The use of solid biofuels alone or the co-combustion of biomass and coal is a technology which has numerous advantages:

• It reduces net CO₂ emissions. The carbon dioxide released during combustion of biomass equals that which is taken in during growth. A full or partial replacement of fossil fuels with biomass would therefore reduce net emissions of CO₂.

• The use of surplus biomass, like municipal waste, sewage and agricultural/forestry residues, alleviates mounting pressures to find disposal routes.

• Biofuels encourage the preservation of diminishing resources of fossil fuels, because most biomass resources are abundant in nature, as shown in Table 1.

Straw has proven to be a significant crop with production values varying by country. Despite the small land area of Denmark, it produces significant quantities of straw. Surplus straw poses a significant problem to which the response has been to attempt to use the surplus as a feedstock for heat-and-power generation. Local farmland in Grenaa, Denmark produces 125,000 tonnes of surplus straw annually, of which, 75,000 tonnes are used by the local cogeneration company and the farmer’s co-operative. As would be expected for Denmark, the percentage of land occupied by forest and woodlands is moderate but not extensive, so indicating that wood and wood residues would perhaps not be the preferred biomass feedstocks for energy production. Ireland, like Denmark, is small in size but produces neither straw nor forest/wood biomass in significant quantities. This, however, has not prevented the Republic of Ireland from investing time and money into the research and assessment of energy potential from biomass. Ireland has a strong agricultural base and an interest in reaping the benefits of socio-economic and employment stimulation via the establishment of a bioenergy industry. CAP reforms, set-aside and the need for alternative economic activities on agricultural land are encouraging land owners to consider energy crops such as willow, poplar, and Miscanthus as a new farming enterprise.

The percentage of cropland coverage with respect to total land area can be calculated using figures from Table 1. These calculations show that 60% of Denmark’s land is used to produce crops. Short and long term phases of crop surpluses and diseases of various sorts have, in the past, resulted in significant biomass wastages. Denmark is presently experiencing straw surpluses, which are partly dealt with using bioenergy routes of disposal and partly through simple open-field incineration.

Over 75% of Finland’s total land area is covered by forest and woodland. Sweden follows closely with over 65% and then Austria with over a third of woodland coverage. Further analysis would determine the portion of these lands managed for wood and timber production along with quantities of resultant wastes (woodchippings, branches, foliage etc.).

With regard to crop and woodland coverage, the three countries showing significant resources are Spain, Portugal and Germany. Although these countries have the resources, it does not follow that they will invest the required time and money into stimulating a market for bioenergy technology. There must be strong motivations and sufficient resources, aside from the natural ones, to make such an investment tempting. Motivations may include: (1) the need to secure additional energy sources, (2) pressure to reduce net harmful emissions and (3) the need to safely dispose of surplus biomass. Resources that enable this are: (1) money to invest, (2) research and technical experience to employ the technology and (3) political support for this alternative energy.

Atmospheric pollution and government are terms often used interdependently. International treaties pertaining to emission guidelines affect all governments involved by setting specific standards for various pollutants. However, these standards are carefully set according to related economic factors and, to a lesser extent, public opinion. Hence, it is extremely important to use any time and money invested into bioenergy research to gain a thorough understanding of the potential and harmful emissions and to develop economically feasible methods for reducing output levels and/or impacts. Research to date has identified a group of potentially harmful emissions from the combustion of biomass including crops and wood. The compounds associated with these emissions are semi-volatile organic compounds (SVOCs). The rest of this paper reviews the basic process of combustion, routes and mechanisms of SVOC formation and the methods used to predict these emissions using fuel input information. The final section uses results from some laboratory tests and predictive model runs to arrive at some general conclusions about SVOCs associated with biomass combustion.

Combustion takes a typical particle of coal or biomass and decomposes it into fractions of char and volatiles, the former burning slowly. In general the initial combustion or gasification process can be separated into two main reactions, one of them being the devolatilisation process which involves the breakdown of initial coal/biomass into light gases and tars, which subsequently form soot. Numerous studies have indicated that the thermochemical conversion of biomass is similar to that of coal, although the amount of char is significantly smaller.

In general the ratio of volatile products and chars in the case of coal gasification is almost unity, but, in the case of biomass, the volatile content is around 80% and rest is char/ash. A comparative elemental analysis of biofuels and various coals in terms of their elemental contents is given in Table 2 and Table 3.

Wood as a fuel is characterised by low ash contents, high calorific values and large amounts of fixed carbon. Straw is low in moisture content with high heating values and a high percentage of hydrogen and oxygen and has proven to have low concentrations, if any, of iron, lead, zinc and copper. However, its high chlorine content can be a significant drawback. Miscanthus is a highly volatile fuel with a high heating value due to its large proportions of carbon and oxygen: it contains almost no iron, lead, zinc or copper, thus making it a fairly clean and effective biomass fuel. Hence, a typical biofuel has a high volatile content and low fixed carbon content, the nitrogen content varies and, generally low chlorine levels altogether except straw which shows the highest chlorine concentration.

The combustion or incineration of various wastes or natural materials containing chlorine can lead to the formation and emission of, polynuclear aromatic hydrocarbons (PAH), dioxins, furans, chlorohydrocarbons and other species. Dioxin is a general term for a group of chemical compounds consisting of 75 polychlorinated dibenzo para dioxins (PCDDs) and 135 polycyclic dibenzofurans (PCDFs). They are structurally very similar, only differing in the number and spatial arrangement of chlorine atoms in the molecule. Fig. 1 shows the basic structure of these two subgroups. Each of these structures represents a whole series of discrete compounds which are present as trace amounts in the atmosphere and some of these isomers have been shown to be extremely toxic, mutagenic and linked to the suppression of the immune system in humans.

As a result of dioxin-contamination in Seveso, Italy in 1976, the European Community introduced the Seveso Directive in 1982 obliging dangerous chemicals manufacturers to identify risks present in their factories and informing the local residents of the potential dangers. This directive also lists the amounts of dangerous chemicals that can be stored safely within 500 m of each other. The United Kingdom complied with this Directive by introducing the Control of Industrial Major Accident Hazards (CIMAH) regulations in 1984.