BOILERS, BOILER FUEL AND BOILER EFFICIENCY
A WIENESE Sugar Milling Research Institute, Durban 4041, South Africa
Abstract This paper describes the modern boilers in the South African sugar industry. A new equation for the calculation of the net calorific value (NCV) of bagasse is suggested and a distinction is made between boiler design efficiency and boiler operation efficiency.Methods to calculate fuel calorific values and boiler efficiencies from first principles are presented. Introduction In the past the fuel requirements for most sugar factories were easily met by the available bagasse, in fact some factories were in the rather embarrassing situation of having a surplus of bagasse. However, with an increase in alternative uses of bagasse, the operation of back endrefineries and the move towards cogeneration, more and more factories find themselves short of bagasse and have to resort to the use of alternative fuel in the form of coal. This has led to an increasing interest in energy efficiency of which the boiler efficiency forms an essential part. The boiler efficiency does not only depend on the boiler configuration and operation but also on the fuel beingused. This paper describes a typical sugar factory boiler, the analysis of boiler fuel and discusses the calculation of boiler efficiency. The figures that are used are generic and are not to be taken definitively. Boilers Typical modern boilers in the South African sugar industry produce superheated steam at a pressure of 3100 kPa (abs) and a temperature of 400°C. They are designed to burn bagasse,coal or a mixture thereof and are equipped with a full heat recovery system. Boiler configuration The main components of a modern boiler are: the grate, fuel feeders, combustion chamber or furnace, water or mud drum, steam drum, main bank, superheater, economiser, air-heater, scrubber, induced draught (ID) fan, forced draught (FD) fan, secondary air (SA) fan, boiler feed water pumps and someauxiliary equipment (Figure 1). Grate: Bagasse burns in suspension and has a relatively low ash content of 4%, part of which leaves with the flue gases. The removal of bagasse ash is therefore rather simple and can be achieved by a dumping or pinhole grate (Misplon et al, 1996). Coal burns on top of the grate, has a much higher ash content of typically 11-16% forming clinker and requires a continuousash discharge (CAD) stoker (Page et al, 2000). Under grate air temperatures should not exceed 250°C for bagasse or 150°C for coal fired boilers to prevent overheating of the grate. Boiler grates
are typically designed for a heat release rate of about 3 MW/m2 for bagasse and 1,5 MW/m2 for coal. Fuel feeders and spreaders: Modern boilers are fitted with tall bagasse chutes to give a stable feed tothe furnace. The throughput is controlled by feeder rolls and spreading is pneumatic. Coal spreaders are either mechanical or pneumatic. Today, dual pneumatic distributors are used successfully provided secondary air is introduced in the right quantities (Magasiner and Naude, 1988). Feeder chutes must never empty since the fuel in the chute provides an air seal and prevents blow backs. Combustionchamber or furnace: While the spaced tube and tile has been the main furnace construction for many years, it is being replaced by membrane walls combined with refractory tiles to give the thermal reserve needed for the ignition of bagasse. This welded wall design has the advantage of providing a gas tight enclosure and is less prone to slagging. The gas leaving the furnace must be at least 200°Cbelow the ash fusion temperature and is approximately 950°C for bagasse and 1050°C for coal. The furnace heat release rate is in the order of 0,24 and 0,12 MW/m3 for bagasse and coal respectively. A typical boiler gas temperature profile is given in Table 1. Superheater: Because of the regular failure from overheating of non-drainable superheaters during start-up fully drainable superheaters...
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