Design Theory of Circulating Fluidized Bed Boilers

Abstract

Studies on circulating fluidized bed (CFB) boilers have being conducted at the Tsinghua University (TH) for about two decades and much of works are done to link the fundamentals with practical application. A full set of design theory was developed and some key elements of this theory are presented in this paper. First, a classification of state of the solid-gas two-phase flow in CFB boiler is given. TH’s studies validated that a CFB boiler can be generally described as the superposition of a fast bed in the upper part with a bubbling bed or turbulent bed in the bottom part. A concept model of material balance for the open system of CFB boiler was developed and later improved as a more comprehensive 1-D model taking ash formation, particle attrition and segregation in bed into account. Some results of the models are discussed. Then the concept of State Specification of a CFB boiler is defined and discussed. The State Specification is regarded as the first step to design a CFB and a base to classify different style of CFB boiler technologies for various CFB boiler manufacturers. The State Specification adopted by major CFB boiler makers is summarized and associated importance issues are addressed. The heat transfer model originally developed by Leckner and his coworkers is adopted and improved. It is further calibrated with experimental data obtained on the commercial CFB boiler measurements. The principle, improvements and application of the model are introduced. Some special tools developed for heat transfer field test are also given. Also, combustion behaviors of char and volatile content are studied, and the combustion difference between a CFB boiler and a bubbling bed is analyzed. The influence of volatile content and size distribution is discussed. The concept of vertical distribution of combustion and heat in CFB boiler furnace is introduced and discussed as well. In the last, the suggested design theory of CFB boiler is summarized.

A 1000 MWth boiler for chemical-looping combustion of solid fuels – Discussion of design and costs

“…The heat balance in Section 3.5 gives the circulation 5.3 tonne/s, which corresponds to 25.5 kg/m 2 ,s. Normally, the circulation in large circulating fluidized beds is, if known, not published, but there are some data in the literature. According to Yue et al [30], the circulations measured in large CFB boilers are around 6-10 kg/m 2 ,s at fluidizing velocities in the range 5-6 m/s. These Table 1 Lower heating value (H i ) over stoichiometric air demand (L 0 ) for normal conditions.…”

Heat extraction from a utility-scale oxy-fuel-fired CFB boiler

“…The extents to which this might pose a problem and how this effect could be limited by additional/modified refractory lining remain to be elucidated. There are reports in the literature of large-scale CFB boilers being operated with solid fluxes of up to 25 kg/m 2 /s (see Yue et al, 2005). cases, i.e., those cases that yield maximum furnace temperatures of o 1273 K. As shown in the figure, there is a continuous decrease in heat flux up through the furnace in all the cases despite that the temperatures for these cases increase with height as can be seen from Fig.…”

Progress of circulating fluidized bed combustion technology in China: a review

“…The lower energy consumption CFB technology based on state specification design theory can be clearly illustrated with the fluidization state map proposed by Yue et al [9], as shown in Fig. 3.…”

“…Moreover, the mass transfer resistance increases between the bubble phase and fuel particle surfaces [60], which leads to an oxygen-lean condition for the fuel combustion. The intensified reducing atmosphere will restrain the transformation of char-nitrogen and volatile nitrogen of coarse fuel particle into NO x [9,61]. As shown in Fig.…”

“…As shown in Fig. 1, the flow pattern in the furnace is composed of the bubbling or turbulent bed formed by large particles at the bottom and the fast bed formed by fine particles in the freeboard [9]. In the light of the gas-solid flow and heat transfer in the furnace, the bed material can be divided into two groups, effective material and ineffective material.…”