A high combustion quality, in terms of maximal combustion of the burning gases, is very important for a low emission level. It mainly depends on the combustion chamber temperature, the turbulence of the burning gases, residence time and the oxygen excess. These parameters are governed by a series of technical details such as:
- combustion technology (e.g. combustion chamber design, process control technology)
- settings of the combustion (e.g. primary and secondary air ratio, distribution of the air nozzles)
- load condition (full- or part-load)
- fuel characteristics (shape, size distribution, moisture content, ash content, ash melting behaviour).
Biomass has a number of characteristics that makes it more difficult to handle and combust than fossil fuels. The low energy density is the main problem in handling and transport of the biomass, while the difficulties in using biomass as fuel relates to its content of inorganic constituents. Some types of biomass used contain significant amounts of chlorine, sulfur and potassium. The salts, KCl and K2SO4, are quite volatile, and the release of these components may lead to heavy deposition on heat transfer surfaces, resulting in reduced heat transfer and enhanced corrosion rates. Severe deposits may interfere with operation and cause unscheduled shut downs. The release of alkali metals, chlorine and sulfur to the gas-phase may also lead to generation of significant amounts of aerosols (sub-micron particles) along with relatively high emissions of HCl and SO2.
The nature and severity of the operational problems related to biomass depend on the choice of combustion technique. In grate-fired units deposition and corrosion problems are the major concern. In fluidized bed combustion the alkali metals in the biomass may facilitate agglomeration of the bed material, causing serious problems for using this technology for herbaceous based biofuels. Fluidized bed combustors are frequently used for biomass (e.g. wood and waste material), circulating FBC are the preferred choice in larger units. Application of biomass in existing boilers with suspension- firing is considered an attractive alternative to burning biomass in grate-fired boilers. However, also for this technology the considerable chlorine and potassium content in some types of biomass (e.g. straw) may cause problems due to deposit formation, corrosion, and deactivation of catalysts for NO removal (SCR).
Currently wood based bio-fuels are the only biomasses that can be co-fired with natural gas; the problems of deposition and corrosion prevent the use of herbaceous biomass. However, significant efforts are aimed at co-firing of herbaceous biomass together with coal on existing pulverized coal burners. For some problematic fuels, esp. straw a separate auxiliary boiler may be required. In addition to the concerns about to deposit formation, corrosion, and SCR catalyst deactivation, the addition of biomass in these coal units may impede the utilization of fly ash for cement production. In order to minimize these problems, various fuel pretreatment processes have been considered, including washing the straw with hot water or using a combination of pyrolysis and char treatment.