| The excessive utilization of fossil fuels,for a long time,has brought a high risk on energy security and caused an urgent environmental pollution.As the commitment of carbon peak and carbon neutrality by the government,more attentions have been paid on alternatives to traditional energy,in which the biomass energy is an effective one.Biomass fuel can be sourced widely,but varies significantly in quality.Compared with the coal-fired boiler,a higher moisture content is witnessed in the biomass-burned boiler.Additionally,the flue gas contains HCl and NH3,while the ash particles are composed by various deliquescent chlorides.Hence,the condensation of acid and deliquescence of chlorides will potentially occur to flue gas surrounding the low-temperature surface in a biomass-fired boiler.This phenomenon can accelerate adhesive or viscous ash deposition,further causing the low-temperature corrosion and even the thermal performance degradation.To conclude,it is crucial to ensure the safety operation of low-temperature surfaces in a biomass-burned boiler through finding a proper calculation method for the moisture content of flue gas-and investigating the ash deposition characteristics under an integrated effects from acid dew point,salt deliquescence and water condensation.In this paper,characteristics of ash deposition covering low-temperature heating surfaces in biomass-fired boilers were systematically studied by theoretical analyses.laboratory tests,field tests and numerical calculation methods.Calculation models of flue gas flow and water content were obtained based on the data of proximate analysis for biomass fuel.A dew point test system with transformable cooling medium was designed to study the characteristics of ash deposition covering low-temperature surface in flue gas of biomass-fired boiler.Four typical ash deposition layers were obtained among the variation of tested tested pipe temperature during cooling process,as well as the effect of typical ash deposition layer on the heat transfer performance.The determination method of acid dew point,water dew point and salt deliquescence temperature in low temperature flue gas of biomass boiler were obtained by the mechanism of typical fouling layer,the NH4Cl crystallizing,acid condensation,deliquescence of deliquescent salt and water condensation.The safety criterion of low temperature heating surface of biomass boiler was established,and the safe operating temperature curve of low temperature heating surface was given.Combined with the studied biomass boiler,the safety design of flue gas waste heat recovery and utilization system was carried out.The safety criterion for low-temperature heating surface in biomass-fired boiler was established by the safe operation temperature curve of low-temperature heating surface.And a safety design of flue gas waste heat recovery and utilization system was performed for the boiler done field experiments.This paper mainly completed the following research contents:(1)The concept oxygen demanding coefficient of low calorific value of biomass fuel was proposed,the relationship between theoretical air volume and low calorific value of biomass fuel was studied,and calculation models of flue gas volume and moisture content based on industrial analysis were proposed.150 biomass fuel samples were used to verify the calculation models.The results showed that the calculation models are suitable for most agroforestry biomass fuels.(2)The characters of biomass fuels,low temperature flue gas and fly ash of biomass-fired boiler were carried out.The biomass fuels,flue gas and fly ash of a 35MW biomass-fired boiler were tested and analyzed.The results showed that the moisture content of raw materials was high,and the drying was not sufficient before combustion,resulting in high moisture content of feed fuel.The content of chlorine in herbs was high,reaching 0.4 wt%which was much higher than coal.150 biomass fuel samples were classified,and the relationship between element content and low calorific value was studied in different kinds.Due to high moisture content of feed fuel,The moisture content of flue gas was high,reaching above 15%.The field tests showed that the contents of HC1,NH3,and SO2 of flue gas were 71.7mg/m3,61.5 mg/m3,and 1.4 mg/m3,respectively.The reasons of high contents of HCl and NH3 were given.The Scanning Electron Microscope(SEM)analysis showed that the shape of fly ash was mostly irregular porous medium.The X-ray Diffraction(XRD)analysis showed that SiO2 in the fly ash was mainly composed of α-SiO2 and glassy amorphous matter;metal oxides,carbonates and hydroxides were the main existing form of metal elements;and feldspar minerals and SiO2 were the main insoluble substances.Ion Chromatography(IC)analysis showed the mass percentage of soluble chlorine in fly ash was 0.40%.By Comparing the results of Inductively Coupled Plasma Mass Spectrometry(ICP-MS)and Electronic Differential System(EDS)analyses,most of Mg,Al,Ca and Fe in fly ash were insoluble in water.The subsequent theoretical exploration and the design of waste heat utilization system were obtained on basis of these characteristics of biomass fuels,flue gas and fly ash.(3)The characteristics of ash deposition covering low temperature surface of flue gas in biomass-fired boiler under different wall temperatures were analyzed by on-site ash deposition experiments.①According to the characteristics of low temperature flue gas and ash samples of biomass-fired boiler,a dew point test system with transformable cooling medium was designed.The test system included a cooling water circuit and a heat transfer oil circuit,which can switch the cooling medium for the test needs.②The ash deposition characteristics of a single test element under different wall temperatures were studied using the test system.The wall temperature of the test element was the main factor affecting ash deposition characteristics.Low wall temperature leaded to adhesive or viscous ash deposition on the wall,and the existence of wet ash might lead to corrosion.The amount of ash on the wall of the test element increased with time and reached dynamic equilibrium after a period of time.The time required for the ash to reach a stable state was related to the wall temperature.③By means of SEM,EDS,XRD,and gravimetric analysis,the ash samples collected from tested pipe at different temperatures were studied,and the change rules of morphology and composition of the ash samples collected from the heating surface during cooling process were obtained.④Based on Chemical Equilibrium Theory and Multi-component Diffusion Effect Theory,the relationship between crystallization temperature of NH4Cl and partial pressures of NH3 and HCl in flue gas was obtained.The formation rate of NH4Cl under different partial pressures of NH3 and HCl was calculated.⑤According to the results of test analysis and theoretical calculation,the ash deposition process on low temperature heating surface of flue gas in biomass-fired boiler can be divided into four stages,accompanied with four typical ash deposition layers,namely loose ash deposition layer,dry NH4Cl crystallization deposition layer,acid-deliquescent salt interaction deposition layer and water condensation deposition layer.(4)The mechanism of formation of acid-deliquescent salt interaction deposition layer was studied.①By calculating the condensation rate of sulfuric acid and the concentration of sulfuric acid solution,it was found that adhensive ash would not appear on the wall immediately when the heat exchange surface temperature reached the thermodynamic sulfuric acid dew point.②The dew point experiment platform was built with the mixture of water vapor and HCl,and the variation law of dew point in the H2O-HCl binary system was obtained through the experimental method.It was found that the moisture content had a greater impact on dew point of the mixture,and the change regulation of the dew point with respect to moisture content is similar between the mixture and pure water vapor.When the concentration of HCl was between 0 ppm and 1000 ppm,the impact of HCl on dew point of the mixture was limited.Hydrochloric acid condensation was not the main factor causing formation of acid-deliquescent salt interaction deposition layer.③ The deliquescence law of deliquescent salt such as LiCl,CaCl2,MgCl2,NH4Cl,KCl and NaCl was studied by simulating the temperature and humidity environment of flue gas in biomass-fired boiler,as well as the changes of non-deliquescent substances in the fly ash such as CaCO3,CaSO4,CaSO4·2H2O and CaO under the same conditions.The water uptake and deliquescence rate of deliquescent salt were studied by experimental method.The deliquescence relative humidity and the theoretical maximum water uptake of deliquescent salt were calculated by theoretical analysis.It was proved that among all kinds of ash deposition components,deliquescent salt was the key factor leading to adhensive deposition on the tested pipe during the cooling process.(5)The mechanism of NH4Cl crystallization,acid condensation,salt deliquescence and water condensation in the process of low temperature flue gas ash deposition of biomass-fired boiler was revealed.The safety criterion for low temperature heating surface in biomass-fired boiler was proposed.According to the dew point formulas of sulfuric acid and hydrochloric acid,as well as the deliquescence relative humidity data of deliquescent salt at different temperatures and the vapor pressure data of their saturated solution,the safe operation temperature curve of low temperature heating surface was obtained.A system for testing inlet water safety temperature of waste heat utilization equipment in biomass-fired boiler was designed.(6)Under the guidance of the safety criterion for heating surface,a case of waste heat recovery and utilization system using low-pressure economizer was designed for the biomass-fired boiler studied.On the basis of safety,the operational mode,inflow and temperature of inlet water of the low-pressure economizer were optimized.In this paper,characteristics of ash deposition covering low-temperature heating surfaces in biomass-fired boilers were studied and safety criterion of low temperature heating surface of biomass boiler was established.The methods used in sample testing,theoretical analysis in this study can be applied to various biomass-fired boilers.The conclusion can provide useful guidance and practical methodology for the safe and efficient operation of the heat exchange equipment in the biomass-fired boiler. |
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