Survey of Energy Resources 2007
Biomass for Electricity Generation
The largest secondary transformation of biomass after charcoal production is in the electricity sector. For many years biomass processing industries such as sugar, wood products and chemical pulping (black liquor) have installed combined heat and power (CHP, also known in the USA as cogeneration) plants. Many of these have been relatively low-steam-temperature installations, with only sufficient electricity to meet the plant processing needs. Since the 1970s there has been a large expansion of biomass-based electricity generation, with an increased emphasis on generating efficiency, resulting in electricity exports into liberalised or deregulated markets. In addition, there has been an expansion of district heating schemes with CHP in Scandinavia, based on straw in Denmark and wood residues in Sweden and Finland. In countries with extensive coal-fired electricity generation there have been incentives under climate schemes to co-fire biomass in order to achieve carbon offsets of up to 15%. Germany and other countries have also stimulated the generation of electricity from urban residue streams in energy from waste (EFW) facilities, from land fill methane, and from anaerobic digestors associated with the animal husbandry sector. India, China and Brazil have also invested in rural electricity generation from producer gas and vegetable oils.
In 2005 estimated total electricity generation was about 180 TWh from an installed capacity in excess of 40 GW. At an average 20% efficiency this corresponds to 3 EJ of primary energy input. The overall rate of growth has been greater than 5% in the last decade as shown in Fig. 9-2 . The recent negative trend for MSW is a consequence of increased material recycling, together with reductions in the amount of biomass-derived materials entering the waste stream.
The eight leading countries with biomass-based electricity production are all members of the OECD, except for Brazil (Fig 9-3). Brazil is also unique in that at present almost all of its input is solid biomass, much of it bagasse from the expanding sugar cane-based alcohol fuel industry. The OECD countries generally have contributions of up to 10% of the total from both MSW (EFW) and biogas, with a large contribution from landfill sites containing biodegradable urban residues. The rising contribution of liquid fuels reflects the increasing use of vegetable oils in combustion turbines and diesel generation.
Biopower trends. A key issue for the biopower sector is efficiency. The move towards co-firing with coal has the advantage that the efficiency when firing the blended fuel is that of the original coal boiler with little or no loss relative to the coal component. CHP installations are efficient in generally satisfying the primary thermal requirement, with the electricity output following
the thermal load. Typically, CHP electrical efficiency is higher, with improved steam conditions of temperature and pressure, a trend which could result in the bagasse sector becoming much more efficient.
A typical sugar mill has an electricity demand of about 20-30 kWh/tc. One tonne of cane (tc) typically provides 150 kg of sugar and 90 kg of bagasse (dry basis). The thermal requirements for refining sugar are easily met from the generated bagasse, such that CHP is the preferred option. Low-efficiency mills generate 21 bar steam and produce about 60-70 kWh/tc for export, while more recent investments have been in 88 bar steam with the capability of exporting 130 kWh/tc.
The World Alliance for Decentralized Energy (WADE) has estimated that the 11 leading countries had 3.9 GW of installed bagasse-based generating capacity with Brazil contributing 1.7 GW (Bell, 2005) due to PROFINA, a programme guaranteeing power sales to the grid. Mauritius (Deepchand, 2001), with 40% of all electricity generated from bagasse, has demonstrated (through a programme taking over 20 years) the restructuring needed in the sugar industry to access sufficient bagasse and capital for this renewable resource to make a major, high-efficiency contribution. The capacity consists of seasonal generation in bagasse-only plants and also two larger more efficient plants that are fired with coal in the off-season. The overall potential of sugarcane bagasse in power generation is clearly dependent on both technical and socio-commercial factors. However, using high-pressure steam technologies with the resources indicated in Table 9-1, the technical potential is more than 130 TWh annually.