 O CH 3 CH 2 COO - + 2 CH 3 COO - +4 H 2 + 2H +

CH 3 CHCH 2 CH 2 CH 2 COO -
|
CH 3

+ 2 H 2 O CH 3 CHCH 2 COO - + CH 3 COO - + 2H 2 + H +
|
CH 3

Table 4-2 Free-Energy Changes for Reactions Involving Anaerobic Oxidation in Pure Cultures or in Co-Cultures with H 2 -Utilizing Methanogens or Desulfovibrio spp.

1. Proton-reducing (H 2 -producing) acetogenic bacteria

A. CH 3 CH 2 CH 2 COO - + 2H 2 O 2 CH 3 COO - + 2H 2 + H +

B. CH 3 CH 2 COO - + 3H 2 O CH 3 COO - + HCO 3 - + H + + 3H 2

2. H 2 -using methanogens and desulfovibrios

C. 4H 2 + HCO 3 - + H + CH 4 + 3 H 2 O

D. 4H 2 + S0 4 2- + H + HS - + 4 H 2 O

A + C 2 CH 3 CH 2 CH 2 COO - + HCO 3 - + H 2 O 4 CH 3 COO - + H + + CH 4

A + D 2 CH 3 CH 2 CH 2 COO - + S0 4 2- 4 CH 3 COO - + H + + HS -

B + C 4 CH 3 CH 2 COO - + 12H 2 4 CH 3 COO - + HCO 3 - + H + + 3 CH 4

B + D 4 CH 3 CH 2 COO - + 3 S0 4 2 " 4 CH 3 COO - + 4 HCO 3 - + H + + 3 HS -

Table 4-3 Energy-Yielding Reactions of Methanogens

CO 2 + 4 H 2 ® CH 4 + 2H 2 O

HCO 3 - + 4 H 2 + H + ® CH 4 + 3 H 2 O

CH 3 COO - + H 2 O ® CH 4 + HCO 3 -

HCOO - + H + ® 0.25 CH 4 + 0.75 CO 2 + 0.5 H 2 O

CO + 0.5 H 2 O ® 0.25 CH 4 + 0.75 CO 2

CH 3 OH ® 0.75 CH 4 + 0.25 CO 2 + 0.5 H 2 O

CH 3 NH 3 + + 0.5 H 2 O ® 0.75 CH 4 + 0.25 CO 2 + NH 4 +

(CH 3 ) 2 NH 2 + + H 2 O ® 1.5 CH 4 + 0.5 CO 2 + NH 4 +

(CH 3 ) 2 NCH 2 CH 3 H + + H 2 O ® 1.5 CH 4 + 0.5 CO 2 + + H 3 NCH 2 CH 3

(CH 3 ) 3 NH+ 1.5H 2 O ® 2.25 CH 4 + 0.75 CO 2 + NH 4 +

Since methanogens, as obligate anaerobes, require a redox potential of less than -300 mV for growth, their isolation and cultivation was somewhat elusive due to technical difficulties encountered in handling them under completely O 2 -free conditions. However, as a result of a greatly improved methanogen isolation techniques developed by Hungate (6), more than 40 strains of pure methanogens have now been isolated. Methanogens can be divided into two groups: H 2 /CO 2 - and acetate-consumers. Although some of the H 2 /CO 2 -consumers are capable of utilizing formate, acetate is consumed by a limited number of strains, such as Methanosarcina spp. and Methanothrix spp. (now, Methanosaeta), which are incapable of using formate. Since a large quantity of acetate is produced in the natural environment (Fig. 4-1), Methanosarcina and Methanothrix play an important role in completion of anaerobic digestion and in accumulating H 2 , which inhibits acetogens and methanogens. H 2 -consuming methanogens are also important in maintaining low levels of atmospheric H 2 .

H 2 /CO 2 -consuming methanogens reduce CO 2 as an electron acceptor via the formyl, methenyl, and methyl levels through association with unusual coenzymes, to finally produce CH 4 (7) (Fig. 4-3). The overall acetoclastic reaction can be expressed as:

Since a small part of the CO 2 is also formed from carbon derived from the methyl group, it is suspected that the reduced potential produced from the methyl group may reduce CO 2 to CH 4 (8).

On the basis of homologous sequence analysis of 16S rRNAs, methanogens have been classified into one of the three primary kingdoms of living organisms: the Archaea (Archaebacteria). The Archaea also include major groups of organisms such as thermophiles and halophiles. Although Archaea possess a prokaryotic cell structure and organization, they share common feature with eukaryotes: homologous sequences in rRNA and tRNA, the presence of inn-ones in their genomes, similar RNA polymerase subunit organization, immunological homologies, and translation systems.

Recombinant DNA technology is one of the most powerful techniques for characterizing the biochemical and genetic regulation of methanogenesis. This necessitates the selection of genetic markers, an efficient genetic transformation system, and a vector system for genetic recombination as prerequisites.

Genetically marked strains are prerequisites for genetic studies: these strains can be employed to develop a genetic-exchange system in methanogens based on an efficient selection system. Since growth of M. thermoautotrophicum is inhibited by fluorouracil, analogue-resistant strains were isolated by spontaneous mutation. Other mutants resistant to DL-ethionine or 2-bromoethane sulfonate (coenzyme M analogue), in addition to autotrophic mutants, were obtained by mutagenic treatment. Several autotrophic strains were also obtained for the acetoclastic methanogen, M. voltae. These mutant strains are listed in Table 4-4.

Although some methanogen genes such as amino acid and purine biosysnthetic genes, transcription and translation machinery genes, and structural protein genes, have been cloned, genes encoding enzymes involved in methanogenesis were chosen as "methane genes" here.

Methyl CoM reductase (MR Fig. 4-3) constitutes approximately 10% of the total protein in methanogenic cultures. The importance and abundance of MR inevitably focused initial attention on elucidating its structure and the mechanisms directing its synthesis and regulation. MR- encoding genes have been cloned and sequenced from Methanococcus vanielli, M. voltae, Methanosarcina barkeri, Methanobacterium thermoautotrophicum and M. fervidus.

Formylmethanofuran transferase (FTR) catalyzes the transfer of a formyl group from formylmethanofuran (MFR) to tetrahydromethanopterin (H 4 MPT) (Fig. 4-3, 4-2). The FTR-encoding gene from M. thermoautotrophicum has been cloned, sequenced, and functionally expressed in E. coli. Formate dehydrogenase (FDH) may sometimes account for 2 to 3% of the total soluble proteins in methanogenic cultures. The two genes encoding the a ± and a ² subunits of FDH have been cloned and sequenced from M formicicum. In addition, the genes encoding F 420 -reducing hydrogenase (Fig. 4-3), ferredoxin, and ATPase have also been cloned.

Table 4-4 Auxotrophic and Drug-Resistant Mutants Applicable To Gene Transfer Experiments


Contents

HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection. It is essentially an Otto combustion cycle. HCCI was popular before electronic spark ignition was used. One example is the hot-bulb engine which used a hot vaporization chamber to help mix fuel with air. The extra heat combined with compression induced the conditions for combustion. Another example is the "diesel" model aircraft engine.

Methods Edit

A mixture of fuel and air ignites when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased in several different ways:

  • Increasing compression ratio
  • Pre-heating of induction gases
  • Forced induction
  • Retained or re-inducted exhaust gases

Once ignited, combustion occurs very quickly. When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures.

Advantages Edit

  • Since HCCI engines are fuel-lean, they can operate at diesel-like compression ratios (>15), thus achieving 30% higher efficiencies than conventional SI gasoline engines. [2]
  • Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Because peak temperatures are significantly lower than in typical SI engines, NOx levels are almost negligible. Additionally, the technique does not produce soot. [3]
  • HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels. [4]
  • HCCI avoids throttle losses, which further improves efficiency. [5]

Disadvantages Edit

  • Achieving cold start capability.
  • High heat release and pressure rise rates contribute to engine wear.
  • Autoignition is difficult to control, unlike the ignition event in SI and diesel engines, which are controlled by spark plugs and in-cylinder fuel injectors, respectively. [6]
  • HCCI engines have a small torque range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions. [7] (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a typical spark ignition engine, caused by incomplete oxidation (due to the rapid combustion event and low in-cylinder temperatures) and trapped crevice gases, respectively. [8]

Control Edit

HCCI is more difficult to control than other combustion engines, such as SI and diesel. In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In Diesel engines, combustion begins when the fuel is injected into pre-compressed air. In both cases, combustion timing is explicitly controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever sufficient pressure and temperature are reached. This means that no well-defined combustion initiator provides direct control. Engines must be designed so that ignition conditions occur at the desired timing. To achieve dynamic operation, the control system must manage the conditions that induce combustion. Options include the compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, or quantity of retained or re-inducted exhaust. Several control approaches are discussed below.

Compression ratio Edit

Two compression ratios are significant. The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This system is used in diesel model aircraft engines. The effective compression ratio can be reduced from the geometric ratio by closing the intake valve either very late or very early with variable valve actuation (variable valve timing that enables the Miller cycle). Both approaches require energy to achieve fast response. Additionally, implementation is expensive, but is effective. [9] The effect of compression ratio on HCCI combustion has also been studied extensively. [10]

Induction temperature Edit

HCCI's autoignition event is highly sensitive to temperature. The simplest temperature control method uses resistance heaters to vary the inlet temperature, but this approach is too slow to change on a cycle-to-cycle frequency. [11] Another technique is fast thermal management (FTM). It is accomplished by varying the intake charge temperature by mixing hot and cold air streams. It is fast enough to allow cycle-to-cycle control. [12] It is also expensive to implement and has limited bandwidth associated with actuator energy.

Exhaust gas percentage Edit

Exhaust gas is very hot if retained or re-inducted from the previous combustion cycle or cool if recirculated through the intake as in conventional EGR systems. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine output. Hot combustion products conversely increase gas temperature in the cylinder and advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally. [13]

Valve actuation Edit

Variable valve actuation (VVA) extends the HCCI operating region by giving finer control over the temperature-pressure-time envelope within the combustion chamber. VVA can achieve this via either:

  • Controlling the effective compression ratio: VVA on intake can control the point at which the intake valve closes. Retarding past bottom dead center (BDC), changes the compression ratio, altering the in-cylinder pressure-time envelope.
  • Controlling the amount of hot exhaust gas retained in the combustion chamber: VVA can control the amount of hot EGR within the combustion chamber, either by valve re-opening or changes in valve overlap. Balancing the percentage of cooled external EGR with the hot internal EGR generated by a VVA system, makes it possible to control the in-cylinder temperature.

While electro-hydraulic and camless VVA systems offer control over the valve event, the componentry for such systems is currently complicated and expensive. Mechanical variable lift and duration systems, however, although more complex than a standard valvetrain, are cheaper and less complicated. It is relatively simple to configure such systems to achieve the necessary control over the valve lift curve.

Fuel mixture Edit

Another means to extend the operating range is to control the onset of ignition and the heat release rate [14] [15] by manipulating the fuel itself. This is usually carried out by blending multiple fuels "on the fly" for the same engine. [16] Examples include blending of commercial gasoline and diesel fuels, [17] adopting natural gas [18] or ethanol. [19] This can be achieved in a number of ways:

  • Upstream blending: Fuels are mixed in the liquid phase, one with low ignition resistance (such as diesel) and a second with greater resistance (gasoline). Ignition timing varies with the ratio of these fuels.
  • In-chamber blending: One fuel can be injected in the intake duct (port injection) and the other directly into the cylinder.

Direct Injection: PCCI or PPCI Combustion Edit

Compression Ignition Direct Injection (CIDI) combustion is a well-established means of controlling ignition timing and heat release rate and is adopted in diesel engine combustion. Partially Pre-mixed Charge Compression Ignition (PPCI) also known as Premixed Charge Compression Ignition (PCCI) is a compromise offering the control of CIDI combustion with the reduced exhaust gas emissions of HCCI, specifically lower soot. [20] The heat release rate is controlled by preparing the combustible mixture in such a way that combustion occurs over a longer time duration making it less prone to knocking. This is done by timing the injection event such that a range of air/fuel ratios spread across the combustion cylinder when ignition begins. Ignition occurs in different regions of the combustion chamber at different times - slowing the heat release rate. This mixture is designed to minimize the number of fuel-rich pockets, reducing soot formation. [21] The adoption of high EGR and diesel fuels with a greater resistance to ignition (more "gasoline like") enable longer mixing times before ignition and thus fewer rich pockets that produce soot and NO
x [20] [21]

Peak pressure and heat release rate Edit

In a typical ICE, combustion occurs via a flame. Hence at any point in time, only a fraction of the total fuel is burning. This results in low peak pressures and low energy release rates. In HCCI however, the entire fuel/air mixture ignites and burns over a much smaller time interval, resulting in high peak pressures and high energy release rates. To withstand the higher pressures, the engine has to be structurally stronger. Several strategies have been proposed to lower the rate of combustion and peak pressure. Mixing fuels, with different autoignition properties, can lower the combustion speed. [22] However, this requires significant infrastructure to implement. Another approach uses dilution (i.e. with exhaust gases) to reduce the pressure and combustion rates (and output). [23]

In the divided combustion chamber approach [1], there are two cooperating combustion chambers: a small auxiliary and a big main.
A high compression ratio is used in the auxiliary combustion chamber.
A moderate compression ratio is used in the main combustion chamber wherein a homogeneous air-fuel mixture is compressed / heated near, yet below, the auto-ignition threshold.
The high compression ratio in the auxiliary combustion chamber causes the auto-ignition of the homogeneous lean air-fuel mixture therein (no spark plug required) the burnt gas bursts - through some "transfer ports", just before the TDC - into the main combustion chamber triggering its auto-ignition.
The engine needs not be structurally stronger.

Power Edit

In ICEs, power can be increased by introducing more fuel into the combustion chamber. These engines can withstand a boost in power because the heat release rate in these engines is slow. However, in HCCI engines increasing the fuel/air ratio results in higher peak pressures and heat release rates. In addition, many viable HCCI control strategies require thermal preheating of the fuel, which reduces the density and hence the mass of the air/fuel charge in the combustion chamber, reducing power. These factors make increasing the power in HCCI engines challenging.

One technique is to use fuels with different autoignition properties. This lowers the heat release rate and peak pressures and makes it possible to increase the equivalence ratio. Another way is to thermally stratify the charge so that different points in the compressed charge have different temperatures and burn at different times, lowering the heat release rate and making it possible to increase power. [24] A third way is to run the engine in HCCI mode only at part load conditions and run it as a diesel or SI engine at higher load conditions. [25]

Emissions Edit

Because HCCI operates on lean mixtures, the peak temperature is much lower than that encountered in SI and diesel engines. This low peak temperature reduces the formation of NO
x , but it also leads to incomplete burning of fuel, especially near combustion chamber walls. This produces relatively high carbon monoxide and hydrocarbon emissions. An oxidizing catalyst can remove the regulated species, because the exhaust is still oxygen-rich.

Difference from knock Edit

Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in an SI engine spontaneously ignite. This gas is compressed as the flame propagates and the pressure in the combustion chamber rises. The high pressure and corresponding high temperature of unburnt reactants can cause them to spontaneously ignite. This causes a shock wave to traverse from the end gas region and an expansion wave to traverse into the end gas region. The two waves reflect off the boundaries of the combustion chamber and interact to produce high amplitude standing waves, thus forming a primitive thermoacoustic device where the resonance is amplified by the increased heat release during the wave travel similar to a Rijke tube.

A similar ignition process occurs in HCCI. However, rather than part of the reactant mixture igniting by compression ahead of a flame front, ignition in HCCI engines occurs due to piston compression more or less simultaneously in the bulk of the compressed charge. Little or no pressure differences occur between the different regions of the gas, eliminating any shock wave and knocking, but the rapid pressure rise is still present and desirable from the point of seeking maximum efficiency from near-ideal isochoric heat addition.

Simulation of HCCI Engines Edit

Computational models for simulating combustion and heat release rates of HCCI engines require detailed chemistry models. [17] [26] This is largely because ignition is more sensitive to chemical kinetics than to turbulence/spray or spark processes as are typical in SI and diesel engines. Computational models have demonstrated the importance of accounting for the fact that the in-cylinder mixture is actually in-homogeneous, particularly in terms of temperature. This in-homogeneity is driven by turbulent mixing and heat transfer from the combustion chamber walls. The amount of temperature stratification dictates the rate of heat release and thus tendency to knock. [27] This limits the usefulness of considering the in-cylinder mixture as a single zone, resulting in the integration of 3D computational fluid dynamics codes such as Los Alamos National Laboratory's KIVA CFD code and faster solving probability density function modelling codes. [28] [29]

As of 2017, no HCCI engines were produced at commercial scale. However, several car manufacturers had functioning HCCI prototypes.

  • The 1994 Honda EXP-2 motorcycle used "ARC-combustion". This had a two stroke engine uses an exhaust valve to mimic a HCCI mode. Honda sold a CRM 250 AR.
  • In 2007–2009, General Motors demonstrated HCCI with a modified 2.2 L Ecotec engine installed in Opel Vectra and Saturn Aura. [30] The engine operates in HCCI mode at speeds below 60 miles per hour (97 km/h) or when cruising, switching to conventional SI when the throttle is opened and produces fuel economy of 43 miles per imperial gallon (6.6 L/100 km 36 mpg‑US) and carbon dioxide emissions of about 150 grams per kilometre, improving on the 37 miles per imperial gallon (7.6 L/100 km 31 mpg‑US) and 180 g/km of the conventional 2.2 L direct injection version. [31] GM is also researching smaller Family 0 engines for HCCI applications. GM has used KIVA in the development of direct-injection, stratified charge gasoline engines as well as the fast burn, homogeneous-charge gasoline engine. [29] developed a prototype engine called DiesOtto, with controlled auto ignition. It was displayed in its F 700 concept car at the 2007 Frankfurt Auto Show. [32] are developing two types of engine for HCCI operation. The first, called Combined Combustion System or CCS, is based on the VW Group 2.0-litre diesel engine, but uses homogeneous intake charge. It requires synthetic fuel to achieve maximum benefit. The second is called Gasoline Compression Ignition or GCI it uses HCCI when cruising and spark ignition when accelerating. Both engines have been demonstrated in Touran prototypes. [33]
  • In November 2011 Hyundai announced the development of GDCI (Gasoline Direct Injection Compression Ignition) engine in association with Delphi Automotive. [34] The engine completely eliminated the ignition plugs, and instead utilizes both supercharger and turbocharger to maintain the pressure within the cylinder. The engine is scheduled for commercial production in near future. [35]
  • In October 2005, the Wall Street Journal reported that Honda was developing an HCCI engine as part of an effort to produce a next generation hybrid car. [36]
  • Oxy-Gen Combustion, a UK-based Clean Technology company, produced a full-load HCCI concept engine with the aid of Michelin and Shell. [37]
  • Mazda's SkyActiv-G Generation 2 has a compression ratio of 18:1 to allow the use of HCCI combustion. [38] An engine model called SKYACTIV-X has been announced by Mazda in August 2017 as a major breakthrough in engine technology. [39]
  • Mazda is undertaking research with HCCI with Wankel engines. [40]

To date, few prototype engines run in HCCI mode, but HCCI research has resulted in advancements in fuel and engine development. Examples include:


Auto-ignition temperature

The auto-ignition temperature (AIT) of the hazardous material in a facility must be known to complete its EAC. NFPA 497 provides AIT values for various combustible substances.

A typical area classification for propane gas would be: Class I, Division 2, Group D, 450C AIT. Here:

  • Class I indicates the presence of vapor.
  • Division 2 indicates that the vapor is present only under abnormal conditions.
  • Group D indicates that propane is a member of this group.
  • 450C is the auto-ignition temperature of propane.

Once an area has been classified, the NEC provides very specific and stringent requirements about the electrical equipment and associated wiring that can be installed within that area. The requirements are intended to prevent electrical equipment from being the ignition source for a flammable mixture. Accordingly, the installation itself must be explosion-proof.

Obviously, a facility’s EAC must be known before any electrical equipment can be specified, designed, or installed. On many CTG power projects, special-purpose mechanical equipment with long lead times (motors and instrumentation and control systems and components, for example) must be specified and ordered early. Failure to determine the EAC for the facility and such equipment in a timely fashion can result in unsafe installations, rework, confusion, delays, and cost overruns.


Contents

The first references to hydrogen fuel cells appeared in 1838. In a letter dated October 1838 but published in the December 1838 edition of The London and Edinburgh Philosophical Magazine and Journal of Science, Welsh physicist and barrister Sir William Grove wrote about the development of his first crude fuel cells. He used a combination of sheet iron, copper and porcelain plates, and a solution of sulphate of copper and dilute acid. [6] [7] In a letter to the same publication written in December 1838 but published in June 1839, German physicist Christian Friedrich Schönbein discussed the first crude fuel cell that he had invented. His letter discussed current generated from hydrogen and oxygen dissolved in water. [8] Grove later sketched his design, in 1842, in the same journal. The fuel cell he made used similar materials to today's phosphoric acid fuel cell. [9] [10]

The Brits who bolstered the Moon landings, BBC Archives. [11]

In 1932, English engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. [11] The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its inventor, is one of the most developed fuel cell technologies, which NASA has used since the mid-1960s. [11] [12]

In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the "Grubb-Niedrach fuel cell". [13] [14] GE went on to develop this technology with NASA and McDonnell Aircraft, leading to its use during Project Gemini. This was the first commercial use of a fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers, which was demonstrated across the U.S. at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt & Whitney licensed Bacon's U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). In 1991, the first hydrogen fuel cell automobile was developed by Roger Billings. [15] [16]

UTC Power was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. [17]

In recognition of the fuel cell industry and America's role in fuel cell development, the US Senate recognized 8 October 2015 as National Hydrogen and Fuel Cell Day, passing S. RES 217. The date was chosen in recognition of the atomic weight of hydrogen (1.008). [18]

Fuel cells come in many varieties however, they all work in the same general manner. They are made up of three adjacent segments: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load.

At the anode a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot. The freed electrons travel through a wire creating the electric current. The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide.

Design features in a fuel cell include:

  • The electrolyte substance, which usually defines the type of fuel cell, and can be made from a number of substances like potassium hydroxide, salt carbonates, and phosphoric acid. [19]
  • The fuel that is used. The most common fuel is hydrogen.
  • The anode catalyst, usually fine platinum powder, breaks down the fuel into electrons and ions.
  • The cathode catalyst, often nickel, converts ions into waste chemicals, with water being the most common type of waste. [20]
  • Gas diffusion layers that are designed to resist oxidization. [20]

A typical fuel cell produces a voltage from 0.6 to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors:

To deliver the desired amount of energy, the fuel cells can be combined in series to yield higher voltage, and in parallel to allow a higher current to be supplied. Such a design is called a fuel cell stack. The cell surface area can also be increased, to allow higher current from each cell.

Proton-exchange membrane fuel cells (PEMFCs) Edit

In the archetypical hydrogen–oxide proton-exchange membrane fuel cell design, a proton-conducting polymer membrane (typically nafion) contains the electrolyte solution that separates the anode and cathode sides. [25] [26] This was called a solid polymer electrolyte fuel cell (SPEFC) in the early 1970s, before the proton-exchange mechanism was well understood. (Notice that the synonyms polymer electrolyte membrane and 'proton-exchange mechanism result in the same acronym.)

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what are commonly referred to as multi-facilitated proton membranes. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water.

In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells and indirect methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water. When hydrogen is used, the CO2 is released when methane from natural gas is combined with steam, in a process called steam methane reforming, to produce the hydrogen. This can take place in a different location to the fuel cell, potentially allowing the hydrogen fuel cell to be used indoors—for example, in fork lifts.

The different components of a PEMFC are

  1. bipolar plates, , ,
  2. membrane, and
  3. the necessary hardware such as current collectors and gaskets. [27]

The materials used for different parts of the fuel cells differ by type. The bipolar plates may be made of different types of materials, such as, metal, coated metal, graphite, flexible graphite, C–C composite, carbon–polymer composites etc. [28] The membrane electrode assembly (MEA) is referred as the heart of the PEMFC and is usually made of a proton-exchange membrane sandwiched between two catalyst-coated carbon papers. Platinum and/or similar type of noble metals are usually used as the catalyst for PEMFC. The electrolyte could be a polymer membrane.

Proton-exchange membrane fuel cell design issues Edit

Phosphoric acid fuel cell (PAFC) Edit

Phosphoric acid fuel cells (PAFC) were first designed and introduced in 1961 by G. V. Elmore and H. A. Tanner. In these cells phosphoric acid is used as a non-conductive electrolyte to pass positive hydrogen ions from the anode to the cathode. These cells commonly work in temperatures of 150 to 200 degrees Celsius. This high temperature will cause heat and energy loss if the heat is not removed and used properly. This heat can be used to produce steam for air conditioning systems or any other thermal energy consuming system. [37] Using this heat in cogeneration can enhance the efficiency of phosphoric acid fuel cells from 40 to 50% to about 80%. [37] Phosphoric acid, the electrolyte used in PAFCs, is a non-conductive liquid acid which forces electrons to travel from anode to cathode through an external electrical circuit. Since the hydrogen ion production rate on the anode is small, platinum is used as catalyst to increase this ionization rate. A key disadvantage of these cells is the use of an acidic electrolyte. This increases the corrosion or oxidation of components exposed to phosphoric acid. [38]

Solid acid fuel cell (SAFC) Edit

Solid acid fuel cells (SAFCs) are characterized by the use of a solid acid material as the electrolyte. At low temperatures, solid acids have an ordered molecular structure like most salts. At warmer temperatures (between 140 and 150 °C for CsHSO4), some solid acids undergo a phase transition to become highly disordered "superprotonic" structures, which increases conductivity by several orders of magnitude. The first proof-of-concept SAFCs were developed in 2000 using cesium hydrogen sulfate (CsHSO4). [39] Current SAFC systems use cesium dihydrogen phosphate (CsH2PO4) and have demonstrated lifetimes in the thousands of hours. [40]

Alkaline fuel cell (AFC) Edit

The alkaline fuel cell or hydrogen-oxygen fuel cell was designed and first demonstrated publicly by Francis Thomas Bacon in 1959. It was used as a primary source of electrical energy in the Apollo space program. [41] The cell consists of two porous carbon electrodes impregnated with a suitable catalyst such as Pt, Ag, CoO, etc. The space between the two electrodes is filled with a concentrated solution of KOH or NaOH which serves as an electrolyte. H2 gas and O2 gas are bubbled into the electrolyte through the porous carbon electrodes. Thus the overall reaction involves the combination of hydrogen gas and oxygen gas to form water. The cell runs continuously until the reactant's supply is exhausted. This type of cell operates efficiently in the temperature range 343–413 K and provides a potential of about 0.9 V. [42] AAEMFC is a type of AFC which employs a solid polymer electrolyte instead of aqueous potassium hydroxide (KOH) and it is superior to aqueous AFC.

High-temperature fuel cells Edit

Solid oxide fuel cell Edit

Solid oxide fuel cells (SOFCs) use a solid material, most commonly a ceramic material called yttria-stabilized zirconia (YSZ), as the electrolyte. Because SOFCs are made entirely of solid materials, they are not limited to the flat plane configuration of other types of fuel cells and are often designed as rolled tubes. They require high operating temperatures (800–1000 °C) and can be run on a variety of fuels including natural gas. [5]

SOFCs are unique since in those, negatively charged oxygen ions travel from the cathode (positive side of the fuel cell) to the anode (negative side of the fuel cell) instead of positively charged hydrogen ions travelling from the anode to the cathode, as is the case in all other types of fuel cells. Oxygen gas is fed through the cathode, where it absorbs electrons to create oxygen ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas at the anode. The reaction at the anode produces electricity and water as by-products. Carbon dioxide may also be a by-product depending on the fuel, but the carbon emissions from an SOFC system are less than those from a fossil fuel combustion plant. [43] The chemical reactions for the SOFC system can be expressed as follows: [44]

Anode reaction: 2H2 + 2O 2− → 2H2O + 4e − Cathode reaction: O2 + 4e − → 2O 2− Overall cell reaction: 2H2 + O2 → 2H2O

SOFC systems can run on fuels other than pure hydrogen gas. However, since hydrogen is necessary for the reactions listed above, the fuel selected must contain hydrogen atoms. For the fuel cell to operate, the fuel must be converted into pure hydrogen gas. SOFCs are capable of internally reforming light hydrocarbons such as methane (natural gas), [45] propane and butane. [46] These fuel cells are at an early stage of development. [47]

Challenges exist in SOFC systems due to their high operating temperatures. One such challenge is the potential for carbon dust to build up on the anode, which slows down the internal reforming process. Research to address this "carbon coking" issue at the University of Pennsylvania has shown that the use of copper-based cermet (heat-resistant materials made of ceramic and metal) can reduce coking and the loss of performance. [48] Another disadvantage of SOFC systems is slow start-up time, making SOFCs less useful for mobile applications. Despite these disadvantages, a high operating temperature provides an advantage by removing the need for a precious metal catalyst like platinum, thereby reducing cost. Additionally, waste heat from SOFC systems may be captured and reused, increasing the theoretical overall efficiency to as high as 80–85%. [5]

The high operating temperature is largely due to the physical properties of the YSZ electrolyte. As temperature decreases, so does the ionic conductivity of YSZ. Therefore, to obtain optimum performance of the fuel cell, a high operating temperature is required. According to their website, Ceres Power, a UK SOFC fuel cell manufacturer, has developed a method of reducing the operating temperature of their SOFC system to 500–600 degrees Celsius. They replaced the commonly used YSZ electrolyte with a CGO (cerium gadolinium oxide) electrolyte. The lower operating temperature allows them to use stainless steel instead of ceramic as the cell substrate, which reduces cost and start-up time of the system. [49]

Molten-carbonate fuel cell (MCFC) Edit

Molten carbonate fuel cells (MCFCs) require a high operating temperature, 650 °C (1,200 °F), similar to SOFCs. MCFCs use lithium potassium carbonate salt as an electrolyte, and this salt liquefies at high temperatures, allowing for the movement of charge within the cell – in this case, negative carbonate ions. [50]

Like SOFCs, MCFCs are capable of converting fossil fuel to a hydrogen-rich gas in the anode, eliminating the need to produce hydrogen externally. The reforming process creates CO
2 emissions. MCFC-compatible fuels include natural gas, biogas and gas produced from coal. The hydrogen in the gas reacts with carbonate ions from the electrolyte to produce water, carbon dioxide, electrons and small amounts of other chemicals. The electrons travel through an external circuit creating electricity and return to the cathode. There, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte, completing the circuit. [50] The chemical reactions for an MCFC system can be expressed as follows: [51]

Anode reaction: CO3 2− + H2 → H2O + CO2 + 2e − Cathode reaction: CO2 + ½O2 + 2e − → CO3 2− Overall cell reaction: H2 + ½O2 → H2O

As with SOFCs, MCFC disadvantages include slow start-up times because of their high operating temperature. This makes MCFC systems not suitable for mobile applications, and this technology will most likely be used for stationary fuel cell purposes. The main challenge of MCFC technology is the cells' short life span. The high-temperature and carbonate electrolyte lead to corrosion of the anode and cathode. These factors accelerate the degradation of MCFC components, decreasing the durability and cell life. Researchers are addressing this problem by exploring corrosion-resistant materials for components as well as fuel cell designs that may increase cell life without decreasing performance. [5]

MCFCs hold several advantages over other fuel cell technologies, including their resistance to impurities. They are not prone to "carbon coking", which refers to carbon build-up on the anode that results in reduced performance by slowing down the internal fuel reforming process. Therefore, carbon-rich fuels like gases made from coal are compatible with the system. The United States Department of Energy claims that coal, itself, might even be a fuel option in the future, assuming the system can be made resistant to impurities such as sulfur and particulates that result from converting coal into hydrogen. [5] MCFCs also have relatively high efficiencies. They can reach a fuel-to-electricity efficiency of 50%, considerably higher than the 37–42% efficiency of a phosphoric acid fuel cell plant. Efficiencies can be as high as 65% when the fuel cell is paired with a turbine, and 85% if heat is captured and used in a combined heat and power (CHP) system. [50]

FuelCell Energy, a Connecticut-based fuel cell manufacturer, develops and sells MCFC fuel cells. The company says that their MCFC products range from 300 kW to 2.8 MW systems that achieve 47% electrical efficiency and can utilize CHP technology to obtain higher overall efficiencies. One product, the DFC-ERG, is combined with a gas turbine and, according to the company, it achieves an electrical efficiency of 65%. [52]

Electric storage fuel cell Edit

The electric storage fuel cell is a conventional battery chargeable by electric power input, using the conventional electro-chemical effect. However, the battery further includes hydrogen (and oxygen) inputs for alternatively charging the battery chemically. [53]

Comparison of fuel cell types Edit

Fuel cell name Electrolyte Qualified power (W) Working temperature (°C) Efficiency Status Cost (USD/W)
Cell System
Metal hydride fuel cell Aqueous alkaline solution > −20
(50% Ppeak @ 0 °C)
Commercial / Research
Electro-galvanic fuel cell Aqueous alkaline solution < 40 Commercial / Research
Direct formic acid fuel cell (DFAFC) Polymer membrane (ionomer) < 50 W < 40 Commercial / Research
Zinc–air battery Aqueous alkaline solution < 40 Mass production
Microbial fuel cell Polymer membrane or humic acid < 40 Research
Upflow microbial fuel cell (UMFC) < 40 Research
Regenerative fuel cell Polymer membrane (ionomer) < 50 Commercial / Research
Direct borohydride fuel cell Aqueous alkaline solution 70 Commercial
Alkaline fuel cell Aqueous alkaline solution 10–200 kW < 80 60–70% 62% Commercial / Research
Direct methanol fuel cell Polymer membrane (ionomer) 100 mW – 1 kW 90–120 20–30% 10–25% [54] Commercial / Research 125
Reformed methanol fuel cell Polymer membrane (ionomer) 5 W – 100 kW 250–300 (reformer)
125–200 (PBI)
50–60% 25–40% Commercial / Research
Direct-ethanol fuel cell Polymer membrane (ionomer) < 140 mW/cm² > 25
? 90–120
Research
Proton-exchange membrane fuel cell Polymer membrane (ionomer) 1 W – 500 kW 50–100 (Nafion) [55]
120–200 (PBI) [56]
50–70% 30–50% [54] Commercial / Research 50–100
Redox fuel cell (RFC) Liquid electrolytes with redox shuttle and polymer membrane (ionomer) 1 kW – 10 MW Research
Phosphoric acid fuel cell Molten phosphoric acid (H3PO4) < 10 MW 150–200 55% 40% [54]
Co-gen: 90%
Commercial / Research 4.00–4.50
Solid acid fuel cell H + -conducting oxyanion salt (solid acid) 10 W – 1 kW 200–300 55–60% 40–45% Commercial / Research
Molten carbonate fuel cell Molten alkaline carbonate 100 MW 600–650 55% 45–55% [54] Commercial / Research
Tubular solid oxide fuel cell (TSOFC) O 2− -conducting ceramic oxide < 100 MW 850–1100 60–65% 55–60% Commercial / Research
Protonic ceramic fuel cell H + -conducting ceramic oxide 700 Research
Direct carbon fuel cell Several different 700–850 80% 70% Commercial / Research
Planar solid oxide fuel cell O 2− -conducting ceramic oxide < 100 MW 500–1100 60–65% 55–60% [54] Commercial / Research
Enzymatic biofuel cells Any that will not denature the enzyme < 40 Research
Magnesium-air fuel cell Salt water −20 to 55 90% Commercial / Research

Glossary of terms in table:

Theoretical maximum efficiency Edit

The energy efficiency of a system or device that converts energy is measured by the ratio of the amount of useful energy put out by the system ("output energy") to the total amount of energy that is put in ("input energy") or by useful output energy as a percentage of the total input energy. In the case of fuel cells, useful output energy is measured in electrical energy produced by the system. Input energy is the energy stored in the fuel. According to the U.S. Department of Energy, fuel cells are generally between 40 and 60% energy efficient. [61] This is higher than some other systems for energy generation. For example, the typical internal combustion engine of a car is about 25% energy efficient. [62] In combined heat and power (CHP) systems, the heat produced by the fuel cell is captured and put to use, increasing the efficiency of the system to up to 85–90%. [5]

The theoretical maximum efficiency of any type of power generation system is never reached in practice, and it does not consider other steps in power generation, such as production, transportation and storage of fuel and conversion of the electricity into mechanical power. However, this calculation allows the comparison of different types of power generation. The theoretical maximum efficiency of a fuel cell approaches 100%, [63] while the theoretical maximum efficiency of internal combustion engines is approximately 58%. [64]

In practice Edit

In a fuel-cell vehicle the tank-to-wheel efficiency is greater than 45% at low loads [65] and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure. [66] The comparable NEDC value for a Diesel vehicle is 22%. In 2008 Honda released a demonstration fuel cell electric vehicle (the Honda FCX Clarity) with fuel stack claiming a 60% tank-to-wheel efficiency. [67]

It is also important to take losses due to fuel production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as liquid hydrogen. [68] Fuel cells cannot store energy like a battery, [69] except as hydrogen, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. As of 2019, 90% of hydrogen was used for oil refining, chemicals and fertilizer production, and 98% of hydrogen is produced by steam methane reforming, which emits carbon dioxide. [70] The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency), using pure hydrogen and pure oxygen can be "from 35 up to 50 percent", depending on gas density and other conditions. [71] The electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore suited for long-term storage.

Solid-oxide fuel cells produce heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as 800 degrees Celsius. This heat can be captured and used to heat water in a micro combined heat and power (m-CHP) application. When the heat is captured, total efficiency can reach 80–90% at the unit, but does not consider production and distribution losses. CHP units are being developed today for the European home market.

Professor Jeremy P. Meyers, in the Electrochemical Society journal Interface in 2008, wrote, "While fuel cells are efficient relative to combustion engines, they are not as efficient as batteries, primarily due to the inefficiency of the oxygen reduction reaction (and . the oxygen evolution reaction, should the hydrogen be formed by electrolysis of water). [T]hey make the most sense for operation disconnected from the grid, or when fuel can be provided continuously. For applications that require frequent and relatively rapid start-ups . where zero emissions are a requirement, as in enclosed spaces such as warehouses, and where hydrogen is considered an acceptable reactant, a [PEM fuel cell] is becoming an increasingly attractive choice [if exchanging batteries is inconvenient]". [72] In 2013 military organizations were evaluating fuel cells to determine if they could significantly reduce the battery weight carried by soldiers. [73]

Power Edit

Stationary fuel cells are used for commercial, industrial and residential primary and backup power generation. Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, communications centers, rural locations including research stations, and in certain military applications. A fuel cell system running on hydrogen can be compact and lightweight, and have no major moving parts. Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability. [74] This equates to less than one minute of downtime in a six-year period. [74]

Since fuel cell electrolyzer systems do not store fuel in themselves, but rather rely on external storage units, they can be successfully applied in large-scale energy storage, rural areas being one example. [75] There are many different types of stationary fuel cells so efficiencies vary, but most are between 40% and 60% energy efficient. [5] However, when the fuel cell's waste heat is used to heat a building in a cogeneration system this efficiency can increase to 85%. [5] This is significantly more efficient than traditional coal power plants, which are only about one third energy efficient. [76] Assuming production at scale, fuel cells could save 20–40% on energy costs when used in cogeneration systems. [77] Fuel cells are also much cleaner than traditional power generation a fuel cell power plant using natural gas as a hydrogen source would create less than one ounce of pollution (other than CO
2 ) for every 1,000 kW·h produced, compared to 25 pounds of pollutants generated by conventional combustion systems. [78] Fuel Cells also produce 97% less nitrogen oxide emissions than conventional coal-fired power plants.

One such pilot program is operating on Stuart Island in Washington State. There the Stuart Island Energy Initiative [79] has built a complete, closed-loop system: Solar panels power an electrolyzer, which makes hydrogen. The hydrogen is stored in a 500-U.S.-gallon (1,900 L) tank at 200 pounds per square inch (1,400 kPa), and runs a ReliOn fuel cell to provide full electric back-up to the off-the-grid residence. Another closed system loop was unveiled in late 2011 in Hempstead, NY. [80]

Fuel cells can be used with low-quality gas from landfills or waste-water treatment plants to generate power and lower methane emissions. A 2.8 MW fuel cell plant in California is said to be the largest of the type. [81] Small-scale (sub-5kWhr) fuel cells are being developed for use in residential off-grid deployment. [82]

Cogeneration Edit

Combined heat and power (CHP) fuel cell systems, including micro combined heat and power (MicroCHP) systems are used to generate both electricity and heat for homes (see home fuel cell), office building and factories. The system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. As the result CHP systems have the potential to save primary energy as they can make use of waste heat which is generally rejected by thermal energy conversion systems. [83] A typical capacity range of home fuel cell is 1–3 kWel, 4–8 kWth. [84] [85] CHP systems linked to absorption chillers use their waste heat for refrigeration. [86]

The waste heat from fuel cells can be diverted during the summer directly into the ground providing further cooling while the waste heat during winter can be pumped directly into the building. The University of Minnesota owns the patent rights to this type of system [87] [88]

Co-generation systems can reach 85% efficiency (40–60% electric and the remainder as thermal). [5] Phosphoric-acid fuel cells (PAFC) comprise the largest segment of existing CHP products worldwide and can provide combined efficiencies close to 90%. [89] [90] Molten carbonate (MCFC) and solid-oxide fuel cells (SOFC) are also used for combined heat and power generation and have electrical energy efficiencies around 60%. [91] Disadvantages of co-generation systems include slow ramping up and down rates, high cost and short lifetime. [92] [93] Also their need to have a hot water storage tank to smooth out the thermal heat production was a serious disadvantage in the domestic market place where space in domestic properties is at a great premium. [94]

Delta-ee consultants stated in 2013 that with 64% of global sales the fuel cell micro-combined heat and power passed the conventional systems in sales in 2012. [73] The Japanese ENE FARM project will pass 100,000 FC mCHP systems in 2014, 34.213 PEMFC and 2.224 SOFC were installed in the period 2012–2014, 30,000 units on LNG and 6,000 on LPG. [95]

Fuel cell electric vehicles (FCEVs) Edit

Automobiles Edit

By year-end 2019, about 18,000 FCEVs had been leased or sold worldwide. [96] Three fuel cell electric vehicles have been introduced for commercial lease and sale: the Honda Clarity, Toyota Mirai and the Hyundai ix35 FCEV. Additional demonstration models include the Honda FCX Clarity, and Mercedes-Benz F-Cell. [97] As of June 2011 demonstration FCEVs had driven more than 4,800,000 km (3,000,000 mi), with more than 27,000 refuelings. [98] Fuel cell electric vehicles feature an average range of 314 miles between refuelings. [99] They can be refueled in less than 5 minutes. [100] The U.S. Department of Energy's Fuel Cell Technology Program states that, as of 2011, fuel cells achieved 53–59% efficiency at one-quarter power and 42–53% vehicle efficiency at full power, [101] and a durability of over 120,000 km (75,000 mi) with less than 10% degradation. [102] In a 2017 Well-to-Wheels simulation analysis that "did not address the economics and market constraints", General Motors and its partners estimated that per mile traveled, a fuel cell electric vehicle running on compressed gaseous hydrogen produced from natural gas could use about 40% less energy and emit 45% less greenhouse gasses than an internal combustion vehicle. [103]

In 2015, Toyota introduced its first fuel cell vehicle, the Mirai, at a price of $57,000. [104] Hyundai introduced the limited production Hyundai ix35 FCEV under a lease agreement. [105] In 2016, Honda started leasing the Honda Clarity Fuel Cell. [106] In 2020, Toyota introduced the second generation of its Mirai brand, improving fuel efficiency and expanding range compared to the original Sedan 2014 model. [107]

Criticism Edit

Some commentators believe that hydrogen fuel cell cars will never become economically competitive with other technologies [108] [109] [110] or that it will take decades for them to become profitable. [72] [111] Elon Musk, CEO of battery-electric vehicle maker Tesla Motors, stated in 2015 that fuel cells for use in cars will never be commercially viable because of the inefficiency of producing, transporting and storing hydrogen and the flammability of the gas, among other reasons. [112]

In 2012, Lux Research, Inc. issued a report that stated: "The dream of a hydrogen economy . is no nearer". It concluded that "Capital cost . will limit adoption to a mere 5.9 GW" by 2030, providing "a nearly insurmountable barrier to adoption, except in niche applications". The analysis concluded that, by 2030, PEM stationary market will reach $1 billion, while the vehicle market, including forklifts, will reach a total of $2 billion. [111] Other analyses cite the lack of an extensive hydrogen infrastructure in the U.S. as an ongoing challenge to Fuel Cell Electric Vehicle commercialization. [65]

In 2014, Joseph Romm, the author of The Hype About Hydrogen (2005), said that FCVs still had not overcome the high fueling cost, lack of fuel-delivery infrastructure, and pollution caused by producing hydrogen. "It would take several miracles to overcome all of those problems simultaneously in the coming decades." [113] He concluded that renewable energy cannot economically be used to make hydrogen for an FCV fleet "either now or in the future." [108] Greentech Media's analyst reached similar conclusions in 2014. [114] In 2015, Clean Technica listed some of the disadvantages of hydrogen fuel cell vehicles. [115] So did Car Throttle. [116]

A 2019 video by Real Engineering noted that, notwithstanding the introduction of vehicles that run on hydrogen, using hydrogen as a fuel for cars does not help to reduce carbon emissions from transportation. The 95% of hydrogen still produced from fossil fuels releases carbon dioxide, and producing hydrogen from water is an energy-consuming process. Storing hydrogen requires more energy either to cool it down to the liquid state or to put it into tanks under high pressure, and delivering the hydrogen to fueling stations requires more energy and may release more carbon. The hydrogen needed to move a FCV a kilometer costs approximately 8 times as much as the electricity needed to move a BEV the same distance. [117] A 2020 assessment concluded that hydrogen vehicles are still only 38% efficient, while battery EVs are 80% efficient. [118]

Buses Edit

As of August 2011 [update] , there were about 100 fuel cell buses in service around the world. [119] Most of these were manufactured by UTC Power, Toyota, Ballard, Hydrogenics, and Proton Motor. UTC buses had driven more than 970,000 km (600,000 mi) by 2011. [120] Fuel cell buses have from 39% to 141% higher fuel economy than diesel buses and natural gas buses. [103] [121]

As of 2019, the NREL was evaluating several current and planned fuel cell bus projects in the U.S. [122]

Forklifts Edit

A fuel cell forklift (also called a fuel cell lift truck) is a fuel cell-powered industrial forklift truck used to lift and transport materials. In 2013 there were over 4,000 fuel cell forklifts used in material handling in the US, [123] of which 500 received funding from DOE (2012). [124] [125] Fuel cell fleets are operated by various companies, including Sysco Foods, FedEx Freight, GENCO (at Wegmans, Coca-Cola, Kimberly Clark, and Whole Foods), and H-E-B Grocers. [126] Europe demonstrated 30 fuel cell forklifts with Hylift and extended it with HyLIFT-EUROPE to 200 units, [127] with other projects in France [128] [129] and Austria. [130] Pike Research projected in 2011 that fuel cell-powered forklifts would be the largest driver of hydrogen fuel demand by 2020. [131]

Most companies in Europe and the US do not use petroleum-powered forklifts, as these vehicles work indoors where emissions must be controlled and instead use electric forklifts. [132] [133] Fuel cell-powered forklifts can provide benefits over battery-powered forklifts as they can be refueled in 3 minutes and they can be used in refrigerated warehouses, where their performance is not degraded by lower temperatures. The FC units are often designed as drop-in replacements. [134] [135]

Motorcycles and bicycles Edit

In 2005, a British manufacturer of hydrogen-powered fuel cells, Intelligent Energy (IE), produced the first working hydrogen-run motorcycle called the ENV (Emission Neutral Vehicle). The motorcycle holds enough fuel to run for four hours, and to travel 160 km (100 mi) in an urban area, at a top speed of 80 km/h (50 mph). [136] In 2004 Honda developed a fuel-cell motorcycle that utilized the Honda FC Stack. [137] [138]

Other examples of motorbikes [139] and bicycles [140] that use hydrogen fuel cells include the Taiwanese company APFCT's scooter [141] using the fueling system from Italy's Acta SpA [142] and the Suzuki Burgman scooter with an IE fuel cell that received EU Whole Vehicle Type Approval in 2011. [143] Suzuki Motor Corp. and IE have announced a joint venture to accelerate the commercialization of zero-emission vehicles. [144]

Airplanes Edit

In 2003, the world's first propeller-driven airplane to be powered entirely by a fuel cell was flown. The fuel cell was a stack design that allowed the fuel cell to be integrated with the plane's aerodynamic surfaces. [145] Fuel cell-powered unmanned aerial vehicles (UAV) include a Horizon fuel cell UAV that set the record distance flown for a small UAV in 2007. [146] Boeing researchers and industry partners throughout Europe conducted experimental flight tests in February 2008 of a manned airplane powered only by a fuel cell and lightweight batteries. The fuel cell demonstrator airplane, as it was called, used a proton-exchange membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which was coupled to a conventional propeller. [147]

In 2009, the Naval Research Laboratory's (NRL's) Ion Tiger utilized a hydrogen-powered fuel cell and flew for 23 hours and 17 minutes. [148] Fuel cells are also being tested and considered to provide auxiliary power in aircraft, replacing fossil fuel generators that were previously used to start the engines and power on board electrical needs, while reducing carbon emissions. [149] [150] [ failed verification ] In 2016 a Raptor E1 drone made a successful test flight using a fuel cell that was lighter than the lithium-ion battery it replaced. The flight lasted 10 minutes at an altitude of 80 metres (260 ft), although the fuel cell reportedly had enough fuel to fly for two hours. The fuel was contained in approximately 100 solid 1 square centimetre (0.16 sq in) pellets composed of a proprietary chemical within an unpressurized cartridge. The pellets are physically robust and operate at temperatures as warm as 50 °C (122 °F). The cell was from Arcola Energy. [151]

Lockheed Martin Skunk Works Stalker is an electric UAV powered by solid oxide fuel cell. [152]

Boats Edit

The world's first fuel-cell boat HYDRA used an AFC system with 6.5 kW net output. Amsterdam introduced fuel cell-powered boats that ferry people around the city's canals. [153]

Submarines Edit

The Type 212 submarines of the German and Italian navies use fuel cells to remain submerged for weeks without the need to surface.

The U212A is a non-nuclear submarine developed by German naval shipyard Howaldtswerke Deutsche Werft. [154] The system consists of nine PEM fuel cells, providing between 30 kW and 50 kW each. The ship is silent, giving it an advantage in the detection of other submarines. [155] A naval paper has theorized about the possibility of a nuclear-fuel cell hybrid whereby the fuel cell is used when silent operations are required and then replenished from the Nuclear reactor (and water). [156]

Portable power systems Edit

Portable fuel cell systems are generally classified as weighing under 10 kg and providing power of less than 5 kW. [157] The potential market size for smaller fuel cells is quite large with an up to 40% per annum potential growth rate and a market size of around $10 billion, leading a great deal of research to be devoted to the development of portable power cells. [158] Within this market two groups have been identified. The first is the microfuel cell market, in the 1-50 W range for power smaller electronic devices. The second is the 1-5 kW range of generators for larger scale power generation (e.g. military outposts, remote oil fields).

Microfuel cells are primarily aimed at penetrating the market for phones and laptops. This can be primarily attributed to the advantageous energy density provided by fuel cells over a lithium-ion battery, for the entire system. For a battery, this system includes the charger as well as the battery itself. For the fuel cell this system would include the cell, the necessary fuel and peripheral attachments. Taking the full system into consideration, fuel cells have been shown to provide 530Wh/kg compared to 44 Wh/kg for lithium ion batteries. [158] However, while the weight of fuel cell systems offer a distinct advantage the current costs are not in their favor. while a battery system will generally cost around $1.20 per Wh, fuel cell systems cost around $5 per Wh, putting them at a significant disadvantage. [158]

As power demands for cell phones increase, fuel cells could become much more attractive options for larger power generation. The demand for longer on time on phones and computers is something often demanded by consumers so fuel cells could start to make strides into laptop and cell phone markets. The price will continue to go down as developments in fuel cells continues to accelerate. Current strategies for improving micro fuel cells is through the use of carbon nanotubes. It was shown by Girishkumar et al. that depositing nanotubes on electrode surfaces allows for substantially greater surface area increasing the oxygen reduction rate. [159]

Fuel cells for use in larger scale operations also show much promise. Portable power systems that use fuel cells can be used in the leisure sector (i.e. RVs, cabins, marine), the industrial sector (i.e. power for remote locations including gas/oil wellsites, communication towers, security, weather stations), and in the military sector. SFC Energy is a German manufacturer of direct methanol fuel cells for a variety of portable power systems. [160] Ensol Systems Inc. is an integrator of portable power systems, using the SFC Energy DMFC. [161] The key advantage of fuel cells in this market is the great power generation per weight. While fuel cells can be expensive, for remote locations that require dependable energy fuel cells hold great power. For a 72-h excursion the comparison in weight is substantial, with a fuel cell only weighing 15 pounds compared to 29 pounds of batteries needed for the same energy. [157]

Other applications Edit

  • Providing power for base stations or cell sites[162][163] are a type of fuel cell system, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific laboratories, data centers, [164]
  • telecommunication [165] equipment and modern naval ships.
  • An uninterrupted power supply (UPS) provides emergency power and, depending on the topology, provide line regulation as well to connected equipment by supplying power from a separate source when utility power is not available. Unlike a standby generator, it can provide instant protection from a momentary power interruption. , pairing the fuel cell with either an ICE or a battery. for applications where AC charging may not be readily available.
  • Portable charging docks for small electronics (e.g. a belt clip that charges a cell phone or PDA). , laptops and tablets.
  • Small heating appliances [166] , achieved by exhausting the oxygen and automatically maintaining oxygen exhaustion in a shipping container, containing, for example, fresh fish. [167] , where the amount of voltage generated by a fuel cell is used to determine the concentration of fuel (alcohol) in the sample. [168] , electrochemical sensor.

Fueling stations Edit

According to FuelCellsWorks, an industry group, at the end of 2019, 330 hydrogen refueling stations were open to the public worldwide. [169] As of June 2020, there were 178 publicly available hydrogen stations in operation in Asia. [170] 114 of these were in Japan. [171] There were at least 177 stations in Europe, and about half of these were in Germany. [172] [173] There were 44 publicly accessible stations in the US, 42 of which were located in California. [174]

A hydrogen fueling station costs between $1 million and $4 million to build. [175]

In 2012, fuel cell industry revenues exceeded $1 billion market value worldwide, with Asian pacific countries shipping more than 3/4 of the fuel cell systems worldwide. [176] However, as of January 2014, no public company in the industry had yet become profitable. [177] There were 140,000 fuel cell stacks shipped globally in 2010, up from 11,000 shipments in 2007, and from 2011 to 2012 worldwide fuel cell shipments had an annual growth rate of 85%. [178] Tanaka Kikinzoku expanded its manufacturing facilities in 2011. [179] Approximately 50% of fuel cell shipments in 2010 were stationary fuel cells, up from about a third in 2009, and the four dominant producers in the Fuel Cell Industry were the United States, Germany, Japan and South Korea. [180] The Department of Energy Solid State Energy Conversion Alliance found that, as of January 2011, stationary fuel cells generated power at approximately $724 to $775 per kilowatt installed. [181] In 2011, Bloom Energy, a major fuel cell supplier, said that its fuel cells generated power at 9–11 cents per kilowatt-hour, including the price of fuel, maintenance, and hardware. [182] [183]

Industry groups predict that there are sufficient platinum resources for future demand, [184] and in 2007, research at Brookhaven National Laboratory suggested that platinum could be replaced by a gold-palladium coating, which may be less susceptible to poisoning and thereby improve fuel cell lifetime. [185] Another method would use iron and sulphur instead of platinum. This would lower the cost of a fuel cell (as the platinum in a regular fuel cell costs around US$1,500 , and the same amount of iron costs only around US$1.50 ). The concept was being developed by a coalition of the John Innes Centre and the University of Milan-Bicocca. [186] PEDOT cathodes are immune to monoxide poisoning. [187]

In 2016, Samsung "decided to drop fuel cell-related business projects, as the outlook of the market isn't good". [188]


Flame Temperatures Table for Different Fuels

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    This is a list of flame temperatures for various common fuels. Adiabatic flame temperatures for common gases are provided for air and oxygen. (For these values, the initial temperature of air, gas, and oxygen is 20 °C.) MAPP is a mixture of gases, chiefly methyl acetylene, and propadiene with other hydrocarbons. You'll get the most bang for your buck, relatively speaking, from acetylene in oxygen (3100°C) and either acetylene (2400°C), hydrogen (2045°C), or propane (1980°C) in the air.


    Methods in Methane Metabolism, Part A

    Katharina Schlegel , Volker Müller , in Methods in Enzymology , 2011

    1 Introduction

    Methanogenic archaea grow on a limited number of C1 substrates, such as methanol, methylamines, formate, and some can also use acetate ( Deppenmeier, 2002 Ferry, 1997 Thauer et al., 2008 ). Central to all those pathways is the Wood–Ljundahl pathway ( Ljungdahl, 1994 Ragsdale, 2008 ). During growth on H2 + CO2, CO2 is first bound to methanofuran and thereby reduced to a formyl group ( Fig. 12.1 ). This endergonic reaction is catalyzed by the formylmethanofuran dehydrogenase and driven by the electrochemical ion gradient across the membrane ( Kaesler and Schönheit, 1989 Winner and Gottschalk, 1989 ). The formyl group is transferred to tetrahydromethanopterin (H4MPT) and reduced to a methyl group. This methyl group is transferred to cofactor M (CoM-SH) by the methyl-H4MPT:HS-CoM methyltransferase. This exergonic reaction (ΔG0′ = − 29 kJ/mol) is used to transfer 1.7 Na + /CH4 over the membrane and thereby generates a primary and electrogenic Na + gradient across the membrane ( Becher et al., 1992b Gottschalk and Thauer, 2001 Lienard et al., 1996 Müller et al., 1987 ). In the next step, methyl-CoM undergoes a nucleophilic attack by the thiolate anion of HS-CoB (Coenzyme B, 7-thioheptanoyl-o-phosho- l -threonine), thus liberating CH4 and generating a disulfide of CoM and CoB, the so-called heterodisulfide. The heterodisulfide is the terminal electron acceptor in methanogenesis and is reduced and thereby cleaved by a membrane-bound heterodisulfide reductase. The electrons necessary for this reaction are provided by F420 (during growth on methylated substrates), a membrane-bound hydrogenase (during growth on H2 + CO2) or ferredoxin (during growth on acetate) and transferred via an electron transport chain. In this reaction, three to four H + are translocated over the membrane ( Blaut et al., 1987 Deppenmeier et al., 1990 ).

    Figure 12.1 . Ion currents during methanogenesis from H2 + CO2. Please note that this model only describes the pathway in M. mazei and M. barkeri. M. acetivorans does not contain hydrogenases. (1) Ech/Eha hydrogenase (2) Na + /H + antiporter (3) methyl-H4MPT coenzyme M methyltransferase (4) H2: heterodisulfide oxidoreductase system (5) A1AO ATP synthase MF, methanofuran H4MPT, tetrahydromethanopterin CoM-SH, coenzyme M CoB-SH, coenzyme B Fd, ferredoxin.

    During growth on methyl-group containing substrates, the methyl groups are channeled to CoM-SH by specific methyltransferases. One quarter of the methyl group is oxidized to CO2 to gain the reducing equivalents to reduce the other 75% to methane ( Deppenmeier, 2002 van der Meijden et al., 1983 Fig. 12.2 ). Methanogenesis from acetate starts with the activation of acetate to acetyl-CoA, followed by an oxidation of acetyl-CoA to CO2 and a coenzyme-bound methyl group that is reduced to CH4 with electrons gained during the oxidation reaction ( Ferry, 1997 Terlesky and Ferry, 1988 Thauer, 1990 ).

    Figure 12.2 . Ion currents during methanogenesis from methanol. Please note that this model only describes the pathway in M. mazei and M. barkeri. M. acetivorans does not contain an Ech hydrogenase but a Rnf complex. The electrons of the hydrogen produced by the Ech/Eha hydrogenase are finally transferred to the heterodisulfide reductase. It is still a matter of debate whether this occurs via a soluble F420 reducing hydrogenase or directly by the H2: heterodisulfide oxidoreductase system. (1) Ech/Eha hydrogenase (2) Na + /H + antiporter (3) methyl-H4MPT-coenzyme M methyltransferase (4) F420: heterodisulfide oxidoreductase system (5) A1AO ATP synthase MF, methanofuran H4MPT, tetrahydromethanopterin CoM-SH, coenzyme M CoB-SH, coenzyme B Fd, ferredoxin.

    A common feature of methanogenesis found in all methanogens investigated so far is its Na + requirement. Na + is required not only for growing cells but also by resting cells for methanogenesis, indicating an involvement of Na + in one of the steps of the pathway ( Müller et al., 1986, 1988 Perski et al., 1981, 1982 ). Indeed, it turned out that the methyl-H4SPT:HS-CoM-methyltransferase is a primary Na + pump ( Becher et al., 1992a Müller et al., 1987 ). Apart from the Na + motive methyl-H4MPT:HS-CoM-methyltransferase, the heterodisulfide reductase system is proton motive. Thus, methanogens are the only organisms that generate a primary proton and sodium ion gradient at the same time. How the two ion motive forces are connected to ATP synthesis is still a matter of debate, but the A1AO ATP synthase is essential for ATP synthesis ( Deppenmeier and Müller, 2008 Müller and Grüber, 2003 Pisa et al., 2007 Saum et al., 2009 ).

    In this chapter, we describe how Na + dependence of growth and methanogenesis is analyzed, how Na + transport is measured in cell suspensions, and how ATP synthesis as well as ATP hydrolysis is measured.


    Electron Behavior

    On a submolecular level, the reason for the difference in energy levels between the reactants and products, lies with electronic configurations. Hydrogen atoms have one electron each. They combine into molecules of two so that they can share two electrons (one each). This is because the inner-most electron shell is at a lower energy state (and therefore more stable) when occupied by two electrons. Oxygen atoms have eight electrons each. They combine together in molecules of two by sharing four electrons so that their outer-most electron shells are fully occupied by eight electrons each. However, a far more stable alignment of electrons arises when two hydrogen atoms share an electron with one oxygen atom. Only a small amount of energy is needed to "bump" the electrons of the reactants "out" of their orbits so that they can realign in the more energetically stable alignment, forming a new molecule, H2O.


    Watch the video: Der Hackl Schorsch erklärt Biogas - Wärme (May 2022).