to read the full report - Ecolateral by Peter Jones

Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
Small industrial-commercial and domestic boilers are small grate burners that burn almost exclusively
clean wood in the form of logs, chips and pellets. Logs are used mostly for domestic applications, pellets
for domestic and commercial and chips for commercial to large commercial and industrial.
Fluidised bed burners
In a fluidised bed furnace, the fuel burns in a bed of sand or other mineral that is violently agitated by the
combustion air. The fuel is fed at a controlled rate to keep the temperature of the sand bed at 800 to
900°C. With moderate air velocities the bed has the appearance of a boiling liquid, hence the name
bubbling fluidised bed (BFB). If a higher velocity is used the sand will be carried out of the furnace and
must be recycled to the base via a cyclone – this is known as a circulating fluidised bed (CFB). Heat is
removed and steam is raised by tubes in the bed of sand (not needed for biomass fuels), the walls of the
furnace and in the exhaust flue. This type of boiler is proving very popular for medium to large industrial
boilers for coal and other solid fuels and is taking an increasing share of this market.
A great advantage of the fluidised bed to the power plant operator is its fuel flexibility. This feature has
been used to great advantage by CHP plants in the Nordic Countries, where it is common practice to fire
wood chip, coal, peat, oil and wastes both together and separately. This flexibility is bought at the cost of
greater complexity however and they are unlikely to be cost effective lower output alternatives.
Good modern examples of fluidised bed burners from the Bioenergy capital grants scheme are at
Lockerbie (43MWe) and Wilton (31MWe). Both are fuelled by a mix of forestry wood chip and graded
waste wood from wood recycling operations.
Steam Boilers
There are two types of boiler in common usage, fire tube and water tube.
Fire tube boilers, as the name suggests, transfer the heat from the fire to the water by passing the hot flue
gasses through tubes inserted in a pressure vessel containing the boiling water. Typically the gas will
pass backwards and forwards three times through the boiler – three tube passes. They are inexpensive
and manufactured in volume. Typically the furnace supplier will purchase the boiler and adapt it to his
system. Fire tube boilers are manufactured in sizes up to 50 tonnes steam/h and a typical steam
pressure of 20bar, although 40bar is possible. They produce only saturated steam but a variant is
available that allows for superheating by installing a heat exchanger after the second tube pass. This is a
cost effective solution for industrial plants that require smaller turbines, typically 1 - 2MWe.
Water tube boilers generate steam inside of tubes that are installed around the furnace and in banks
across the flue gas. The temperatures and pressure of the steam can be much higher than in water tube
boilers. Utility boilers can reach supercritical conditions. For large biomass boilers 520°C and 120 bar is
not uncommon. Smaller units that might be appropriate for a CHP installation would have temperatures
of 450°C and 50 bar. They can be manufactured in much larger sizes than fire tube boilers and tend to
overlap the top end of the fire tube range.
Grate burners can be installed with either fire tube or water tube boilers. Fluidised beds are normally
installed with water tube boilers.
Control of emissions
The combustion gases are cleaned by a sequence of process stages that remove particulates, acid gases
and trace organic compounds such as dioxins and furans. The number of stages their type and
complexity depends upon the composition of the waste, its legal classification under the Waste
Incineration Directive and the size of the installation. The main species to be removed and abatement
technologies are summarised below in Table 55.
109