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Rapid technological development of small and medium scale pellet combustion systems is occurring globally as a result of the pressing need to develop clean and green renewable fuels. In particular, technological advances have taken place in countries like Sweden, Denmark, Austria and Germany. Pellet boilers have been widely introduced, particularly into these northern European countries. Sweden is among the countries most actively pursuing pellet boilers for energy security and rural development. Some initial investigations from surveys performed in Europe and North America have indicated consumer satisfaction with pellet appliances and their appreciation for the technology. The major advances in combustion technology are overviewed in this section as they relate to improving performance and ambient air emissions from combustion appliances. The main approach at creating more efficient and clean combustion has been to improve combustion appliance geometries and to create appropriate air staging.

Combustion basics

The three main factors to consider in developing clean combustion design technologies are the three Ts: time, temperature and turbulence. Complete combustion of gases occurs through effective air mixing in the secondary combustion, achieving long residence times for gases to burn in the appliances along with reaching adequately high temperatures in the combustion process.

Research and experience has shown that the volume, and supply method of combustion air is of paramount importance to achieving optimization of the combustion process. All clean combustion appliances now divide the combustion chamber into primary and secondary air to enable two stage combustion. The primary combustion zone is the area where pyrolysis occurs and the dry fuel is transformed such that combustible volatile components are rapidly released while the residual char is slower to oxidize. Effective secondary combustion is achieved through developing a well designed geometry of the combustion chamber to create turbulence and through an effective arrangement and design of the air nozzles. The air also needs to be introduced with adequate velocity. The main objectives are to create effective turbulence for gas mixing and residence time to ensure complete gas combustion.

Many combustion systems are now being designed to be multi-fuel appliances or boilers burning wood, agricultural feedstock and coal. However there are major differences between coal and biomass as combustion fuels. A primary difference between the fuels, which affects the combustion process, is the amount of volatiles. In coal most of the carbon is fixed and is more slowly released during the combustion processes. In contrast biomass fuels have high levels of volatiles and hence the fuel releases its energy more rapidly. Consequently biomass feedstock requires significantly more overfire air than coal to enable clean combustion by thoroughly burning the volatiles.

Residuals in softwood, hardwood and bituminous coal (%)

The importance of the secondary air is not only the quantity but also how it is strategically introduced to create turbulence. In some models, cylindrical walls provide a confined central gasification chamber and air is injected from holes located in two spiral paths extending up the wall as mirror images of each other. This approach, completed at high temperatures of 1,200 to 1,370°C, creates a vertically swirling and turbulent motion which results in near complete combustion of the gases emanating from the solid fuel.

A major problem in burning high ash fuels that have significant levels of aerosol forming compounds like potassium, chlorine, sodium and sulfur are that they have a low ash melting point. This causes several problems including clinker formation and corrosion of appliances. A major problem with burning fuels at high temperatures is the occurrence of alkali species migration where potassium compounds form on the inside walls of the combustion unit. One of the main strategies to address this problem is to use a lower temperature to initiate the release of gases in the primary combustion area. The problematic compounds will remain on the fuelbed where they are eventually removed through the bottom ash avoiding the secondary combustion area and boiler tubes. For example in grain combustion systems in Denmark, the feedstock is pyrolised at less than 700°C. In larger, more sophisticated combustion systems, staged combustion can occur where the temperature is gradually increased through the combustion process. The more successful combustion units that are burning higher ash fuels have fuel bed systems which continuously remove ash and avoid a remixing of the burning char and burnt out ash with newly introduced fuel.

This is particularly important in small boilers and stoves where the fuelbed is quite small and fuels are commonly fed from above. Ensuring there is limited to no mixing of fresh fuel and burnt out fuel, will help prevent the formation of soft surface bridging on the fuelbed, which has been widely reported when burning higher ash fuels.

It has also been well documented that the volume of excess air needs to be minimized to prevent excessive heat losses out of the combustion stack that commonly occurs when excess air required for effective combustion is employed. To gain better control of the combustion process, lamda (oxygen) monitoring controls can be used in the combustion process to maintain effectiveness. Both Lamda and CO controlled combustion appliances are now on the market which are overall providing considerably better results than temperature controlled combustion systems. It is extremely important that small scale appliances are well designed as installation of air clean up technologies are not affordable with small scale systems under 70 kilowatts.

Pellet stoves and boilers for agricultural feedstock

There now is an increasing number and diversity of pellet stoves and small boiler designs suitable for household, small commercial, and farm-based heating. A list of technology providers is provided in the final section. All these companies are incorporating features which make them better adapted to burning higher ash agricultural fuels. A list of some of the design features that stoves and boilers can include to help them achieve efficient and clean combustion of agricultural feedstock such as feed grains or grass pellets can include:

• Moving grate with limited to no mixing of fresh and already burning or burnt
• Lamda or oxygen control.
• Turbulence created through a well designed combustion chamber geometry and strategically introduced secondary air
• Low excess
• Use of ceramic refractory brick and high quality smooth surfaced stainless steel (to prevent adhesion by alkali compounds) in key areas where degradation from corrosion and high temperatures can
• Modest initial combustion zone temperatures in the primary combustion zone area followed by high temperature secondary
• Flue gas condensing system to recover additional heat and reduce particulate

The overall progress in improving the combustion process in Europe in recent years has been remarkable. Progress has been made in small pellet boilers in efficiency, particulate load and CO emissions. The better technologies are generally incorporating many of the aforementioned improved design features. However some design features such as use of a flue gas condensing system for heat recovery may create excessive capital costs in smaller units.