BFB Boiler is a kind of boiler that can handle low calorific value fuel or nonflammable fuel. The fuel is fed into the sand mixture at high temperature, so that the fuel can be effectively burned.
As shown in Figure 1, the BFB boiler is vertical cylindrical structure or square structure. The metal shell is lined with refractory materials, and there is a supporting structure above the bottom of BFB boiler body to accommodate the sand bed. The supporting structure has a series of tuyeres that become air distributor plate. The function of air distributor plate is to bear the sand bed, allow air to flow upwards and prevent sand from falling into the air box. The structure of the air distributor plate can be refractory materials or steel alloy.
The fuel of this project consists of carbon black and tire pyrolysis oil. The carbon black is put into the fluidized bed sand bed area by a feeder and feed chute, and the pyrolysis oil is introduced into the freeboard via the burner.
The fluidized bed boiler is a single-chamber combustion device, and the air box is below the air distributor plate. A start-up burner is set here, which can quickly heat the bed temperature and reach the feeding conditions. The sand bed area is made of refractory materials, and the carbon black feeding and combustion process all take place in the fluidized bed. It lasted for several seconds between 760oC to 850oC at sand bed area, in which black carbon particles is burned. After leaving the surface of sand bed, unburned carbon black particles are ejected to upper freeboard area to continue burning until they are completely burned out. Pyrolysis oil is injected into the freeboard area and ignited by burner. The freeboard area is composed of membrane walls, and high-pressure boiler water is introduced into the tube, which is heated and evaporated to generate steam. Four burners are respectively arranged at the eccentric position of four membrane walls. The four burners can form a simulated tangential flame in a square furnace, promote mixing in the combustion chamber, enhance combustion efficiency and uniform heat release. The flue gas will be removed by air pollution control device immediately after leaving the combustion chamber. During operation, the sand in the sand bed must be replaced or supplemented. The sand loss is usually 5% of the bed capacity every 300 hours. The carbon black can be fed from the upper side of the bed or injected from the side of the hearth.
The airflow in the fluidized bed is determined by several factors, and it must meet both fluidization air and combustion air at the same time. The airflow is enough to expand the bed to an appropriate density, but it must be low enough to prevent unburned carbon black particles from rising to and floating on the bed. Excessive air will blow out sand and unburned carbon black particles, and excessive combustion exhaust gas will waste stored heat energy and increase fuel consumption. The air supply must ensure that oxygen meets the minimum oxidation requirement for of all combustible carbon black.
The operation of fluidized bed incineration system is relatively simple, with the fewest mechanical parts, and the equipment investment capital is low. The normal exhaust gas temperature exceeds 760oC(1400oF). After the exhaust gas is exposed to this temperature for a few seconds, the odor, carbonyl and unburned hydrocarbon emissions will be completely burned and disappear. Without using an independent secondary combustion chamber, the minimum and strict emission regulations can be met. However, it is important that operating conditions shall be optimal to ensure compliance with strict local emission standards.
