8/3/2019 Circulating Fluidized Bed Combustion Technology

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Coal Fired Power Generation Technologies (Combustion Technologies)

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1A2. Circulating Fluidized Bed Combustion Technology (CFBC)

Outline of technology

1.FeaturesThe features of circulating fluidized bed boiler are described

below.

1)Wide range of fuel adaptability

Conventional boilers for power generation can use only fossil fuel

such as high grade coal, oil, and gas. The CFBC also uses low

grade coal, biomass, sludge, waste plastics, and waste tire as fuel.

2)Low pollution

Emissions of pollutants such as NOx and SOx are significantly

decreased without adding special environment measures. For

the case of fluidized bed boiler, the desulfurization is intrafurnace

desulfurization using mainly limestone as the fluidizing material.

For the denitration, ordinary boilers operate at combustion

temperatures from 1,400OC to 1,500OC, and the circulating

fluidized bed boiler operates at lower temperatures ranging from

850OC to 900OC so that the thermal NOx emissions (generated

NOx depending on temperature) are suppressed. In addition, the

operation of circulating fluidized bed boiler is conducted by two-

stage combustion: the reducing combustion at the fluidized bed

section; and the oxidizing combustion at the freeboard section.

Then, the unburnt carbon is collected by a high temperature

cyclone located at exit of the boiler to recycle to the boiler, thus

increasing the denitration efficiency.

3)High combustion efficiency

High combustion efficiency is attained by excellent combustion

mechanism of circulating fluidization mode.

4)Space saving and high maintenance ability

Space saving is attained because there is no need of separate

desulfurization unit, denitration unit, and fuel finely crashing unit.

Accordingly, sections of trouble occurrence become few, and

maintenance becomes easy.

4.Study period

Most of the technologies of circulating normal pressure fluidized

bed boiler (CFBC) are introduced into Japan from abroad

beginning from around 1986.

5.Progress and result of the development

The CFBC was introduced from abroad as the coal fired boiler,

and is used in power companies, iron making companies, paper

making companies, and other sectors. The CFBC is planned to

distribute in China under the Green Aid Plan (GAP). The issues

of CFBC include further investigations of and efforts for initial cost

and power generation efficiency in the boilers using fuel such as

RDF, industrial waste, and wood-base biomass.

2.Outline of technology

Figure 1 shows a typical process flow of CFBC

Figure 2 shows rough structure of CFBC. Generally CFBC is

structured by the boiler and the high temperature cyclone. The

intrafurnace gas velocity is as high as 4 to 8 m/s. Coarse fluidizing

medium and char in the flue gas are collected by the high

temperature cyclone, and are recycled to the boiler. The recycle

maintains the bed height and increases the denitration efficiency. To

increase the thermal efficiency, a preheater for fluidizing air and

combustion air, and a boiler feed water heater are installed. Most of

the boiler technologies are introduced from abroad: mainly from

Foster Wheeler, Lurgi, Steinmuller, ALSTOM, and Babcock & Wilcox.

Fig.2 Schematic drawing of CFBC structure

Fig.1 Process flow of circulating fluidized bed boiler

3.Study site and application field

Photograph 1 shows overview of the CFBC boiler facilities. The CFBC

became popular mainly as coal fired boilers. Recently, however, CFBC

using RDF and wood-base biomass as the fuel has drawn attention.

Typical applications of coal fired biogas are Tamashima plant (70 t/h) of

Kuraray Co., Ltd, Chiba worksoil refinery (300 t/h) of Idemitsu Kosan

Co., Ltd., and Isa plant (210 t/h) of Ube

Industries, Ltd. Example of RDF fired boiler is

Tomakomai plant of Sanix Inc. As for the

biomass fuel, mixed combustion with coal is

adopted for decrease of CO2 emission.

Generated steam

CycloneCirculatingfluidized

bed furnace

Boiler feed water

ID fan

Ash storage tank

Stack

Flue gas

Secondary airPrimary air

Heatrecoverysection

Electricprecipitator

LimestoneCoal

CFBC boiler bodyHot cyclone

Boiler feed water heater

Air preheater

Photo 1 CFBC appearance

Part2 Outline of CCT