Co-combustion

Co-firing biomass with coal in traditional coal-fired boilers is becoming increasingly popular, as it capitalizes on the large investment and infrastructure associated with the existing fossil-fuel-based power systems while traditional pollutants (SOx, NOx, etc.) and net greenhouse gas (CO2, CH4, etc.) emissions are decreased.

The least expensive way of cofiring is by directly adding biomass to the coal belt

The R&D demands arising from co-firing cover the proper selection and further development of appropriate co-combustion technologies for different fuels, possibilities of NOx reduction by fuel staging, problems concerning the de-activation of catalysts, characterisation and possible utilisation of ashes from co-combustion plants, as well as corrosion and ash deposition problems.

Fuel Characteristics

The biomass fuels usually considered range from woody to grassy and straw-derived materials and include both residues and energy crops. The fuel properties differ significantly from those of coal and also show significantly greater variation as a class. For example, ash contents vary from less than 1% to over 20% and fuel nitrogen varies from around 0.1% to over 1%. Other properties of biomass which differ from those of coal are a generally high moisture content, potentially high chlorine content, relatively low heating value, and low bulk density. These properties affect design, operation, and performance of co-firing systems.

Fuel Preparation and Handling

A biomass fuel handling facility, which directly meters biomass onto the coal conveyor belts at the Wallerawang Power Station, Australia (Courtesy of Delta Electricity, Australia)

Because biomass fuels are hygroscopic, have low densities, and have irregular shapes, they should generally be prepared and transported using equipment designed specifically for that purpose. In some cases, however, they can be directly metered on the coal belt conveyor. Care must be taken to prevent skidding, bridging, and plugging in pulverizers, hoppers, and pipe bends.

Effect on NOx emissions when cofiring wood (top) and switchgrass (bottom) with coal. NOx emissions can both increase and decrease when cofiring biomass. Fuel nitrogen content: wood = 0.18, switchgrass = 0.77; coal = 1..1.2 lb N/MMBtu. (Courtesy Larry Baxter, USA)

Pollutant Emissions

Co-firing biomass with coal can have a substantial impact on emissions of sulphur and nitrous oxides. SOx emissions almost uniformly decrease when biomass is fired with coal, often in proportion to the biomass thermal load, because most biomass fuels contain far less sulphur than coal.

An additional incremental reduction is sometimes observed due to sulphur retention by alkali and alkaline earth compounds in the biomass fuels. The effects of co-firing biomass with coal on NOx emissions are more difficult to anticipate (see figure).

Ash deposition rate for various
fuels in g deposit per kg fuel. (Courtesy Larry Baxter, USA)

Ash Deposition

Rates of ash deposition from biomass fuels can greatly exceed or be
considerably less than those from firing coal alone. This is attributable only partially to the total ash content of the fuels. Deposition rates from blends of coal and biomass are generally lower than indicated by a direct interpolation between the two rates. Experimental evidence supports the hypothesis that this reduction occurs primarily because of interactions between alkali (mainly potassium) from the biomass and sulphur from the coal.

Carbon Conversion

Experiments on carbon burnout of biomass fuels in coal power plants show that large, wet or high-density biomass particles may undergo incomplete combustion. However, this biomass-derived carbon does not always figure prominently in fly ash analyses because of the relatively low amount of carbon in biomass, the limited share of biomass usually co-fired, and the fact that large biomass particles are more likely to collect in the bottom ash than in the fly ash.

The molar ratio of sulphur to available alkali and chlorine is an indicator of the chlorine corrosion potential (Courtesy Larry Baxter, USA)

Chlorine-based Corrosion

High-temperature corrosion of superheaters is of great concern when
burning high-chlorine or high-alkali fuels, such as herbaceous or
intensely cultivated fuels, since species containing chlorine (generally alkali chlorides) may deposit it on heat transfer surfaces and greatly increase surface chlorine concentration. However, research has indicated that the corrosion potential can be reduced if alkali chlorides (primarily from the biomass) can interact with sulphur (primarily from the coal) to form alkali sulphates. As a result, highly corrosive alkali chlorides on
superheater tubes are converted to HCl and other gas-phase products that are less corrosive and that leave the surface relatively easily. The HCl may condense on lower-temperature surfaces such as air heaters. However, this problem is generally less serious and more manageable than superheater corrosion.

Fly Ash Utilization

The majority of the fly ash generated from coal combustion world-wide, is used as a concrete additive or for other purposes. However, current standards preclude the use of fly ash as a concrete additive from any source other than coal.

The technical case for precluding the use of fly ash from co-firing
wood with coal appears to be unjustified. However, the less comprehensive data available for herbaceous biomass fuels suggest that alkali, chorine, and other properties may compromise several important concrete properties.

Strict interpretation of many standards that are the basis for
regulations and policy for many institutions would preclude all fly ash from use in concrete if it contains any amount of non-coal-derived material, including co-fired fly ash. Though these standards are under active revision, this may take many years to complete.