Experimental Investigation Of CFB Ash Discharge Valve Characteristics

Coal gasification is being developed worldwide to provide environmentally clean power and chemical production.The main direction of coal gasification technology development is to improve the conversion efficiency,the flexibility and reliability.Circulating Fluidized bed(CFB)gasifiers can operate at variable loads which gives them a high turndown flexibility.CFB gasifiers,which have agglomerated ash operation,can handle higher ranks of coals because they have higher cold gas efficiency than dry ash systems.In CFB gasification,ash discharge is required to regulate its solid circulation rate.The discharge process remains challenge due to the poor reliability of the solid valve at high temperature and pressure,especially for CFB gasifier.The main challenge of CFB gasification is still the reliability of ash valves in long-term operation.In this work,a novel ash valve for CFB was proposed to overcome such a challenge.It features three supply chambers and one discharge chamber.Solids flow rate of ash valve can be well adjusted to suit a wide variety of operating conditions with three independent supply chambers.In this study,the hydrodynamics and control principle of the novel valve was characterized using Geldart type B particles in a cold flow model.The effects of gas aeration,pressure drop of standpipe and backpressure on the performance of the valve were investigated.Carbon dioxide is adopted to trace the gas repartition in the ash valve.The result demonstrated that the 60%gas flow of the supply chamber flowed into the discharge chamber to increase the gas velocity in discharge chamber.The experiment results show that as the constant gas velocity in supply chamber,the increase of gas velocity in the discharge chamber resulted in a linear increase of solids flow rate,the solids flow rate kept stable until it greater than 2.5Umf.As the gas velocity in discharge chamber fixed at 3Umf,solids flow rate increased linearly with the gas velocity of supply chamber,which had an operated region is range from 0.8 Umf to 2Umf.A wide linear operation range of solids flow rate can be continuously acquired by adjusting the aeration of three supply chambers.The specific regulation of the three supply chambers is as follows:ⅰ)When the scope of Gs is less than 90 kg/m2/s,supply chamber No.1 is used to precision tuning for an accurate Gs;ⅱ)When the scope of Gs is between 90 and 140 kg/m2/s,the gas velocity of supply chamber No.2 is fixed at 2.1 Umf,and supply chamber No.1 is used to precision tuning for an accurate Gs;ⅱ)When the scope of Gs is between 140 and 200 kg/m2/s,the gas velocity of supply chamber No.2 is fixed at 2.1 Umf,meanwhile the gas velocity of supply chamber No.3 is fixed at 2.2 Umf.And supply chamber No.1 is used to precision tuning for an accurate Gs.Pressure drop between the standpipe and the ash valve promoted the solids flow rate of the ash valve,while the backpressure affects oppositely.Furthermore,the specific feasible operated schemes of the novel high-temperature ash valve were discussed.