i

Ts inlet steam temperature Qpr absorbed heat of the platen reheater

Ts outlet steam temperature  radiation heat coefficient

’ heat of flue gas away at the outlet of the hot

yx cyclone Ifh

heat of fly ash away at the out let of the hot cyclone

Q1 total absorbed heat in the furnace

surface, steam- cooled cyclone separator。 The heat trans- fer to heating surfaces is a key parameter for the control of the furnace temperature and for the arrangement of the heating surface in a CFB boiler[11-15]。

Properly selecting the heat transfercoefficient from the bed to waterwall is important in CFB boiler de- sign[16-18], especially in supercritical cases。In addition, the frequency of tube explosion accidents of platen heat- ing surface, up to approximately 40%, is the highest among all kinds of heating surfaces[19]。 Over temperature is an important factor leading to tube damage accidents, and the heat transfer process in the cyclone is not similar to that in the bed, because in cyclone the solid particles swirl over the wall surface with high velocity。 Many lit- erature exist for the evaluation of cyclone performance in the CFB[20-22], but few literature is available to the heat transfer of cyclone [23]。 The heat transfer coefficient cal- culation of EHE is also important to boiler design and operation。 So accurate calculation of the heat transfer coefficients of these heating surfaces mentioned above has the vital practical significance。

This paper researched on actual operation of three 300MWe subcritical pressure CFB boilers, in view of different loads, it is to provide the operation results of the heat transfer coefficients of the different heating surfaces。 The ultimate objective of the above work is to provide valuable information for scaling-up of supercritical pres- sure CFB boilers, for example, the design and commer- cialization of 600 MWe and 800 MWe supercritical pressure CFB boilers。

Brief introduction of commercial 300 MWe CFB boilers

The types of main arrangement of the heating sur- faces in 300 MWe CFB boilers

Among the operational commercial CFB boilers, the heating surface arrangement can be approximately di- vided into three major types。 These surfaces could be in the forms of (1) waterwall in furnace+ platen heating surface in furnace + hot cyclone separator in the solids

circulation loop (2) waterwall in furnace + EHE in the solids circulation loop + hot cyclone separator in the sol- ids circulation loop (3) waterwall in furnace + platen heating surface in furnace + steam-cooled cyclone sepa- rator in the solids circulation loop。 It is necessaryto know the heat transfer characteristics of these main sur- faces for design and operating considerations。 The result was obtained from the calculation and the analysis of three commercial 300 MWe CFB boilers。 Different ar- rangements of heating surfaces in CFB boilerscan be seen on Fig。1。 No。1 boiler, No。2 boiler and No。3 boiler represent type 1 boiler, type 2 boiler and type 3 boiler respectively。 Three main arrangement types of the heat- ing surfaces are showed on Fig。1。

The general arrangement of the three boilers

No。1 boiler: A commercial 300 MWe CFB boiler with a pantleg type furnace, natural circulation and circulating fluidized bed combustion (Fig。 2)。 The boiler is com- posed of three main parts - a furnace with a waterwall membrane, two hot cyclone separators on each sideof the furnace, a twin-exit loop seal at the end of each cy- clone standpipe and the convection pass。