Middle Pressure Circulating Fluidized Bed Boiler Non Uniformity Flow Patterns

Coal Fired Middle Pressure CFB Boiler for Power Plant circulating fluidised bed boiler

Main process characteristics

1. The circulating fluidized bed reactors have been widely used in various industrial processes such as gasification and coal combustion.[18] Though the circulating fluidized beds are used widely, the CFD, which can be, describe by non-uniformity flow patterns and a thorough back mixing still possess significant radial gradients in the particle density and a lower solid holdup inside the riser interior compared to the wall of the reactor. These events will then result in low contact efficiency.

2. For the case of catalytic gas-phase reaction process, gas back mixing should be avoided thus the reacted product is the gas phase. Another characteristic of the circulating fluidized bed is, as it required promoting the small contact time of gas and solid catalyst and plug flow, a significant high gas velocity in the riser is needed. The significant high gas velocity in the riser is also desired to fulfill the necessity in the catalytic gas-phase reaction.

Fluidized Bed Introduction

1. Circulating fluidized bed is a relatively new technology with the ability to achieve lower emission of pollutants. Extensive research has been conducted on this technology within the past 15 years due to increasing concerns over pollution caused by traditional methods of combusting coal and its sustainability. The importance of this technology has grown recently because of tightened environmental regulations for pollutant emission.

2. The Mercury and Air Toxic Standards enacted in December 2011 by the EPA have forced all the countries in Europe and America to strictly adhere to this policy. This means that emissions such as metals, acid gases, organic compound, flue gas acids and other pollutants from power plants or industrial facilities have to meet the requirements set by EPA and upgrades have to be done for facilities that do not meet the standards. As a result, the demand for circulating fluidized bed technology is predicted to sky rocket.

Circulation

1. Fine particles of partly burned coal, ash and bed material are carried along with the flue gases to the upper areas of the furnace and then into a cyclone. In the cyclone the heavier particles separate from the gas and falls to the hopper of the cyclone. 2. This returns to the furnace for recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from the cyclone pass to the heat transfer surfaces and go out of the boiler.

Bed Material

1. To start with the bed material is sand. Some portion is lost in the ash during the operation and this has to be made-up. In coal fired boilers the ash from the coal itself will be the makeup material.

2. When firing bio fuels with very low ash content sand will be the makeup bed material. For high Sulphur coals Limestone addition to the bed material reduces SO2 emissions.

Main Specifications of Circulation Fluidized Bed Boiler