Numerical Simulation Of Flue Gas Quench And Sludge Drying

For biomass/waste fueled power plants,stricter regulations require a further reduction of the negative impacts on the environment caused by the release of pollutants and withdrawal of fresh water externally.To handle this challenge,it becomes more common to add a water scrubbing process,which is called flue gas quench(FGQ),before flue gas condenser(FGC).Meanwhile,a large amount of sludge is produced during wastewater treatment process.Researches shows that it is important to dry sludge before reuse the sludge.And,drying sludge with waste heat of the flue gas(FG)will be beneficial to energy saving.Flue gas quench(FGQ)is playing an important role in biomass or waste fueled combined heat and power(CHP)plants,as it can link the flue gas cleaning,energy recovery and wastewater treatment.Enhancing water evaporation can benefit the concentrating of pollutant in the quench water;however,when FG condenser(FGC)is not in use,it results in a large consumption of fresh water.Moreover,drying sludge with FG will affect the heat recovery in the FGC,thereby,affecting the energy efficiency of the CHP system.In order to deeply understand the operation of FGQ,and the effect of FGQ and sludge drying on the CHP system,the mathematic models of FGQ and sludge drying were developed and validated against the measurements.Then,to study the effect of FGQ and sludge drying on the CHP system,those two models were integrated into the CHP model.Based on simulation results key parameters affecting FGQ have been identified,such as the flow rate and temperature of recycling water and the moisture content of FG.A guideline about how to reduce the discharge of wastewater to the external and the withdrawal of external water can be proposed.There exists one water to FG ratio(L/Gmax)that can result in the highest amount of water evaporation.When it is needed to concentrate the pollutants in the quench water,a higher recycling water temperature,a L/G closer to L/Gmax and/or a lower FG moisture content should be used;whereas,when it is needed to reduce the consumption of fresh water,a lower recycling water temperature,a larger L/G and a higher FG moisture content should be adopted.The mathematic model was also implemented into an ASPEN Plus model about a CHP plant to assess the impacts of FGQ on CHP.Results show that the energy efficiency decreased when the FGQ was running,nevertheless,the wastewater discharged to wastewater treatment plant decreased by 8.6%.Meanwhile,the insights regarding optimizing the heat recovery of the FG can be proposed by studying the sludge drying with flue gas.