A study of the heat transfer about the heating surface of three commercial 300 MWe CFB boilers was conducted in this work。 The heat transfer coefficients of the platen heating surface, the external heat exchanger (EHE) and cyclone separator were calculated according to the relative operation data at different boiler loads。 Moreover, the heat transfer coefficient of the waterwall was calculated by heat balance of the hot circuit of the CFB boiler。 With the boiler capacity increasing, the heat transfer coefficients of these heating surface increases, and the heat trans- fer coefficient of the water wall is higher than that of the platen heating surface。 The heat transfer coefficient of the EHE is the highest in high boiler load, the heat transfer coefficient of the cyclone separator is the lowest。 Be- cause the fired coal is different from the design coal in No。1 boiler, the ash content of the fired coal is much lower than that of the design coal。 The heat transfer coefficients which calculated with the operation data are lower than the previous design value and that is the reason why the bed temperature is rather high during the boiler operation in No。1 boiler。 83511

Keywords: CFB boiler; heating surface; heat transfer coefficient; calculation

Introduction

Circulating Fluidized Bed (CFB) combustion has been considered as one of the prevailing clean coal combus- tion technologies in China since the mid 1980s。 CFB boiler, which has been widely applied, has many advan- tages over conventional combustion systems, such as satisfactory desulfurization efficiency, high combustion efficiency, low NOx emission, flexibility for different types of coal, good performance for control and appro- priate capital and operation costs[1-4]。 The effective NOx control and high sulfur capture efficiency are two of its very attractive features when using low-grade coals。 As the global environmental association imposes more stringent regulations on atmospheric emissions, CFB boilers have become one of the best technologies for the

production of clean energy from coal, and worldwide applications of CFB boilers are increasing[5-8]。 Up to now, a lot of 300 MWe CFB boilers have been put into com- mercial operation in China, the first supercritical pa- rameter 460 MWe CFB boiler has been put into operation at Łagisza Power Plant in Poland, and a 600 MWe CFB boiler is being erected at Baima Power Plant in China。

With the increasing of boiler capacity, it requires more heating surface area to control the furnace temperature and necessitates the increase of boiler heights。 However, in practice the furnace height of a 300 MWe CFBboiler is limited to 40 m due to commercial considerations[9], hence additional heat transfer surfaces must be provided to control the furnace temperature。 Of course, there are various heating surface types, such as External Heat Ex- changer (EHE)[10], Ω type heating surface, platen heating

Received: March 2012 Zhang Man: Senior Engineer

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Q absorbed heat of platen heating surface q4 unburnt carbon loss

H total heating surface for each chamber Q6 sensible heat los

Δt logarithmic temperature difference Qk heat from the hot air

Tfi inlet flue gas temperature Qw absorbed heat of the waterwall

Tfo outlet flue gas temperature Qps absorbed heat of the platen superheaters