In-situ Study Of Alkali Metals Release During Coal And Biomass Co-pyrolysis/gasification

Co-gasification of coal and biomass is an important technology to realize the clean and efficient utilization of coal and biomass.It can not only alleviate the contradiction between the increasing demand and the decreasing reserves of coal,but also solve the problems caused by low energy density,dispersion and seasonality of biomass.However,equipment corrosion caused by alkali metals volatilization seriously restricts the development of coal and biomass co-gasification technology.Therefore,it is crucial to deeply understand the release and migration mechanisms of alkali metals during co-gasification of coal and biomass.In this dissertation,rice straw and Yangchangwan coal are used as raw materials,a visual fixed bed(VFB)combined with laser-induced breakdown spectroscopy(LIBS)is applied to investigate the in-situ release characteristics of alkali metals during co-pyrolysis/co-gasification of coal and biomass.The effects of temperature,coal and biomass mixing ratio,minerals and O-containing functional groups in coal on the release of alkali metals during co-pyrolysis/co-gasification of coal and biomass are investigated.The relationship between the chemical structure changes of chars and the release of alkali metals is analyzed by X-ray diffractometry(XRD),Fourier transform infrared spectroscopy(FT-IR)and Raman spectroscopy(Raman).The main contents and conclusions are summarized as following:(1)A visual fixed bed combined with laser-induced breakdown spectroscopy was applied to investigate the in-situ release characteristics of alkali metals during co-pyrolysis of coal and biomass.The results showed that the release concentration of alkali metals first increased and then decreased with pyrolysis time.The pyrolysis temperature,mass ratio of biomass to coal,and minerals in coal had little effect on the temporal releasing trend but seriously affected the release concentration extremity of alkali metals.The maximum release concentration of potassium increased from 111.6 mg/Nm3 to 530.5 mg/Nm3 and sodium increased from 29.9 mg/Nm3 to 81.4 mg/Nm3 as the pyrolysis temperature increased from 700℃ to 900℃ during rice straw pyrolysis.The release ratio of potassium and sodium increased with the increase of pyrolysis temperature.The presence of minerals such as SiO2 and Al2O3 in coal limited the release of alkali metals during co-pyrolysis of coal and biomass,and the addition of SiO2 and Al2O3 reduced the release concentration and release ratio of potassium and sodium.A negative correlation was obtained between the disorder degree of co-pyrolysis char and the temporal release concentration of alkali metals.(2)Pre-oxidized coals with different O-containing functional groups were prepared by low temperature pre-oxidation to study the effect of O-containing functional groups in coal on the release of alkali metals during co-pyrolysis of coal and biomass.The results showed that pre-oxidation increased the content of O-containing functional groups such as carbonyl and carboxyl groups in coal,and the content of O-containing functional groups was related to the degree of pre-oxidation.The maximum release concentration of potassium decreased from 345.35 mg/Nm3 to 307.89 mg/Nm3 and sodium decreased from 104.56 mg/Nm3 to 83.12 mg/Nm3 as the degree of pre-oxidation increased.The release concentration and release ratio of potassium and sodium decreased with the degree of pre-oxidation increased.The layered pyrolysis increased the interaction time between gas-phase alkali metals and O-containing functional groups,further reducing the release concentration and release ratio of alkali metals.(3)A visual fixed bed combined with laser-induced breakdown spectroscopy was applied to investigate the in-situ release characteristics of alkali metals during CO2 co-gasification of coal and biomass.The results showed that as the gasification temperature increased from 700℃ to 900℃,the maximum release concentration of potassium increased from 39.4 mg/Nm3 to 147.0 mg/Nm3 and the maximum release concentration of sodium increased from 13.2 mg/Nm3 to 50.4 mg/Nm3.The release ratio of potassium increased from 15.63%to 25.33%and sodium increased from 24.21%to 35.62%.When the gasification temperature was below 800℃,mainly organic potassium and organic sodium were released,and when the gasification temperature was above 800℃,the evaporation of KCl and NaCl became the main way of alkali metals release.The release ratio of alkali metals during gasification was lower than that of pyrolysis,which may be due to the reaction of K and Na released into the gas phase with CO2 to form K2CO3 and Na2CO3.