Plasma assisted coal combustion. Theory and experiment
Keywords:
, Coal combustion, coal thermo chemical preparation, plasma-fuel system, simulation, full-scale experiment.Abstract
Coal fired utility boilers face two problems, the first being the necessity to use expensive oil for start-up and the second being the increased commercial pressure requiring operators to burn a broader range of coals, possibly beyond the quality envisaged by the manufacturer’s assurances for the combustion equipment. Each problem produces a subsequent negative environmental impact. Oil-firing for start-up increases the gaseous and particulate burden of the plant. The firing of poorer quality coals has two disadvantages: reduced flame stability performance necessitating oil support and its consequential emissions and cost implications; and reduced combustion efficiency due to a higher amount of carbon in the residual ash, resulting in an increase in the amount of emissions per MW of power generated. Plasma assisted coal combustion represents a new effective and ecological friendly technology, which is equally applicable to alternative ‘green’ solid fuels. One of the prospective technologies is Thermochemical Plasma Preparation of Coals for Burning (TCPPCB). This technology addresses the above problems in Thermal Power Plants (TPP). The realisation of the TCPPCB technology project comprises two main steps. The first is the execution of a numerical simulation and the second involves full-scale trials of plasma supported coal combustion through plasmafuel systems (PFS) mounted at TPP boilers. For both the numerical simulation and the further full-scale trials, the boiler of 200 MW power of Gusinoozersk TPP (Russia) was selected. Four PFS are mounted on the furnace and used for boiler start-up and low-rank coal flame stabilisation. The numerical simulation was fullfilled with the help of the Cinar ICE ‘CFD’ code. Cinar ICE has been designed to provide computational solutions to industrial problems related to combustion and fluid mechanics. The Cinar code solves equations for mass, momentum and energy conservation. Physical models are employed for devolatilisation, volatiles combustion (fast un-premixed combustion), the char burnout and the turbulence (k-). Comparison of the calculations with the trials data showed good agreement. The maximal devergency between mesured and calculated temperatures in the outlet of the furnace does not exeed 15%. Note: this is bound to be true from a simple heat balance if the data are any good. Numerical simulation and full-scale trials enabled the following technological recommendations for improvement of existing conventional TPP to be made. It is concluded that the developed and industrially tested PFS improves coal combustion efficiency, decreases harmful emission from pulverized coal fired TPP. Prior to the wider implementation of PFS, additional data relating to further coal types and their blends are required.
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