Numerical Simulation Of Combustion Process Of An Oil-fired Boiler Being Converted To Burn Gas

Owning to more and more attention to environment issues and the necessity of building international environmental protection cities of our nation, limitation of oil used in boilers has been broadened and gas is promoted as the fuel to substitute coal which is widely used but with high energy consume and serious pollution. On the other hand, with the expanding of manufacture of petrochemical factories, gas, a by-product, is more and more. Same oil-fired and coal-fired boilers are converted to burn this gas, and have got good result of saving energy and protecting environment. However, because of the greatly different combustion characteristics of gas from which of oil and coal, and lack of practical experience and theoretic direction in modification and operation, the combustion is inefficient and serious accident may happen as pipe blast. So it is necessary to research the gas burning in such boilers. As it is very difficult and expensive to collect data from in-situ experiment, numerical simulation is now widely used in engineering science and plays an important role.In the present paper, the combustion process of an oil-fired boiler with gas fuel is simulated with CFD software FLUENT. Realizable k-ε model is employed to simulate gas turbulent flow. Equilibrium Mixture Fraction/PDF Model is employed to simulate gas turbulent combustion. P1 model is used to simulate radiation heat transfer. The governing equations are discretized using the finite volume method and the equations are solved by SIMPLCE algorithm. Velocity and temperature field as well as wall heat flux density are obtained. The influences of air inlet condition (swirl number and direction), excessive air coefficient and combustor structure on combustion are discussed and optimized control parameters of combustion are obtained.The research shows that the temperature and velocity field obtained by FLUENT with suitable mathematic model is accordant to the practical combustion and can reflect the practical problem. Thus the simulation is feasible. The swirl number of secondary air has a great influence on the combustion. When swirl number is small(S<0.8), the fire is long, and may rush the back wall. When swirl number is big(S≥1.3), the fire is dispersed. Suitable swirl number is 1.0~1.2. Excessive air coefficient affects the fire length and wall heat flux. When two side-by-side burners run at the same time, the swirl direction should be opposite, and then the fire will be centralized. Under low load, the temperature field and wall heat flux will reduce, especially under a load of 26t/h and it is unsuitable to run.The result can be used as a theoretical guide to adjust and control combustion of gas fuel in present oil burners, as well as effective technology retrofit scheme in practice.