Your choices:
1 Process heat (> 1000 °C); 2 Biomass (solid); 3 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 |
| 2 Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | 3 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) |
Some processes, such as the large scale production of container glass in glass tanks or steel reheating prior to rolling, will have a large number of burners mounted in the main furnace and these burners need be controlled group-wise.
For solid biomass to be an attractive fuel in such applications the first criterion will be that not every single burner has to be equipped with a complicated and expensive solid-fuel feeding system. Therefore, pulverised fuel or pellet firing will be out of question for such large-scale installations. Thus it will be advantageous to first gasify the biomass using – in cases like these where the demands on gas quality are not too high – a simple, air-blown, fixed-bed gasifier in direct conjunction with the furnace. It will be important that the gasifier is cheap and easy to operate since the main business idea will not be the gasification but this will only be a means to adapt the fuel to the process.
What will become crucial in cases like this will be the gas seals in the gasifier and the pressure control with the open-flame furnace: It must be remembered that gasifier gas is highly toxic and when it leaves the gasifier it will not only be hot but also flammable.
So the risks must be considered but there are no real technical problems with an installation like this.
The solid biomass should mainly be selected among low-price, low-chlorine and low-sulphur biomasses, so woody biomass in the form of chips would be preferred unless corrosive gases may be accepted in the furnace atmosphere and agricultural biomass be an alternative.
It must also be kept in mind that 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.
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.
The demand on flame temperature will 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.