Fuel Feeders

Fuel feeder/distributors that evenly feed the fuel over the entire grate surface are necessary for even energy rerelease. These feeder/distributors can be mechanical, pneumatic, or a combination of both and must be placed across the width of the front of the stoker in sufficient quantity to achieve even lateral distribution of the fuel and have the means to longitudinally adjust fuel distribution for various types of fuels and sizing. They should be able to bias the feed rate one feeder to another, and to adjust for segregation of fuel sizing from one feeder to another. The performance of the fuel feeder/ distributors can adapt to the different characteristics of solid fuels which plays a major part in the ability to operate at lowest possible emissions and highest combustion efficiency (Johnson, 2002). 

Coal feeders should have a nonsegregating distributor interfacing between the coal bunker and the stoker feeder. A coal scale is recommended between the nonsegregating spout and the coal bunker. A coal scale provides a method for tracking daily, weekly, or monthly coal usage. All modern coal-scale electronics provide for real-time usage in terms of coal rate per hour, which is useful for tracking efficiency. 

For maximum efficiency, best load following characteristics, and lowest emissions, it is recommended that there be a separate metering device for each fuel distributor and that the metering devices be kept full of fuel at all times. It is also important that the metering device be kept in a vertical plane from the front to prevent lateral poor distribution of the feedstock in the furnace (Johnson, 2002). 

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Drying Fhiidized-bed units for drying solids, particularly coal, cement, rock, and limestone, are in general acceptance. Economic-considerations make these units particularly attrac tive when large tonnages of solids are to be handled. Fuel requirements are 3.3 to 4.2 MJ/kg (1500 to 1900 Btu/lb of water removed), and total power for blowers, feeders, etc., is about 0.08 kWh/kg of water removed. The maximum-sized feed is 6 cm (IV2 in) X 0 coal. One of the major advantages of this type of dryer is the close control of conditions so that a predeterminea amount of free moisture may be left with the solids to... 

Moderator make-up capability from the reactor building floor to allow fuel cunimg in ihe event of feeder stagnation break and failure of the ECC system... 

NOTE Where the primary problem is to improve combustion efficiency, the product typically is added to the fuel handling system via an automatic feeder and used continuously at a rate of 0.5 and 1.5 lb per ton of fuel. 

Co-combustion involves partial replacement of a fossil fuel such as coal with biomass. Reviews of co-combustion have been published by Leckner [46] and Sami [47]. Co-combustion is being done successfully at the commercial scale [48, 49]. For partial substitution of coal by biomass, the biomass can be fed with the coal through the same bumer/feeder, or it can be fed to a separate burner. Alternatively, the biomass can be pyrolyzed/gasified first, and then the products can be burned in the boiler. Biomass gasification followed by combustion in a boiler has been commercially applied and is illustrated in Figure 2.3.6. 

The test rig is equipped with a feedstock hopper suitable to low-bulk-density biofuels and the biomass is fed continuously into the sand bed of the reactor via an injector screw which can feed fuel either into the bottom of the bed or to about 1 meter above the air distributor. The maximum fuel mass flow rate is 90 kg/h. The feeding system consists of two screw feeders in series separated by a rotary valve. The second screw has a higher feeding rate thus it will remain almost empty and therefore is not likely to be blocked by pyrolysis products. Steam can be added to the primary air as a gasification agent. 

The DWSA installation can be divided into two main parts. The first part consists of an air preheater, fluidised bed reactor, solid fuel dosing vessel with on-line mass determination system and a hot gas cleaning section, containing a cyclone and a ceramic candle filter (Schumacher type). In the fluidised bed reactor the solid fuel is gasified with air to produce a low calorific value (LCV) gas that is cleaned of fly ash and unreacted solid carbonaceous material. Air and also additional nitrogen can be preheated and is introduced into the reactor by four nozzles just above the distributor plate. The reactor is electrically heated in order to maintain a constant temperature over bed as well as freeboard section. The solid fuel is fed into the bed section in the bottom part Just above the distributor by a screw feeder from beside. The hot gas cleaning section ensures a good gas-solid separation efficiency, with filter temperatures of about 500 C. 

The fuel is fed from a hopper to a weigh belt conveyor which measures the feed flowrate, it then passes through a rotary valve system equipped with sealing air and into the gasifier through a screw feeder. The screw feeder controls the fuel feed flowrate. 

The dried and crushed wood fuel is pressurised in a lock-hopper system to a level which basically is determined by the pressure ratio of the gas turbine, and is fed by screw feeders into the gasifier a few meters above the bottom. The operating ten erature of the gasifier is 950 - 1000 C and the pressure is approximately 17 bar, The gasifier is of a circulating fluidized bed type and consists of the gasifier itself, cyclone and cyclone return leg. The three parts are totally refractory lined. 

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