Almost anything organic will burn, but low moisture-content biomass is best suited for combustion. Combustion refers to the rapid oxidation of the feedstock as it is exposed to high heat.  Most of todays biomass-powered plants are direct-fired systems, similar to fossil-fueled plants. The feedstock is burned in a boiler to produce high-pressure steam that is pumped into a turbine, over a series of blades that roate and power an electric generator. There are three general areas of combustion technology that are being used: Fixed-bed combustion, fluidised-bed combustion, and dust combustion.

Steam powered technologies have proven to be very dependable, but efficiency has at times been limited. Biomass power boilers typically are in the production range of 20-50 MW compared with coal-fired plants in the 100-1500 MW range. Small-capacity plants generally have lower efficiencies because the equipment needed to increase energy-efficiency is not economically viable (Brown 2003). The most economic near-term solution is co-firing furnaces with fossil and biomass feedstocks. Much of the existing power plant equipment can be used with little to no major modifications thus making this much more economically attractive than building new plants. Compared to the coal it might replace, biomass use reduces sulfur dioxide, nitrogen oxide, and other harmful air emissions resulting from combustion (Hustad et al. 1998).  

Many power plants have been burning or co-firing biomass for decades. Most recently the Dunkirk Power Station in New York has started producing energy from local willow plantations, assisting the regional forest products economy (Spaeth 2004). Advances in fluidized beds that promote full oxidation of the feedstock have seen energy efficiencies increase, but one of the largest impacts has been the adoption of pelletized biomass as a commodity fuel. Bixby Energy Systems has desinged a furnace especially for pellets with a 99.7 percent fuel combustion ratio, maximizing value at the same time as limiting emissions and ash residue. The UK-based Talbotts has been a world leader in the development of biomass generators, producing its first wood-fired system during the mid 1970s.

Talbott's Biomass Generator

Talbotts is currently promoting their biomass generator, BG100. The BG 100 is capable of producing 100kW of electricity and 150 kW of heat, utilizing a wide range of fuels including wood chips, pellets, and waste. With a potential reduction of 600 tonnes per unit of carbon dioxide per year compared with an equivalent amount of energy produced from fossil fuels, the BG100 can greatly benefit the environment and take advantage of a common fuel source. The system is small and compact and designed for on-site power production making it ideal for farms, hotels, estates, or commerical properties. Each system can be operational for approximately 8000 hours per year, making it 92% efficient.

A fully-automated, continuous system ensures proper fuel feeding to maintain the required energy output and the step-grate system helps to ensure even burn thoughout the combustion chamber to improve the units efficiency. For more information about the Talbott system and to review the companys past successes, please visit their website here.

In fixed-bed combustion systems, primary air passes through a fixed bed, in which drying, gasification, and charcoal combustion takes place. The combustible gases produced are burned after addition air has been introducted in a combustion zone separated from the fuel bed. When biomass fuels have a high moisture content, varying particle sizes, or high-ash content, grate furnaces can be utilized. Mixtures of wood fuels can be used, but current technology does not allow for mixtures of wood fuels and straw, cereals and grass, due to their different combustion behavior, low moisture content, and low ash-melting point (van Loo and Koppejan 2003).

A homogeneous distribution of the fuel and layer of embers over the whole grate surface is important in order to guarantee an equal primary air supply across the grate surface. Heterogeneous air supply may cause slagging, higher fly-ash amounts, and may increase the excess oxygen needed for complete combustion (Baxter and Koppejan 2004). There are various grate furnace technologies available that will guide the complete combustion of fuel while minimizing the negative externalities.

Underfeed stokers represent a cheap and operationally safe technology for small-and medium-scale systems up to a nominal boiler capacity of 6 MWth (Knoef 2003). They are suitable for biomass fuels with low ash content such as wood chips, sawdust, pellets and particle sizes to 50 mm. An advantage of underfeed stokers is their good partial-load behaviour and their simple load control. Load changes can be achieved more easily and quickly than in grate combustion plants because the fuel supply can be controlled more easily The fuel is fed into the combustion chamber by screw conveyors from below and is transported upwards on an inner or outer grate. Outer grates are more common in modern combustion plants because they allow for more flexible operation and an automatic ash removing system can be attained easier. Primary air is supplied through the grate, and secondary air usually at the entrance to the secondary combustion chamber. A new development is an underfeed stoker with a rotational post-combustion, in which a strong vortex flow is achieved by a specially designed secondary air fan equipped with a rotating chain (van Loo and Koppejan 2003).

Fixed Fuel Beds

Circulating Fluidized Bed

Within a fluidised bed furnace, biomass fuel is burned in a self-mixing suspension of gas and solid material into which combustion air enters from below. A fluidised bed consists of a cylindrical vessel with a perforated bottom plate that is filled with a suspension bed of hot, inert, granular materials, commonly silica sand and dolomite (van Loo and Koppejan 2003). Primary combustion air enters the furnace from below through the air distribution plate and fluidises the bed so that it becomes a mass of particles and bubbles. The intense heat transfer and mixing provides good conditions for a complete combustion with low excess air demand. The combustion temperature has to be kept low, usually between 800-900°C, in order to prevent ash sintering in the bed. This can be achieved by internal heat exchanger surfaces, by flue gas recirculation, or by water injection (Orjala et al. 2000). Fixed-bed combustion plants usually have operating temperatures 100 to 200°C higher than fluidised bed combustion systems (van Loo and Koppejan 2003).

Due to the good mixing achieved, fluidised bed combustion plants can deal flexibly with various fuel mixtures such as mixtures of wood and straw, but are limited when it comes to fuel particle size and impurities contained in the fuel. Therefore, appropriate fuel pre-treatment system achiving particle size reduction and separation of metals is necessary for low-maintenance operation (Nieminen 2004).

Fuel materials with an average diameter smaller than 2 mm are suited for dust combustion. This includes fine shavings and sawdust that can be pneumatically blown into a furnace. Combustion occurs while the fuel is in suspension, thus a constant and regular fuel-to-air mixture must be maintained. Dust furnaces are using waste mainly from the chipboard industry. A thermal capacity of 2 to 8 MW is possible with this technology (van Loo and Koppejan 2003).

Dust Combustion Plant