Your choices so far:
1 Fuel: gaseous
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | 1 Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Just because a fuel is gaseous, it needs neither be good nor clean. Gaseous fuels can be produced from biomass in two ways, by the biochemical process known as anaerobic digestion or by the thermochemical process known as gasification.
In both cases, the aim of the process is to change the state of the solid (in case of digestion is the solid often found in a water suspension) biomass into a gas while at the same time retaining as much as possible of the solar energy originally bound in solid biomass by the photosynthesis. It is obvious that the conversion process cannot proceed without a loss of energy:
- The biochemical process makes use of bacteria and fungi first to split up the original carbohydrates, fats and proteins in the organic material by hydrolysis, turning them into sugars, fatty acids and amino acids. These are then recombined into organic acids and alcohol releasing hydrogen and carbon dioxide during the acidogenesis phase. The next step is to form acetic acid, a process during which carbon dioxide is again released (acetogenesis) and the final step is to form methane, again releasing carbon dioxide. Methane gas is the prime energy-carrying component in the biogas formed and typically amounts to some 50-70% by volume of the dry gas. One will realize that to perform this work, a number of different micro-organisms are employed and the reason that they perform all this work is because they gain energy from the different stages. That energy is taken from the feedstock. Depending on the nature of the feedstock (substrate in this context) approximately 50-60% energy fed with the solid material may be recovered in the form of gas, some 20-40% of the energy will be found in the output slurry (the digestate) and about 10-20% will be lost in the form of carbon dioxide and heat losses.
- Using the thermochemical route, the primary components in solid biomass, cellulose, hemicellulose, lignin, fats and proteins, are broken into smaller molecules by the use of heat. The heat is supplied by combusting part of the fuel and to run, the process requires temperatures in the range 700 – 1000 °C. The prime energy-carrying gas components in gasifier gas (or product gas) are carbon monoxide, hydrogen and methane, typically for air-blown gasifiers containing 10-15% carbon monoxide and similar amounts of hydrogen, methane about 5% on a dry gas basis. Using pure oxygen instead of air to run the process so that nitrogen no longer dilutes the gas will raise the gas quality significantly but the price will be the cost for oxygen. The losses in the process will amount to, depending on the system layout, anything from 10 to more than 30%.
The energy content (lower heating value) of the primary gas will be, in the case of biogas some 15-25 MJ/m3, for product gas from an air blown gasifier some 3-7 MJ/m3 and for an oxygen-blown gasifier some 7-15 MJ/m3.
Now it must be remembered that the gases will be contaminated:
- During the biochemical processes, hydrochloric acid as well as hydrogen sulphide and ammonia are formed and they will to a certain extent be present in the raw gas. The biogas will leave the digester at the digester temperature, typically 35-40 °C and since the digestion requires the presence of water, the water content in the gas will be some 5-7%. Since the gas will also contain corrosive and water soluble components, cooling the gas may become problematic.
- During the thermochemical process a number of heavy hydrocarbons ("tar") will be formed together with hydrogen sulphide, hydrochloric acid and a number of nitrogen compounds. Also, part of the inorganic ash will be evaporated and found in gas phase in the hot gas. The gasifier gas will leave the gasifier at the process temperature and as long as the gas is maintained at that temperature, gaseous contaminants and ashes remain gaseous, but the moment the temperature drops, tars as well as ashes will start condensing.
Biogas can easily be up-graded either by pressurised scrubbing where water is used to wash out the contaminants and to raise the methane content to 90-95% or in a pressure-swing-absorption (PSA) process yielding a similar quality. Since the fossil gas distributed throughout Europe in pipelines (natural gas) consists to the main part of methane but with a bit of heavier hydrocarbons (3-5%) in it, the addition of a little bit of LPG to the upgraded biogas makes it a copy of the fossil gas and it may then be injected into the pipeline system for distribution. Obviously a strict quality control will be necessary prior to injection, but given that, there will be no major problems.
The upgrading of gasifier gas is a much more complicated process since the gas composition is much more complex and singe the starting point is a hot gas still containing complex hydrocarbons and inorganic ash components. The bottleneck for thermal gasification has been – and still is – to clean the gas from its contaminants without having to cool it down. If the gas is cooled down from process temperature to ambient for cleaning, this will represent a very significant heat loss and loss of total efficiency. Once this problem is solved, however, there will be multiple options for a subsequent processing and upgrading of the gasifier gas for numerous purposes. This is known as the “biorefinery concept” but has so far not been realized on a commercial basis.
Hence, gasifier gas should be used on-site and combusted either in an industrial process or in a boiler without first having been cleaned. So the question becomes what is the added value from thermal gasification? Part of the solid ash will be retained in the gasifier and if this part of the ash is the most problematic for the subsequent process, then the answer is clear. It may also be the case that the industrial process where the gas is to be used requires a gaseous fuel. But it must be remembered that unless the gas has been cleaned, the gasifier gas will basically contain everything except part of the ashes and part of the energy that was present in the feedstock.