It is recognised that combustion with heat recovery is not the best environmental or economic option for dealing with wood waste and that it is always preferable to look at the reduction of waste at source followed by reuse and recycling. However, wood processing industries will always create residues and in the case of board materials, these will often have limited potential for reuse and recycling, especially where they are generated at end user sites rather than by board producers. Consequently, combustion remains an important outlet for less desirable forms of wood waste.
Benefits of combustion include a self-contained solution to the problem of waste, the generation of heat leading to reduced fossil fuel consumption and less reliance on landfill. Despite the rises in the associated tax, landfill remains an important waste management option for manufacturers using wood.
The economic argument for combustion in the UK continues to get stronger with significant increases in energy costs, landfill tax, fuel duty (for waste transport) and waste management fees. In addition, the government’s Carbon Trust is providing interest-free loans which were used to finance at least 60 wood combustion installations during 2004. In the longer term, there may also be the option of generating revenue from Renewable Obligation Certificates if problems can be sorted out regarding the definition of “biomass”.
One continuing problem for UK wood combustion is that our winters are not particularly long or cold. Consequently, space heating is only required for four to five months of the year leading to the wastage of heat from April to October.
Power potential
The potential to generate electricity as well as heat from wood waste is an attractive one. When heat is not required, the thermal energy can be converted into electricity for use on site. Large installations can even sell electricity to the National Grid, though there are significant connection costs.
A variety of technologies can create electricity from wood waste. Steam turbines are powered by steam generated from the heat given off by the combustion process. Turbines are well established and can range in size from 0.5MWe (megawatts of electrical energy) to 500MWe. However, in smaller plants under 2MWe the electrical efficiency is low at 8-18%.
A newer version of the traditional turbine is offered by the organic Rankine cycle (ORC) which uses organic oil instead of water as the process medium, thereby enabling operation at relatively low temperatures. ORC processes can operate at partial load, and efficiencies of 13-17% have been achieved. The technology appears promising for plant between 0.4MWe and 1.5MWe.
Steam engines offer a cheaper way to generate small amounts of electricity. Units are typically less than 1MWe in size and they have electrical efficiencies of 6-20%. The technology is less sensitive to contaminants and is suited to boiler plant with a varying workload.
Stirling engines are closed systems which use air, helium or hydrogen as a medium. Heat is transferred to the medium, which is subsequently subjected to forced cooling. The expansion and contraction inside a chamber is used to power a piston and efficiencies of 15-30% are claimed. The best efficiency is obtained where the temperature in the hot heat exchanger is as high as possible without leading to the formation of clinker in the boiler. Therefore, it is necessary to preheat the combustion air, with the flue gas leaving the hot heat exchanger by means of an air pre-heater. The temperature of the combustion air is typically raised to 500-600OC, resulting in very high temperatures in the combustion chamber.
Austrian hotel
The potential of the Stirling engine technology is demonstrated by the Burg Hotel, Oberlech in Austria. The hotel operates in an exclusive ski resort and must ensure that energy creation has a minimal impact on the environment. In 2003, the hotel installed two 440kW Mawera boilers.
One has a ceramic filter, allowing it to burn a variety of wood waste including treated timber. The other has a 35kW Stirling engine attached for electricity generation. The heat is distributed through 3km of pipework to 25 hotels at a cost of 5.5 to 7 cents per kWh. Around 40-50 tonnes per week of wood is consumed in winter.
Another Austrian operation, the Biostrom plant, demonstrates the potential use of energy from wood combustion. Each year it processes 22,000 tonnes of mixed wood waste including pallets, furniture and treated timber.
By taking in treated timber or “condemned wood”, the plant can charge a gate fee of up to €150 per tonne. In order to burn off the associated contaminants, the plant has a combustion chamber which is 20m high to ensure a residence time of two seconds at 1,100OC. A ceramic filter is also used to clean the gases at the rear of the process.
Economics
The plant can produce 10MW total heat, of which 2MW is kept in reserve (this is destined for a future district heating scheme). Six MW of heat energy is used to produce green electricity, 1.2MW of which is sent into the grid. Further revenue is generated through the sale of heat to a neighbouring bottling plant.
Historically, the uptake of combustion technology has made better economic sense on the Continent. For example, in Austria it is possible to sell green electricity to the grid for up to 15 cents per kWh compared to the standard tariff of 7 cents per kWh. This premium, coupled with landfill charges of e80-100 per tonne for general waste and a high rate of carbon tax on fuels such as coal, provides an incentive for the initial investment.
While the UK does not have the same parameters, the continued increases in energy costs, landfill tax, fuel duty and waste management fees continues to improve the payback of combustion schemes making better use of energy.
