Biomass and biofuels

The material of plants and animals, including their wastes and residues, is called biomass. It is organic, carbon-based, material that reacts with oxygen in combustion and natural metabolic processes to release heat. Such heat, especially if at temperatures >400 C, may be used to generate work and electricity. The initial material may be transformed by chemical and biological processes to produce biofuels, i.e. biomass processed into a more convenient form, particularly liquid fuels for transport. Examples of biofuels include methane gas, liquid ethanol, methyl esters, oils and solid charcoal. The term bioenergy is sometimes used to cover biomass and biofuels together. The initial energy of the biomass-oxygen system is captured from solar

radiation in photosynthesis, as described in Chapter 10. When released in combustion the biofuel energy is dissipated, but the elements of the material should be available for recycling in natural ecological or agricultural processes, as described in Chapter 1 and Figure 11.1. Thus the use of industrial biofuels, when linked carefully to natural ecological cycles, may be nonpolluting and sustainable. Such systems are called agro-industries, of which the most established are the sugarcane and forest products industries; however, there are increasing examples of commercial products for energy and materials made from crops as a means of both diversifying and integrating agriculture. The dry matter mass of biological material cycling in the biosphere is

about 250× 109 t y−1 incorporating about 100× 109 t y−1 of carbon. The associated energy bound in photosynthesis is 2×1021 J y−1=07×1014W. Of this, about 0.5% by weight is biomass as crops for human food. Biomass production varies with local conditions, and is about twice as great per unit surface area on land than at sea. Biomass provides about 13% of mankind’s energy consumption, includ-

ing much for domestic use in developing countries but also significant amounts in mature economies; this percentage is comparable to that of fossil gas. The domestic use of biofuel as wood, dung and plant residues

for cooking is of prime importance for about 50% of the world’s population. The industrial use of biomass energy is currently comparatively small for most countries, except in a few sugarcane-producing countries where crop residues (bagasse) burnt for process heat may be as much as 40% of national commercial supply. Nevertheless, in some industrialised countries, the increasing use of biomass and wastes for heat and electricity generation is becoming significant, e.g. USA (about 2% of all electricity at 11GWe capacity); Germany (at 05GWe capacity) and in several countries for co-firing with coal. If biomass is to be considered renewable, growth must at least keep pace

with use. It is disastrous for local ecology and global climate control that firewood consumption and forest clearing is significantly outpacing tree growth in ever increasing areas of the world. The carbon in biomass is obtained from CO2 in the atmosphere via

photosynthesis, and not from fossil sources. When biomass is burnt or digested, the emitted CO2 is recycled into the atmosphere, so not adding to atmospheric CO2 concentration over the lifetime of the biomass growth. Energy from biomass is therefore ‘carbon neutral’. This contrasts with the use of fossil fuels, from which extra CO2 is added to the atmosphere. The use of biomass in place of fossil fuels leaves the fossil fuel underground and harmless; the use of biomass ‘abates’ the extra CO2 otherwise emitted. Thus use of renewable biofuels, on a large scale, is an important component of most medium-to long-term policies for reducing greenhouse gas emissions. The energy storage of sunshine as biomass and biofuels is of fundamental

importance. All of the many processes described in this chapter have the aim of producing convenient fuels, at economical prices, for a full range of end-uses, including liquid fuel for transport. The heat energy available in combustion, equivalent in practice to the enthalpy or the net energy density, ranges from about 8MJkg−1 (undried ‘green’ wood) and

15MJkg−1 (dry wood), to about 40MJkg−1 (fats and oils) and 56MJkg−1

(methane). Biomass is, however, mostly carbohydrate material with a heat of combustion of about 20MJkg−1 dry matter; refer to Table B.6, Appendix B for detail. The success of biomass systems is regulated by principles that are often

not appreciated:

1 Every biomass activity produces a wide range of products and services. For instance where sugar is made from cane, many commercial products can be obtained from the otherwise waste molasses and fibre. If the fibre is burnt, then any excess process heat can be used to generate electricity. Washings and ash can be returned to the soil as fertilizer.