For example, there are many types of coal and the gross heating value of these types varies from 8,600-12,900 Btu/ lb. However, nearly all kinds of biomass feedstocks destined for combustion range from 6,450 to 8,200 Btu/lb. For most agricultural residues, the heating values are even more uniform – approximately 6,450 to 7,300 Btu/lb; the values for most woody materials are 7,750 to 8,200 Btu/lb. Moisture content is probably the most important determinant of heating value. Air-dried biomass typically has approximately 15 to 20% moisture, whereas the moisture content for oven-dried biomass is around 0%. Moisture content is also an important characteristic of coals, varying in the range of 2 to 30%. However, the bulk density (and hence energy density) of most biomass feedstocks is generally low, even after densification, approximately 10 and 40% of the bulk density of most fossil fuels. Liquid biofuels have comparable bulk densities to fossil fuels.
Combustion offers the most direct route for energy recovery from biomass, and is an effective means of utilizing the total energy content of whole wood and other biomass. Technology for small-scale combustion is certainly well developed. Larger-scale operation for electricity generation at the 5 to 50 MW level should be feasible, although it is not expected to be economically competitive with crude oil- or natural-gas-fired systems. Due to the large land area over which wood must be harvested, the seasonal nature of the supply, and the large volume of material to be transported and stored, the reliable provision of wood to a large plant can present considerable problems.
Biofuels are derived from biomass and have the potential to produce fuels that are more environmentally benign than crude oil-based fuels. In addition, ethanol, a crop-based fuel alcohol, adds oxygen to gasoline thereby helping to improve vehicle performance and reduce air pollution. Biodiesel, an alternative or additive to crude oil diesel, is a nontoxic, renewable resource created from soybean or other oil crops.
Direct biofuels are biofuels that can be used in existing unmodified crude oil engines. Because engine technology changes all the time, direct biofuel can be hard to define; a fuel that works well in one unmodified engine may not work in another. In general, newer engines are more sensitive to fuel than older engines, but new engines are also likely to be designed with some amount of biofuel in mind.
See also: Biofuels.
Biomass Ash
The mineral matter in biomass is reflected in the yield of mineral ash that is produced during combustion of the biomass. Biomass ash is naturally alkaline. The mineral matter content of wood is small wood but can be as high as 20% w/w for some types of biomass. The alkali nature of ash tends to lower the fusion point of the ash thereby leading to fouling and slagging. An increase in the ash content can also arise from the presence of non-indigenous contaminants such as soil, rocks, plastic, metals, and various chemical treatments of the biomass. Those contaminants can lead to severe problems like pollutant formation, fouling, and slagging.
Biomass with high content of alkali (and chlorine) has often caused problems with the formation of deposits and corrosion. Using additives rich in silicon, aluminum, calcium, potassium, sulfur, or calcium can reduce these problems.
Slagging in combustion units is caused by molten ash and is one of the main drawbacks with using biomass fuels, especially when waste biomass fuels originating from industry or agricultural production are used. The problems are caused by the typically higher content of mineral matter in these fuels but also due to the typically lower ash melting temperatures compared to fossil fuels or pure wood.
The key technical ash-related problems encountered by operators of biomass combustors and boilers have been associated with (i) the formation of fused or partly fused agglomerates and slag deposits at high temperatures within furnaces and stoves, (ii) the formation of bonded ash deposits and accumulations of ash materials at lower temperatures on surfaces in the convective sections of boilers, (iii) the accelerated metal wastage of furnace and boiler components due to gas-side corrosion under ash deposits, and due to ash-particle impact erosion or ash abrasion of boiler components and other equipment, (iv) the formation and emission of sub-micron aerosols and fumes, (v) the effect of biomass ash on the performance of flue gas cleaning equipment, and (vi) the handling, utilization, and disposal of ash residues from biomass combustion plants and mixed ash residues from the co-firing of biomass in coal-fired boilers.
Currently, there are solutions for combusting fuels with lower ash melting temperatures, such as wheat straw, but they have to be adapted specifically to a specific quality of the fuel. Fuel quality, however, varies widely between different types of biomass and biomass waste and can also vary with the season and especially during the handling of the fuel, which can cause contamination with soil, dirt, or other waste materials.
For biomass gasification and pyrolysis systems, the ash-related issues are largely similar to those for combustion, i.e., the accumulation of ash material within the reactor and associated equipment, the effect of ash on the integrity of the process plant and heat exchangers and the ash-related environmental effects of the process.
See also: Ash.
Biomass Chemistry
