TwinFluid fluidised bed combustion

After being active in the field of fluidised bed combustion for more than 35 years, we have started a project in 2006 to develop a fluidised bed combustion system ideally   suited for industrial energy production from a wide range of fuels and waste, in capacities of up to 200 MW. With this system, the following goals are met

• Fuel flexibility (biomass, waste, coal, sewage sludge etc.) with varying moisture- and ash content as well as calorific values.
• Environmentally friendly and efficient combustion with changing operational parameters and low emissions, also added by the direct desulphurisation with limestone dosing into the fluidised bed.
• High efficiency with low excess air and low flue gas volume.

• Option of fly ash recirculation with horizontal integral high temperature cyclone.
• Compact low investment plant with high degree of automation.
• Small heat transfer surface through modulating secondary fluidised bed immersed tubes.
• Simple system with low maintenance requirements.

A fluidised combustion bed made of sand, fuel ash and limestone for sulphur capture is used to burn a wide variety of fuel. Coarse fuel is distributed over the shallow combustion bed by air spreaders and dust is injected pneumatically from the bottom into the bed.

Apart from limestone for desulphurisation, ammonia or urea can be injected into the furnace to further reduce Nitrogen Oxides which are already low with this low   temperature combustion system. The combustion bed is operated at temperatures between 850 and 950 °C. Because of the good mixing of combustion air and fuel, CO and hydrocarbon emissions are very low

For part load operation and to accommodate the varying heating value of different fuels, hot bed material from the shallow combustion bed flows into the deeper heat transfer fluidised bed which is located next to the combustion bed.

The heat transfer bed is equipped with in bed tubing which are heated by the hot bed material. Because the heat transfer coefficient between the fluidised solids   and the tubes is very high only a small heating surface is needed.

The cooled ash is then recirculated back to the front end of the combustion bed by pneumatic   transport. The rate of recirculation controls the heat load of the system.

The furnace can either be build up from refractory material or boiler water walls. The freeboard temperature is controlled by secondary air which further enhances  the burn out of small fuel particles and volatile matter. The long residence time in the furnace ensures complete burn out.

For difficult fuels, a horizontal cyclone can be added at the exit of the furnace for the recirculation of unburned, where a secondary air venturi sucks the hot gases from the cyclone skimmer which are so blown back into the furnace for secondary combustion.

This TWINFLUID fluidised bed system is protected by international patents in many countries and a number of industrial size reference plant are already in operation.