Your choices:
1 Process heat (> 1000 °C); 2 Fuel: gaseous; 3 Biomass (solid)
| 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 |
| 3 Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | 2 Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | 1 Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
To be feasible for a specific process, the theoretical flame temperature of the fuel should be at least 200, preferably 3-400, degrees higher than the process temperature. LPG and light fuel oil has theoretical flame temperatures about 1850-1900 °C while wood pellets (i.e. 10% moisture content) end up 25-50 °C lower. As the moisture content in woody biomass increases, the attainable flame temperature drops and at 30% moisture content the flame temperature has dropped below 1600 °C. Agricultural biomass generally exhibits flame temperatures about 50-100 °C lower than those of woody biomass at the same moisture content.
During gasification 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
The fact that the biomass is gasified does not per se increase the attainable flame temperature. Rather the other way around: During the gasification process there will inevitably be a loss of energy and that loss of energy will correspond to a drop in the final combustion (i.e. flame) temperature. On the other hand will the fact that the fuel becomes gaseous improve the possibility for combustion control and that may provide for a compensation of the energy loss so that the flame temperature is again restored.
The demand on flame temperature will also reflect backwards in the supply chain: With a properly planned supply chain, there will be no major problem to guarantee a moisture content of 30% with wood chips or agricultural biomass supplied using only open-air drying. To guarantee moisture contents below 30%, though, there will be a need of forced drying to be included in the fuel supply chain and that will, of course, have an influence on the price for the biomass supplied.
The conclusion becomes that the gasifier should be placed close to the main process and the gas distributed hot. This will call for burner modifications and also the control system will have to be modified. These problems can all be overcome and none of them is major, but before taking the decision to change, the consequences need be thoroughly evaluated.