Unlike SO2 and NOx there is no cleaning technology to reduce carbon emissions, but financial and operational measures can be taken to deal with carbon produced, each making a contribution to the reduction of carbon emissions. There are three broad options:
Improvement in combustion efficiencies
These efficiencies are possible with relatively simple measures such as improved steam turbines, more efficient boilers, more efficient gasifiers or new techniques. New techniques include circulating fluidised bed technology (CFB) and pressurised fluid bed combustion (PFBC), advanced concepts of plant integration such as integrated gasification combined cycle (IGCC) technologies and supercritical firing (at higher temperatures and pressures).
Co-firing with CO2 neutral biomass fuels
Co-firing biomass with coal still emits carbon, but the biomass is carbon neutral because in replacing it when new biomass is grown, carbon is taken from the atmosphere.
The first two of these options can yield quick reductions of 10-20% in emissions and are now available.
Biomass can be co-fired directly in coal-fired boilers either together in the same boiler or via the gasification or combustion of biomass in a separate chamber. Co-firing plants may be eligible for carbon credits for the biomass proportion of the fuel, which could be cheaper than coal. Most plants use direct co-firing as separate gasification or combustion is expensive. However, technical issues make direct co-firing challenging:
biomass is more reactive than coal;
quality of the biomass ash, which has a relatively low melting point and is ‘very aggressive’ when melted;
biomass fuels can be very fibrous;
biomass can produce high tar levels at low gasification temperatures.
The type of fuel accepted depends upon the type of gasifier: moving bed gasifiers use fuel that is briquetted or pelletised with coal, fluidised bed gasifiers have less stringent feedstock requirements and are dry fed, entrained flow gasifiers use dry and fine biomass and waste solids such as sewage sludge solids and wood dust. Plants using less conventional feedstocks include:
Polk IGCC plant in Florida, which co-gasified eucalyptus and bahai grass;
Buggenum IGCC plant in the Netherlands, which co-gasifies numerous feedstocks including sewage sludge, waste wood and chicken litter;
Puertollano IGCC plant in Spain, which has co-gasified meat and bone meal and olive oil waste;
Schwarze Pump plant in Germany, which has co-gasified multiple feedstocks.
Biomass components of the fuel mix can contribute towards renewable portfolio standards (RPS), and are likely to play an important role in states in the US meeting their targets.