Biomass Combustion and Co-firing

Large-scale combustion

Large-scale combustion

Different biomass combustion systems are available for industrial
purposes. Broadly, they can be defined as fixed-bed combustion, fluidised
bed combustion, and dust combustion.

Fixed-bed combustion

Fixed-bed combustion systems include grate furnaces and underfeed
stokers. Primary air passes through a fixed bed, where drying,
gasification, and charcoal combustion take place in consecutive stages.
The combustible gases are burned in a separate combustion zone using
secondary air.

Two 3.2 MWth grate
furnaces for wood chips, used for district heating in Interlaken,
Switzerland. (Courtesy of Schmid AG, Switzerland)

Grate furnaces are appropriate for burning biomass fuels with high
moisture content, different particle sizes, and high ash content. Usually,
the capacity goes up to around 20 MWth. Mixtures of wood fuels can be used
but straw, cereals, and grasses may cause problems due to their different
combustion behaviour, their low moisture content, and their low ash
melting point. The grate and walls can be water-cooled to avoid slagging
problems.

The design and control of the grate are aimed at guaranteeing smooth
transportation and even distribution of the fuel and a homogeneous primary
air supply over the whole grate surface. Irregular air supply may cause
slagging, and higher amounts of fly ash, and may increase the oxygen
needed for complete combustion.

Underfeed stokers represent a cheap safe technology for small- and
medium-scale systems up to about 6 MWth. The fuel is fed into the
combustion chamber by screw conveyors from below and is transported
upwards on a grate. Underfeed stokers are suitable for biomass fuels with
low ash content (wood chips, sawdust, pellets) and small particle sizes (up
to 50 mm). Underfeed stokers have a good partial load behaviour and simple
load control. Load changes can be achieved more easily and quickly than in
grate furnaces because there is better control of the fuel supply.

Fluidised bed combustion

In a fluidised bed, biomass fuel is burned in a self-mixing suspension
of gas and solid bed material (usually silica sand and dolomite) in which
air for combustion enters from below. Depending on the fluidisation
velocity, bubbling and circulating fluidised bed combustion can be
distinguished.

The intense heat transfer and mixing provide good side conditions for
complete combustion with low excess air demand. Using internal heat
exchanger surfaces, flue gas re-circulation, or water injection, a
relatively low combustion temperature is maintained in order to prevent
ash sintering in the bed.

Due to the good mixing achieved, fuel flexibility is high, although
attention must be paid to particle size and impurities contained in the
fuel. Partial load operation of fluid bed combustion plants is limited
because of the demand for the process.

A 25 MWe
woodchip fired power plant in Cuijk, Netherlands with BFB boiler

Low NOx emissions can be achieved by good air-staging, good mixing, and
a low requirement for excess air. Moreover, additives (eg limestone for
sulphur removal) work well due to the good mixing behaviour. The low
excess air amounts required reduce the flue gas volume flow and increase
combustion efficiency. Fluid bed combustion plants are of special interest
for large-scale applications (normally exceeding 30 MWth). For smaller
plants, fixed bed systems are usually more cost-effective. One
disadvantage is the high dust loads taken in with the flue gas, which make
efficient dust precipitators and boiler cleaning systems necessary. Bed
material is also lost with the ash, making it necessary to periodically
add new bed material.

Dust combustion

Dust combustion is suitable for fuels available as small, dry particles
such as wood dust. A mixture of fuel and primary combustion air is
injected into the combustion chamber. Combustion takes place while the
fuel is in suspension; the transportation air is used as primary air. Gas
burnout is achieved after secondary air addition. An auxiliary burner is
used to start the furnace. When the combustion temperature reaches a
certain value, biomass injection starts and the auxiliary burner is shut
down. Due to the explosion-like gasification process of the biomass
particles, careful fuel feeding is essential.

Fuel/air mixtures are usually injected tangentially into a cylindrical
furnace to establish a rotational vortex flow. This motion can be
supported by flue gas re-circulation in the combustion chamber. Due to the
high energy density at the furnace walls and the high combustion
temperature, the muffler should be water-cooled. Fuel gasification and
charcoal combustion take place at the same time because of the small
particle size. Therefore, quick load changes and efficient load control
can be achieved. Since the fuel and air are well-mixed, only a small
amount of excess air is required. This results in high combustion
efficiencies.

Need for research

Combustion technologies can be improved to further reduce the total
costs of heat and/or power produced and to maximise safety and ease of
operation. The need for innovation is also driven by the wish to burn new
biomass fuels, such as pellets, energy crops, and waste wood.

Instead of performing expensive and time-consuming test runs, CFD
modelling is increasingly used to calculate flow, temperature, and
residence time distributions as well as two-phase flows (flue gas and ash
particles) in biomass furnaces and boilers, and to evaluate the impact of
design on combustion quality and emissions.

Ash-related technical problems such as particulate formation, deposit
formation, and corrosion as well as slagging require ongoing R&D. To
reduce maintenance and repair costs and to increase the availability of
installations, the mechanisms responsible for problems as well as
appropriate primary and secondary measures to prevent them have to be
thoroughly understood.

There are still gaps in our knowledge about the thermodynamic and
physical properties of certain elements, compounds, and multi-component/multi-phase
systems, as well as the chemical reactions related to ash, NOx and SOx
formation. Improvement of basic data and models is therefore necessary.

A
computer simulation program, used to evaluate the dynamic reaction of a step
grate combustion system on sudden fluctuations in woodfuel properties.