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Denmark Considers Producing Electricity From Straw
B
y the year 2000, the Danish electricity industry intends to burn 1.2 million tons of straw and 200,000 tons of wood per year by converting some of its coalfired plants to accept biomass. The plan, a result of a political agreement last year in the Danish Parliament, means that biomass will account for 7-8% of the total fuel burned by Danish electricity utilities. The development reflects Denmark's radical policy on climate change. Denmark is one of the few countries to have set a target for reducing emissions well into the next century. By 2005, emissions must fall 20%; by 2030 a further fall of 50% is required. By contrast, the 1992 C o n v e n t i o n on Climate Change, signed in Rio de Janeiro, commits developed nations to return greenhouse gas emissions to 1990 levels by 2000. Only now are UN negotiators daring to discuss longer term emission cuts. Denmark has adopted a radical energy policy ever since the oil price shocks of the early 1970s. At that time the country was dependent on imported oil for more than 90% of its energy requirements. The government resolved to reduce its dependency on oil, which fell below 50% by 1988. (Ironically, with the discovery and exploitation of North Sea oil, the country now exports more energy than it consumes.) During the 15 years since 1973, energy consumption per square meter for space heating fell by more than half. Many energy-saving district heating and cogeneration projects were embarked upon, and renewable energy production, particularly from biomass, tripled. Biomass today supplies 6% of gross energy consumption. Some three-quarters of Denmark's available wood and municipal solid waste residues are now used for energy. But, according to a paper (1) by Hakon Mosbech of Denmark's ELKRAFT Power Company, less than a third of the nation's 3.5 million tons of surplus straw is cur-
BY J U L I A N
ROSE
rently used as fuel. Mosbech also points out that stubble-burning on fields was recently p r o h i b i t e d throughout the European Union. According to "The Feasibility of Biomass Production for the Netherlands Energy Economy" by the Dutch energy and environment agency NOVEM, burning straw as a fuel can yield 52.9 gigajoule (GJ) per hectare per year. Only 5.6 GJ is consumed in managing and harvesting the crop and preparing it for combustion, giving an output—input ratio of more than 10. Similar results can be obtained with wood burning from poplar plantations, the Dutch researchers found. By contrast, the ratio for bioethanol from grain is only 0.9; more energy is used in making the fuel than is released in combustion. Thus straw and wood burning are considered good ways of reducing COz emissions. And, according to Mosbech, straw has the added attraction of being readily available without any radical changes to farming practices. There are, nevertheless, significant technical problems to overcome before the Danish power companies can hope to meet their strawburning targets, despite investing some DKr2.2 billion ($355 million) to date on research and development. The work is aimed, ultimately, at using biomass on a very large scale. So far, 15 cogeneration plants, fully or partly fuelled with straw, wood chips, waste, or biogas from animal manure, have been tested. The biomass cogeneration plants produce less electrical power per unit of heat than hoped. This is because slag and clinker form in the boiler and combustion chamber, demanding temporary shutdowns for maintenance. These stop—start cycles lead, in turn, to corrosion because of the alternate heating and cooling. In addition, straws from rape and grass seed crops were
0013-936X/94/0927-465A$04.50/0 © 1994 American Chemical Society
found to be u n s u i t a b l e u n l e s s mixed with other types of straw. And straw with a high moisture content posed further problems. Finally, the variable consistency of the feedstock led to fluctuations in steam production of up to 25%, so more advanced control systems are needed to utilize the energy. A new 10-MW plant is scheduled to open in 1996 to take advantage of the lessons learned. The power companies also intend to scale up the straw-burning technology for use in the much larger, centralized generating plants, where the straw will be burned with coal in the same combustion chamber. Further research is under way to improve the available crops, which to date have been bred to optimize their food quality and yield. ELKRAFT is also developing new c o n t r a c t u a l arrangements w i t h farmers to improve the security of supplies. Straw remains much more expensive than coal as a fuel, but the power companies hope to reduce costs significantly. The largest costs are in handling, baling, and transporting the feedstocks. Current straw prices are DKr23—40 per GJ compared with DKrl2 for coal. The utilities expect to cut C0 2 emissions by some 2 million tons as a result of the straw-burning program, at a cost of DKr250 per ton of avoided C0 2 . According to Mosbech, meeting the target of generating 7—8% of Denmark's electricity from straw will cost DKr575 million ($92 million) per year. Thanks to a government subsidy for alternative fuels, the state will pay some DKr225 million of the additional costs. Reference (1)
"Environmental Impacts of Bioenergy," ISBN 1 872691 21 8, is available from CPL Press, 43 Kingfisher Court, Newbury RG14 5SJ, England.
Julian Rose is an environmental and technology writer based in London.
Environ. Sci. Technol., Vol. 28, No. 11, 1994
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