Biomass CFB Steam Boiler environmental regulations fluidized beds

Design and operation

1. Design: Recirculating loop of particles occurred when entrained particles, which possess a substantial amount of flux, are separated efficiently and externally to the reactor from a giant core reactor from its carrying fluid and will then be circulated back to the bottommost of the riser. The carrying fluid will circulate around this loop only once however the particle will pass through several times before finally leaving the system.

2. Operational: The system is usually operated under high particle flux and high superficial gas velocity, which are typically (10–1000 kg/m2s), and (2–12 m/s) respectively.This operational condition is chosen to avoid a distinct interface between the dilute region and the dense bed inside the riser. Thus gas velocities above the bubbling point is chosen for contacting.

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.

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

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