Gasification route is somewhat more recent and less known. Combustion route is much better known for a long time technically and commercially.
Gasification process converts solid fuel into a gaseous fuel through a process of high temperature oxidation-reduction reactions. Combustion process converts solid fuel into gaseous products of combustion through high temperature oxidation reactions.
Gasification packages heat into chemical bonds by converting the energy into those in gaseous fuels. Combustion releases the energy into high temperature product gas.
The product of gasification is a fuel. It can be further used for combustion purposes (or others like fuel cell, methanol production etc). Compared to direct combustion process, gasification route to combustion can be interpreted as a two stage combustion process.
The thermo-chemical conversion process of biomass leads to a gas called producer gas with typical composition of 20 % CO, 20 % H2, 2 % CH4, 12% CO2, and rest nitrogen. This gas is cooled to ambient temperature and can be transported over economically meaningful distances of 50 to 100 m for being used in one location or several locations. In contrast when combustion process is performed, the hot gases consisting of CO2 and H2O along with some fraction of undesirable emissions will need to be used locally and the exhaust gas treated to maintain emission standards.
It is possible that even producer gas needs to be treated for difficult fuels like used wood, for instance. The throughput of the gas to be treated is slightly less than half of the throughput of a combustion process for treatment purposes. This will make it a more economical proposition for treatment purposes.
Gaseous fuel combustion process can be managed for high environmental compatibility through minimizing undesired emissions. For achieving these, two stage combustion processes are adopted (even for gaseous fuels).
If two-stage combustion process is adopted for biomass based gaseous fuel, it implies that we have a three-stage biomass – to – heat/electricity conversion process.
Such conversion processes promise the ability to fulfill even extreme demands of environmental compatibility using renewable biofuels at reasonable costs.
It is conventionally understood that combustion – steam power generation process is economical for large power levels, typically in excess of 3 MWe. The cost per MWe installed is about 1 million USD. The actual performance shows a fuel-to-electricity efficiency of ~ 30 %. Even 200 MWe steam power generation systems deliver fuel-electricity efficiency of 35 % (using coal with higher calorific value).
The primary reason for this feature is that while the flame temperatures from combustion of fuels is about 1400 to 1600 °C, it is transferred to a working fluid, namely steam, with operating temperatures of 600 to 800 °C and hence the thermodynamic conversion efficiencies will be limited.
If on the other hand, one uses the high temperatures of combustion directly as will happen in reciprocating internal combustion engines or continuous internal combustion based gas turbine engines, one can derive higher efficiencies in open cycle mode. For instance, reciprocating engines of 1 MWe capacity using biogas (75 % CH4 and rest CO2) give conversion efficiencies of 36 to 40 % on an open cycle basis. There is still heat at 350 – 400 C to be captured by using waste heat boilers.
At IISc, in a gas-alone mode, even a 58 kWe gas engine has given an efficiency of 24 % (from wood chips-to-electricity).
In a field dual-fuel installation (using light diesel oil and biomass) of 1 MWe capacity, overall conversion efficiencies of 30 % have been clocked at delivered loads of 750 kWe. The efficiency will reach 35 % when the system is run at full load.
Use of internal combustion engines, thus promise achieving of efficiencies of the order of 40 % (from biomass – to- electricity) even on open cycle at power levels of 1 to 3 MWe.
One can add at large power levels ~ 5 MWe, a downstream steam power generation segment that can generate additional 2 MWe of steam power taking the overall conversion efficiencies of 43 to 45 %, making the system an IGCC (integrated gasification combined cycle) with investment costs of the order of 1.5 to 1.75 million dollars per MWe.
The benefit of the above package is that it can be achieved today with no risk – just employing modules of the existing 1 MWe class systems or a combination of a few gasifiers providing gas to a single larger size dual-fuel or gas engines of 2.5 or 3.5 MWe ; it is the perceived risk that needs to be managed by putting up a demonstration system.
In doing all these, the emissions – (a) gaseous – NOx, CO, HC, and SOx can be shown to meet international norms, (b) liquid effluents – can be treated by water treatment processes that are standard and the disposal of the sludge meeting international norms and (c) solid residues – like ash and char used for landfill after further treatment that would be not be necessary in most cases [except when one uses old wood (altholz)].
In summary, classical combustion technology with steam power generation is a subset of the multi-stage process with moderate biomass-to-electricity conversion efficiency and either limited interventional capabilities at reasonable cost or expensive interventional capabilities for emissions. Gasification technology is multistage combustion process with high conversion efficiency (from biomass –to – electricity) and moderate costs from very low power levels ~ tens of kilowatts to several megawatts in which one derives the benefit of eliminating the undesirables at several stages between the staring point and the end point using the currently available Technologies for a variety of intermediate interventions.
In short it is the equivalent of Clean Coal Technology – it is Clean Biomass Technology.