Numerical Investigation On Bed Combustion Characteristics Of Biomass Under Oxygen-enriched Condition

Biomass energy has rapidly developed and widely applied as an important alternative energy of fossil fuel in recent years in the world.It is an important to optimize the transformation of industrial chain boilers or design new-type biomass fuel boilers by using biomass fuel instead of coal in view of the low efficiency of combustion and the excessive emission of pollutants in domestic industrial chain boilers.Furthermore,the introduction of oxygen-enriched combustion technology is important to strengthen the biomass fuel combustion taking into account the problem of low combustion temperature and low fuel efficiency of biomass fuel.Therefore,it this paper introduces the numerical simulation method to investigate the combustion properties of biomass fuel bed under oxygen-enriched condition.Firstly,the combustion process of biomass bed in chain boiler is studied.Considering the process of mass transfer,heat transfer and chemical reaction in fuel bed,the particle of non-isothermal,the changes of bed height and the energy transfer along the grate horizontal direction in bed combustion progress,an one-dimensional unsteady improved model of bed combustion is established,and the numerical calculation is carried out by FLUENT software.The calculated results of the bed combustion are in good agreement with the experimental data,which verifies the correctness of the model.Compared with the original model,the improved model shortens the bed combustion time and has good effect in accordance with the actual operating conditions.Secondly,the process of bed combustion under different oxygen-enriched conditions and different fuel parameters is numerically studied.It is found that the combustion efficiency of bed combustion increases as oxygen content increases,both the volatile release rate and the char burning rate also increase,and the reaction time of bed combustion will be shortened;The increasing rates of the volatile release rate and the char burning rate both increase in the early stage of bed combustion,the ignition position advances,and the reaction time of bed combustion will be shortened.The volatile release rate and the char burning rate decreases with the increase of the moisture content,the ignition position delays and the reaction time of bed combustion will be prolonged.The influence of different fuel parameters on the bed combustion under oxygen-enriched condition is basically the same as the air condition.Finally,the process of bed combustion fronted-enhanced air supply mode and rear-enhanced air supply mode is numerically studied.It is found that the influence of the two different air supply modes on the initial stage of bed combustion is relatively small,and the volatile release rate slightly increases compared with the uniform air supply mode.The position of the main reaction zone advances.the char burning rate and the gas flow velocity on the bed surface all increase as the primary air flow increases.In the later stage of bed combustion,there exists incomplete combustion under fronted-enhanced air supply condition.And the fuel is completely consumed,the reaction time of bed combustion will be shortened under rear-enhanced air supply condition.Under oxygen-enriched condition,the efficiency of bed combustion is improved,and the variation trends of relevant parameters of bed combustion are basically the same as the air condition.