3.49 A heat producing plant needs a local heat distribution network servicing its customers. This will usually mean constructing the plant reasonably close to housing or commercial or industrial premises that can make use of the heat. This implies that particular attention needs to be paid to emission control, for reasons both of public and environmental health and of public acceptability. Gas cleaning and particulate removal technologies are readily available, and would be incorporated into the initial design for new-build facilities. Condensers and re-heaters can be fitted to remove steam, plumes ofwhich are unsightly but do not otherwise affect the environmental impact.
Figure 3-IV Regulated pollutant emissions from Swedish CHP plant fuelled with biomass or coal70
VOC CO NOx PM SO2 Biomass Technology
I I Fuel production I I Clean-up
VOC CO NOx PM SO2 Reference Technology
I I Conversion I I Conversion/indirect
VOC Volatile Organic Compound
CO Carbon monoxide
NOx Nitrogen oxides
PM Particulate matter
3.50 The emissions of most concern (Figure 3-IV) are Volatile Organic Compounds (VOCs). Carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), sulphur dioxides (SO2) and chlorinated organics (principally dioxins). In gasification plants, the gas can be treated before combustion to remove VOCs. Carbon monoxide emissions are low if the combustion conditions are adequately controlled. Lower combustion temperature compared to other fuels (paragraph 3.3) means that the production of nitrogen oxides is lower. Well designed and operational gas cleaning equipment filters particulate matter and thereby concentrates heavy metals into the fly ash (paragraph 3.54). The sulphur content of wood is much lower than coal, leading to much lower sulphur oxide emissions. Thus, compared on the basis of electrical output, biomass leads to generally lower emissions than coal; example data from Sweden are shown in Figure 3-IV.
3.51 Chlorinated organic emissions can arise if the fuel contains chlorine. Many forms of biomass have very low chlorine content, and therefore give rise to very low quantities of dioxins. However, the presence of chlorine in the biomass can lead to dioxin production. Therefore timber treated with organochlorine wood preservatives, or wood mixed with PVC, should not be used as a source of biomass fuel in the sorts of generators being described here. Such materials would be classified as waste (paragraph 2.73) and should be burned only in a properly authorised waste incinerator. The combustion ofvirgin wood will result in the formation of much lower levels of dioxins, but even these small quantities have the potential to be significant on the scale of wood burning that would be necessary to meet the targets for biomass energy that we have proposed. It is important, therefore, to ensure that wood-burning heat and power plants are designed to reduce dioxin levels to the lowest practicable level. Guidance on best available technology for firing installations for wood and biomass is being prepared by the Expert Group on Best Available Techniques of the Intergovernmental Negotiating Committee of the Stockholm Convention on Persistent Organic Pollutants71.
3.52 A modern wood burning plant should, therefore, with careful design, be able to meet all air pollution control standards at reasonable costs. Even so, siting of the plant must be carried out with care, and in particular it is important that biomass plants should not be located in areas where they would exacerbate existing poor air quality. Plant burning any fuel in a boiler or furnace with a net rated thermal input of 50 megawatts or more is authorised by the Environment Agency (SEPA in Scotland and the Environment and Heritage Service in Northern Ireland) under the Integrated Pollution Prevention and Control (IPPC) regulations Part A. All plant involving pyrolysis, gasification or other heat treatment of carbonaceous material would also fall under Part A. Plant with a thermal input of between 20 and 50 MW would be authorised by local authorities under IPPC Part B73. Emissions of nitrogen oxides may represent a significant contribution to poorer local air quality. On the other hand, in some areas, heat made available from a biomass plant could displace more polluting heat sources (paragraph 3.6).
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