The present invention relates to a new design of-in-bed surfaces for burning fuels of wide variation in quality in a bubbling fluidized bed combustion boiler.
Fluidised bed is a system consisting of an enclosure, where solid particles are in a suspended state. This state is achieved by passing a gas through a packed bed of solid particles. At low velocities, the pressure drop across the bed is small and the particle system will be undisturbed. But as the gas velocity is increased steadily, the pressure drop increases till a point where the pressure drop across the bed equals the weight of the bed particles per unit area. At this stage the bed particles are in a suspended state and it is called "minimally fluidized bed". WJipnjfcegas velocity is increased further, the bed becomes highly turbulent and rapid mixing of particles takes place. At this stage the bed of solids attain pseudo fluid properties.When the solid particles in a fluidized bed are preheated to the ignition temperature of fuel, the fuel fed will bum and impart heat to the whole volume of the bed. The temperature in the entire bed would be uniform. This process is called fluidized bed combustion. Bubbling Fluidized Bed Combustion (BFBC) boilers are designed for fuels of varying quality. This technology is well suited for the present conditions due to their eco friendly nature.
BFBC Boilers are designed to burn fuels of widely varying quality such as -
Lignite with high moisture
Washery rejects with high ash and low moisture
Sub - bituminous coal
Imported coal with very low ash and
Petcoke with low volatiles.

The BFBC technology requires operating temperature in the range of 750 to 950°C. This requires provision of varying amount of heat transfer surfaces in the fluidized bed. While the low quality fuels require less heating surfaces in the fluidized bed, the better quality fuels require more surfaces.
The arrangement of the invention is illustrated in Figure 1 of the accompanying drawing.
Generally evaporative, superheat and reheat surfaces are immersed in the bed. The amount of effective evaporative surface (1) is to contain the bed temperature at all loads in the range. The amount of superheat (2) anchor reheat (3) effective surfaces is to maintain superheat / reheat temperatures at the required loads.
The present invention involves an arrangement that would provide required heating surfaces in the same fluidized bed by varying thebed height. The innovation is varying the bed height by arrangement of ash overflow discharge system (4) and heat transfer surfaces are located in a such way that fuels of varying quality can be burnt in the same fluidized bed to maintain the required bed temperature.
In the existing design the limits are exceeded to accommodate all the fuel operating conditions. While the lower temperature results in defluidization and slumping of bed, the higher temperature results in agglomeration and clinker formation.
The invented design is provided with more than one automatic ash discharge system to eliminate the shortcomings of the existing design. The invented design features maximize the utilization of different fuels in an environmentally acceptable way in a single BFBC boiler.

WE CLAIM;
1. A bubbling fluidized combustion system comprising means to vary expanded bed height by way of more than one automatic ash discharge system to vary the effective immersed area, said means provides heating surfaces in same fluidized bed by varying bed height, wherein bed height is capable of being varied by arrangement of ash overflow discharge system and heat transfer surfaces both so located that fuels of varying quality can be burnt in said fluidized bed to maintain required bed temperature.
2. A bubbling fluidized combustion system substantially as herein described and as illustrated in the drawings accompanying this specification.