Energy and chemicals from biomass

Biomass was one of humanity’s earliest energy sources, and it remains an important resource today and for the future. Traditional plant biomass, particularly fuelwood, currently supplies a significant part of the energy needs of the developing world. Used and regrown sustainably, such energy sources could become an important component in a future COz-neutral energy economy. In the USA, biomass provides nearly 4% of final energy consumption. In the EU, biomass provided 7% of total primary production in 1997. Methane produced by landfill wastes is used to generate electricity as a matter of good practice in many countries.
 

Plant biomass resources include wood and wood wastes, agricultural crops and their residues, and aquatic plants and algae. Energy can be derived from biomass in three main ways:

• by direct combustion to provide heat and light as such, or to raise steam and hence generate electricity;  
• by gasification to provide ‘biogas’, a combustible gas mixture predominantly consisting of H2, CO and C02, that can be used for heating or electricity generation or converted to useful chemicals such as methanol;
• by fast pyrolysis at temperatures around 480-550 C, giving a high yield of ‘biooil’, an espresso-coffee-like liquid that can substitute for conventional fuel oil in transport and static applications

Wood is substantially the largest source of biomass energy and indeed of renewable energy, providing more than twice the contribution of hydroelectricity worldwide. Energy crops (herbaceous plants and forest plantations grown specifically for their energy content) and biofuels'' (solid, liquid or gaseous fuels made from
biomass) are established or emerging features in several parts of the world. At the moment, biofuels and electricity made from biomass are expensive compared with conventional alternatives, and their use generally needs to be stimulated by subsidy or regulation. Future cost improvements should come from volume production, the development of more efficient chemical processes and the production of value-added chemicals.
 

The chemical composition of dry biomass varies somewhat with species but is roughly 75% carbohydrates or sugars and 25% lignin, with the empirical formula C3H4O2. The main carbohydrates are cellulose-a polymer of glucose and the single most abundant product of photosynthesis-and hemicellulose. Because of their oxygen content, biomass and biofuels have a much lower calorific value (- 18 GJ tonne-' @ 100% dry matter) than conventional fuel oils (4244 GJ tonne-'). Biomass generally has low (<0.1%) sulphur content. Its nitrogen content depends on the protein content, which should be kept low to minimise the emission of NO, on combustion.
 

Wood and straw are widely used as fuels. However, most other plant products are not suitable to be directly burned because they are too wet. Even fresh wood contains a considerable amount of water (30-50% by weight) and is best dried before use. Charcoal, produced by slow pyrolysis of wood in limited air, is used as a domestic and industrial fuel. In Brazil, the world's largest charcoal producer and consumer, charcoal is used in heavy industries such as pig-iron, steel making and cement manufacture.
 

Mike Bullard discusses energy crops in Chapter 9. These are (generally perennial) species that grow rapidly and can be harvested for their biomass annually or every few years. In northern Europe, the most promising energy crops are perceived to be coppiced willow or poplar, often referred to as short rotation coppice (SRC) or arable energy coppice (AEC). Other energy crops include conventional arable crops, novel annual crops such as sweet sorghum and perennial species such as elephant grass (Miscanthus). Although a native of Asia and Africa, Miscanthus grows well in temperate climates. Its C4 photosynthetic pathway permits biomass yields of up to 55 t ha-' in the UK (compare wheat, for example, with a theoretical maximum UK yield of 33 t ha-'). Field trials of such energy crops have been underway for some years in Europe and the USA. In developed countries, the economic viability of energy crops is generally marginal, or even poor at times of low energy prices. However, the growth of energy crops on idle or set-aside farmland could represent a major future opportunity if robust supply chains and markets can be established and costs held down.
 

Tony Bridgwater and Kyriakos Maniatis discuss biofuels in Chapter 10. The most widely used biofuel is ethanol, made by fermentation of starch crops. Brazil and USA have pioneered large-scale ethanol fuel programmes. Nearly 1.5 billion gallons of ethanol are produced from corn annually in the USA to blend with gasoline and reduce air pollution. Brazil began its Pro-Alcohol programme, making ethanol from sugar cane, to counter the oil price hikes of the 1970s and its dependency on imported oil. It is by far the world's major producer of cane alcohol, producing 10-15 million cubic metres annually and with significant export opportunities now opening up.
 

Biodiesel is another significant biofuel. Chemically, this consists of a range of fatty acid alkyl esters. It can be made from any vegetable oil, produced from any oilbearing crop or microalga, by esterification with ethanol or methanol. The properties of biodiesel are quite similar to those of conventional diesel oil, enabling it to be used neat or in blends in conventional diesel engines. Rape methyl ester (RME) produced from oilseed rape is the main form of biodiesel in Europe and Canada. The USA produced about 5 billion gallons of biodiesel in 2000 from recycled cooking oils and soy oil.
 

Hydrogen, an important energy vector of the future, can be produced biologically under certain growth conditions by a range of photosynthetic microorganisms that contain either the hydrogenase enzyme or the closely related nitrogenase enzyme. Some strains of cyanobacteria have developed intracellular protective mechanisms that enable them to do this in air. Photosynthetic bacteria and algae have not developed protective systems, and can produce H2 under anaerobic but not aerobic conditions. Historically, these cultures have been easily poisoned and short-lived, but recent cyanobacterial cultures are encouragingly robust and other tricks are being used to enhance H2 evolution from green algae. Boichenko, Greenbaum and Seibert discuss progress in Chapter 8.
 

Many other chemicals and products can be derived from biomass. Cellulose fibres from wood are used to make paper and textiles, natural pharmaceuticals can be extracted from many plants. In Chapter 7, Rosa Martinez and Zvi Dubinsky discuss the range of products that can be derived from algae.
 

Statistics on the use and trends in use of biomass for energy generation are needed in energy and COz emissions modelling, particularly where developing countries are moving from traditional biomass use to fossil fuel use. Unfortunately, reliable data do not generally exist. Traditional biomass (fuelwood and crop and animal residues) is either collected by the user or traded in highly informal markets. The volume of these transactions is not metered and can only be roughly estimated from spot consumer surveys. Statistics on commercial use of biomass in forestry, paper and sugar industries and large-scale CHP and power generation are better.
 

Table 1.1 showed what are probably still the most reliable statistics on global and regional biomass use, derived from two workshops held by the IEA in 1997 and 1998. According to this survey, biomass accounts for -1 1% of world total primary energy supply (TPES) and -14% of world total final consumption (TFC). Nearly all of this consumption is in non-OECD (developing) countries, where biomass comprises -20% of TPES and -27% of TFC. (Most biomass is simply burned in such countries, rather than being converted to derived biofuels or used to generate electricity, so the proportion of biomass in final energy use is higher than in primary supply.) The main sources of national biomass data are the UN’s Food & Agriculture Organisation (wwwf.a o.org) and Energy Statistics Division and, for Asia, the Center for Energy-Environment Research and Development (www.ceerd.ait.uc.tW). The FA0 (www.fao. org/waicent/faoinfo/forestry/energy/) has made considerable efforts to gather statistics on forest woodfuel, through its series of publications Wood Energy Today for Tomorrow (FAO, 1997-1999). Other useful sources of information are the latest biennial survey of the World Energy Council (WEC, 2001), the world biomass scenarios of Hoogwijk et al. (2001) and the final paper by that most indefatigable of champions of biomass, the late Professor David Hall of King’s College, London (Hall et al., 2000).