This project is primarily concerned with the thermodynamic, environmental and economic performance or potential of fuel cells. However, the project team are interested in these factors coupled with the feasibility of biofuels for use with fuel cells. It is therefore useful to explain some details of the types of biofuels which we will consider for use with our fuel cell "models".
Biofuels are solids, liquids or gases derived from organic materials such as plant life, industrial or domestic waste including sewage, or agricultural waste (both animal and crop) . These biofuels can be obtained in a number of ways, and can be processed to give refined fuels, or, in some cases can be used directly. This project is interested in deriving fuels suitable for use with fuel cells.
The primary focus of our fuel survey is aimed at hydrogen yielding compounds and mixtures such as "biogas", synthesis gas and methanol. The analysis is broken down into the following sections -
The sources of biofuels, their availability and processing requirements for fuel cells are considered here. The potential for use of biofuels and environmental aspects are considered in a separate
review.
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| Biogas Generation Via A Digestor With Stripping or Membrane Purification |
As described above the important factor in the use of biogas is the ability to obtain the maximum practical yield of hydrogen from biogas production. Organic material is introduced to the digestor where anaerobic digestion or fermentation (that is in the absence of oxygen) occurs. This produces a "biogas" which consists mainly of methane and carbon dioxide, but which also contains quantities of hydrogen sulphide and water. In addition to cleaning increasing the concentration of methane in the biogas mixture it is also necessary to remove the hydrogen sulphide which is a highly corrosive compound.
In the first of the purification methods, hydrogen sulphide is removed from the biogas by a process known as chemisorption usualy using iron oxide or zinc oxide. The resultant product is sulphur, relatively harmless. The use of the carbon dioxide scrubbing column increases the yield of methane in the resultant biogas.
In the second method, water absorption removes the impurities.
In the third process, membrane separation in the membrane element follows removal of excess water in the dehydrator. On one side methane gas is yielded, on the other, carbon dioxide and hydrogen sulphide.
The clean biogas is then stored at pressure for use in the fuel cell.
The results of information searches on the relative yields of biogas of various raw agricultural biomass materials are shown on the table below.
| BIOMASS | CH4Yield/ m3 per Kg VS | m3 Gas per m3 VS per Culture day |
| Cow Manure | 0.265 | 2.21 |
| Pig Manure | 0.140 | 1.3 |
| Kelp | 0.281 | 0.445 |
| Grass | 0.184 | 0.370 |
| Distillery Waste | 0.149 | 2.5 |
| Energy Crops | 0.184 - 0.274 | 0.7 - 1.0 |
* Food wastes in this table are generally considered to have a high sugar content
Sophisticated techniques allow fast pyrolysis of organic matter (usually wood or other plant life). With these techniques, the process is carried out at temperatures of 800 - 900 oC. This leaves only about 10% of the material as solid char and can give up to 60 % of the product as gas, rich in hydrogen and carbon monoxide. Other organic matter such as rubber and plastics, can be treated in this way. If the temperatures of the process are controlled appropriately, the composition of the gas can be influenced.
Synthesis gas can be produced in a gasification process, of which there are many. Here, a solid fuel is reacted with hot steam and air or oxygen (best results are achieved using oxygen rather than air). If air is used with steam, the resulting gas will contain around 50% of nitrogen and carbon dioxide, giving a relatively low energy content, of around 10% of that available from methane. If the gasification process uses oxygen rather than air, the energy content of the resulting gas increases, since there is a negligible amount of nitrogen in the process.
Methanol can be manufactured from synthesis gas produced as described above. Others have proposed a mixed process where, gas produced from gasification of woody biomass is further reacted by using steam to shift some CO to CO2 thus producing sufficient H2 to produce methanol. Another proposal was to use natural gas as a co-feedstock. Natural gas when reformed with steam yields more H2 than is required for methanol synthesis. It may be possible to consider mixing biogas from anaerobic digestion as co-feedstock, to improve the yield of suitable synthesis gas for methanol production. See reference 2
The advantages of methanol over methane include, low cost storage for seasonal load variations and biomass crop yields. The possibility of using direct methanol fuel cells should not be overlooked.
Ethanol can be produced by a number of fermentation processes using cellulosic biomass as the feedstock. Such biomass energy crops might be switchgrass, hybrid poplar trees or sugar cane for example. See reference 2
Biogas can be produced in sewage plants using anaerobic sludge digestion. Usually, the process is carried out at 30 to 38oC. It is not intended to go into the full detail of anaerobic digestion processes here, however, the main types are detailed below
- Standard-Rate Digestion. Usually carried out in a single stage where digestion, sludge thickening and supernatant formation (formation of a clear liquid above a settled precipitate) occur simultaneously. Untreated sludge is added to the area where sludge is actively digesting. Heat is applied via external heat exchangers. Gas rises to the surface and is collected and stored.
- Single-Stage High-Rate Digestion. The differenc between this process and standard-rate process, is that the sludge is added at a much faster rate. There is more agitation of the sludge by gas recirculation and mechanical mixers, pumping or draft tube mixers. Again, the sludge is heated to attain optimal digestion rates. Gas may be stored in the digester with a floating gas holder roof, or drawn off and compressed for storage.
- Two-Stage Digestion. A high-rate digester is coupled in series with a second digestion tank. The first tank is used for digestion and is heated and fitted with meaans of agitation. The second tank is used for storage and concentration of digested sludge. Again, gas may be stored in the digester with a floating gas holder roof, or drawn off and compressed for storage.
PROCESS DESIGN
For sewage plants (***ref from Jorge Here***) the volume of CH4 produced from a digestor can be calculated from the following formula
VCH4= (5.62) [ (So- S) (Q) (8.34)-1.42 Px]
Where
VCH4 = the volume of methane gas produced at standard conditions (0oC and 1 atm), ft3/day
5.62 = theoretical conversion factor for the amount of meyhane produced from the compplete conversion of 1 pound of BODL to methane and carbon dioxide, ft3BODL oxidised.
Q = flowrate, Mgal/day
So = ultimate BODL in influent, mg/L
S = ultimate BODL in effluent, mg/L
8.34 = conversion factor, lb/Mgal.(mg/L)
Px = net mass of cell tissue produced per day, lb/day
For a comp[lete mix high-rate digester without recycle the mass of biological solids synthesized daily, Pxj can be estimated using
Px = Y[ (So- S) (Q) (8.34) ] / ( 1 + kdDc )
Where
Y = yield coeeficient, lb/lb
kd = endogenous coefficient, per day
Dc = mean cell-residence time, day
The following table gives some suggested mean cell-residence times for use in the design of complete-mix digesters
| Operating Temperature, oC | DcM, day (minimum) | Dc, day ( suggested for design) |
| 18 | 11 | 28 |
| 24 | 8 | 20 |
| 30 | 6 | 14 |
| 35 | 4 | 10 |
| 40 | 4 | 10 |
For tank design other factors must be taken into account and incorporated into any sizing calculations.
Biogas produced from anaerobic digestion of sewage sludge, contains 60 - 70 % CH4, by volume. The other constituents are 30 % CO2, with residual amounts of N2, H2, H2S and water vapour. As described above for agricultural biogas, some cleaning of the gas will be required.
GAS PRODUCTION
Total gas production from the breakdown of volatile solids can be estimated as 0.75 - 1.12 m3/kg of volatile solids.
Since a large proportion of municipal solid waste contains biological matter, in the right conditions anaerobic digestion occurs. This yields gas consisting mainly of CH4 and CO2 with between 50 and 60% of the gas being CH4. The same requirements for cleaning of the gas prior to reforming if a fuel is to be used, apply.
In theory, 5-6GJ per tonne of waste is achievable, but practical yields are well below this. The team did not attempt a case study involving landfill gas.
- Sofer, S S and Zaborsky, O R (editors), Biomas conversion processes for energy and fuels,Plenum Press
- Borgwardt, R H,Transportation fuel from cellulosic biomass: a comoparitive assessment of ethanol and methanol options, Proc Instn Mech Engrs Vol213 Part A 1999
- Kinoshita, C M; Turn, S Q; Overend, R P; Bain, R L ,Power generation potential of biomass gasification systems, Journal of Energy Engineering, December 1997
- Boyle, G ,Renewable energy, power for a sustainable future, Oxford University Press, 1998
- Palz, W; Coombs, J; Hall, D O,Energy from Biomass, , Proceedings of the International Conference on Biomass, Held in Venice, Italy 25- 29 March 1985, Elsevier Applied Science Publishers
- Bridgewater, A V; Kuester, J L,Research in Thermochemical Biomass Conversion, International Conference on Research in Thermochemical Biomass Conversion April 1988, Pheonix Arizona
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