James G. Speight - Encyclopedia of Renewable Energy

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.

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 is typically composed of 75 to 90% by weight sugar species, the other 10 to 25 wt% being mainly lignin. The energy in biomass is the chemical energy associated with the carbon and hydrogen atoms contained in oxidizable organic compounds which are the source of the carbon and hydrogen is carbon dioxide and water. The conversion by plants of carbon dioxide and water to a combustible organic form occurs by the process of photosynthesis in which solar energy and chlorophyll are the important players.

Chlorophyll, present in the cells of green plants, absorbs solar energy and makes it available for the photosynthesis, which may be represented by the simplified chemical reaction:

The oxidizable organic materials that are produced by photosynthesis and which determine the properties of the plant matter of relevance to biomass energy utilization are carbohydrates and lignin.

All of the carbohydrates present are saccharides (i.e., sugars) or polymers of sugars (i.e., polysaccharides) that fall into three types: (i) starch, (ii) cellulose, and (iii) hemicellulose. The simple sugars include glucose, fructose, and the like, while the polymeric sugars such as cellulose and starch can be readily broken down to their constituent monomers by hydrolysis, preparatory to conversion to ethanol or other chemicals.

Starch is a granular polysaccharide which accumulates in the storage tissues of plants such as seeds, tubers, roots, and stem pith. It is an important constituent of corn, potato, rice, and tapioca. Starch consists of 10 to 20% amylose, which is water soluble, and 80 to 90% amylopectin, which is insoluble. Both the constituents of starch are polymers of glucose, with amylose linked in chain structures, while amylopectin is a highly branched structure. Starch is not as chemically resistant as cellulose, and can be readily hydrolyzed by dilute acids and enzymes to fermentable sugars.

Hemicelluloses are polysaccharides that occur in association with cellulose. They are chemically different from cellulose, are amorphous, and have much lower molecular weight mass. While cellulose is built from the single sugar glucose, most hemicelluloses contain two to four different sugars as building blocks. Glucose is a component of some hemicelluloses, although xylose is a dominant sugar in hardwood hemicellulose, and mannose is important in softwood hemicellulose. Unlike the other sugars described so far, xylose contains only 5 carbon atoms and is a pentose.

The fraction of the cellulose containing xylose polymers is often referred to as pentosan. Hemicellulose is more soluble than cellulose, is dissolved by dilute alkaline solutions, and can be relatively readily hydrolyzed to fermentable sugars.

In contrast, lignin is a complex structure containing aromatic groups and is less readily degraded. Although lignocellulose is one of the cheapest and most abundant forms of biomass, it is difficult to convert this relatively unreactive material into sugars. Among other factors, the walls of lignocellulose are composed of lignin, which must be broken down in order to render the cellulose and hemicellulose accessible to acid hydrolysis. For this reason, many programs focused on ethanol production from biomass are based almost entirely on the fermentation of sugars derived from the starch in corn grain.