Medium to large scale combustors

 

The major types of large scale biomass boilers use one of the following technologies: grate combustion systems (stationary or travelling), pulverised fuel (PF) systems or fluidised bed combustion (FBC) systems.

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Grate combustors   

Moving grate with primary air in two stages in the grate and secondary air

With the techniques based on grates (such as inclined grate, traveling grate, chain grate and vibrating grate), the fuel is usually fed automatically onto the grate by gravity. As the fuel bed moves, moisture is driven off initially, followed by ignition, burning and finally cooling when the ash is removed. The air supply below the grate is often sectioned so that the flow rates and pressures of the primary combustion air to each section can be independently controlled. Those systems also require a fairly high proportion of the air supplied above the grate as secondary air. Temperatures above the bed and in the freeboard normally range between 800 and 1000°C. In the stationary grate design, ashes fall into a pit for collection. In contrast, a traveling grate system drops the ash into a hopper.

Moving grate with primary air in two stages in the grate and secondary air

 

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Stokers   

 

Spreader stokers, which feed the fuel by distributing it on the top of the bed throughout the furnace, and retort (underfeed) stokers are successfully used in some boilers that have been converted from coal firing.

Understoker furnace with primary and secondary air, mixing zone and post combustion chamber

 

 

Pulverised fuel (PF) systems

 

Wood and agricultural residues powder is attractive for larger plants, since it allows conversion to biomass fuels with minimal investment for boiler conversion. Burners for wood powder are available in the range of 1 to 30 MW. In pulverised fuel combustion, the fuel is introduced with air in burners similar to those used for oil or coal burners. The particle size must be small (generally below 1 mm) to complete a rapid combustion. Boilers equipped with oil burners or coal powder burners can be converted to use biomass powder fuel, or blends of biomass and fossil fuels. The PF process is somewhat difficult to control and may suffer from too high combustion temperatures (>1200°C) which may result in high emissions of nitrogen oxides (NOx).

cfb 

Fluidised bed systems   

 

Profile of a circulating fluidised bed (CFB)

Fluidized bed boilers are well known for their inherent fuel flexibility which is unmatched by any other combustion technology. Fluidized bed boilers can burn fuels with a wide range of calorific values, ash and moisture content. The fluidized bed combustion process has been in use for more than 25 years and is now established as an efficient and environmentally friendly technique.

Fluidised-bed combustors (FBC) burn biomass fuel in a hot bed of granular material, such as sand. The primary air keeps fluidised the bed of sand (making it resembling a boiling liquid), while secondary air, and in some cases tertiary air, may be introduced higher up in the furnace to achieve a staged and more complete combustion. The bed is normally operated at 750-950°C, which are considerably lower temperatures than for grate and pulverised systems. The lower temperatures inhibits the formation of nitrogen oxides (NOx) from the combustion air and allows fuels with lower ash melting temperatures to be fired. These systems also permit the removal of sulphur dioxide (SO2) from combustion of high-sulphur fuels simply by addition of sulphur absorbents such as limestone or dolomite in the bed. Besides many different coals, wood, other biomasses and also waste derived fuels have been successfully demonstrated in a number of FBC installations around the world. As this technology allows handling high-ash fuels, it is therefore a very suitable conversion technology for a large variety of agricultural biomass residue.

Two types of FBC are commercially available: the stationary fluidised bed (SFB) and the circulating fluidised bed (CFB). The CFB system process provides excellent conditions for the burning of

Profile of a circulating fluidised bed (CFB)

several different fuels in the same boiler – this has been almost impossible in earlier technologies. Bed material circulation and high turbulence in the combustor ensures good mixing of fuel and combustion air, and also efficient heat transfer to the heat surfaces inside the furnace. An efficient particle separator, cyclone, is the heart of the process and differentiate it from SFB units. The cyclone separates out all particles larger than a certain size from entering flue gas – the particles are characterized by each cyclone type – and returns back to the bottom part of the furnace via the loop-seal located in the bottom part of the cyclone. The CFB process is shown in figure below. The duty of the cyclone is to separate unburned fuel particles and return them to the furnace, as well as to give the limestone particles used for sulphur removal more time react.

Main differences between the two FB technologies.

 

 

Stationary fluidised bed

 

Circulating fluidised bed

 

Fuels

 

Good for biofuels (w<63%)
Limited with coal proportion (max. 30%)
Full capacity on oil and gas

 

Limited with biofuel moisture (w<58%)Full coal capability
Limited capacity on oil and gas (max. 40 %)

 

Process

 

Coarse bed material
Low fluidizing velocity

 

Fine bed material
High fluidizing velocity

 

Operation

 

Low power consumption
Low erosion
Low maintenance

 

Higher power consumption
Possibility for erosion
More refractory => more maintenance

 

Performance

 

Good efficiency
Low NOx
Low N2O
Limited SO2 removal with limestone

 

Good efficiency
Very low NOx
Higher N2O
Efficient SO2 removal with limestone