This invention relates to a fluidized bed combustion system and method and, more particularly, to a method for controlling the temperature in the furnace section of the system.
Fluidized bed combustion systems are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. Water is passed in a heat exchange relationship to the fluidized bed to generate steam. The combustion system includes a separator which separates the entrained particulate solids from the gases from the fluidized bed in the furnace section and recycles them back into the bed. This results in an attractive combination of high combustion efficiency, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Circulating fluidized beds are characterized by relatively high solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. In some of these arrangements a recycle heat exchanger is located between the solids separator and the furnace section for cooling the solids before they are recycled back to the furnace section.
The heat transfer, and therefore the temperature, in the furnace section is dependent on the solids loading pattern along the entire furnace height and the furnace is usually conservatively sized from a thermal standpoint to achieve better combustion and sulfur reduction. The solids loading is, in turn, a function of several parameters such as ash and sulfur content in the fuel, fuel and sorbent (limestone) size distribution, furnace gas velocities, combustion air flow distribution, cyclone efficiency and furnace configuration. As a result, it is not always possible to accurately predict the heat transfer rate and therefore the furnace temperature. This is undesirable since in order to ensure optimum sulfur capture the furnace temperature should be within a fairly narrow range which typically is 1500-1640.degree. F. When the furnace temperature is outside this range the sulphur capture efficiency plummets resulting in high sulfur sorbent consumption. Also, fuel burnup efficiency is affected at low furnace temperatures.
Although the furnace absorption and temperature can be varied by varying the external heat exchanger duty, the flue gas recirculation, the amount of spray water, or the amount of sand feed, these techniques are expensive and less desirable from an operational standpoint.