Lower bed temperature in FB waste combustors

■ Start date: April 2008

■ End date: November 2008 Completed

Project description and results
Waste incineration generally serves two purposes; 1) to get rid of waste and 2) generation of heat and power. In the process of power production from waste fuels, the steam temperatures in super heaters are generally limited by the severe fouling and corrosion that occurs at elevated material temperatures, caused by high concentrations of alkali metals and chloride in the flue gas and fly ash.

The overall aim of a continuation of present project is to determine if a reduced temperature of the bed zone in a fluidized bed waste incinerator reduces the amount of alkali chlorides in the flue gas. If so, a reduced bed temperature might enable increased steam temperature in super heaters, or, at unchanged steam temperature, improve the lifespan of the super heaters. The results from the project are of interest for plant owners wishing to improve performance of existing plants. The results may also be used to modify the design of future plants by boiler manufacturers.

The aim of present pre-study was to determine how far the bed temperature can be reduced in a waste fired fluidized bed boiler in Borås while maintaining a stable operation with sufficient combustion temperature in the freeboard to fulfil the directives of waste incineration. A continuation of the project will be based on the results from present study.

The work is based on experiments at the test boiler. During the present study, no other measurements were performed apart from some sampling of bed material and ashes at different modes of operation.

The experiments show that it is possible to alter the air and recycled flue gas in such a manner that the bed temperature is reduced from about 870°C to 700°C at 100% load and normal fuel mixture, while fulfilling the directive of 850°C at 2 seconds. Within normal variations of the fuel properties, however, the bed temperature increases to somewhat above 700°C if the fuel turns dry, while it falls below 650°C when the fuel turns wet.

With constant O2 concentration in the flue gas, the emissions of CO were unaffected by changed ratios of air and recycled flue gas to reduce the bed temperature. However, the emissions of NOx tended to increase due to raised temperatures of about 40°C at the top of the furnace at maximum load.

When increasing the amount of recycled flue gas to the bed, the concentration of oxygen is reduced and its implications upon the chemistry in the bed and possible accumulation of unburned fuel have not been investigated due to the short experimental period.

Chemical analysis of sand from the bed showed that the concentration of chlorides in the sand increased more that 4 times after 30 hours of operation at reduced bed temperatures. This finding indicates that the chemical balance in the bed is significantly altered when the temperature is reduced. To determine the consequences for the operation of the boiler, further experiments and measurements that are more extensive are necessary. No measurable changes of alkali contents in the sand were observed.

Participating partners