Experimental Study On Desulfurization Characteristics Of CaO And Silica-calcium Hydration Sorbents Under Circulating Fluidized Bed Combustion-like Conditions

Circulating fluidized bed?CFB?combustion technology is a new type of clean coal combustion technology.The cost for in-situ desulfurization process is low and the desulfurization efficiency is high,but the calcium utilization conversion of limestone is low.It is critical to identify the reaction mechanism between calcium-based desulfurization sorbents and SO₂,and to develop high-performance hydration sorbents.In current work,the desulfurization characteristics of two CaO samples?commercial CaO,CaO calcined from Ca CO₃?were evaluated based on a thermogravimetric analysis technique under conditions of circulating fluidized bed combustion.In addition,the effects of preparation parameters on the specific surface area and desulfurization conversion of silica?diatomite and rice straw ash?/CaO hydration sorbents were also addressed.Based on experimental research and kinetic theory,the effects of temperature?800-950oC?,SO₂ concentration?1000-4000 ppm?,CO₂ concentration?0-45%?and particle size?0.053-0.25 mm?on the conversion of two CaO samples and the kinetic characteristics were determined.With the increase of SO₂ concentration and temperature,the conversion degree of two CaO samples increased,while the conversion degree of CaO calcined from Ca CO₃ decreased with the increase of CO₂ concentration and particle size.At 850oC,when the SO₂ concentration increased from 1000 ppm to4000 ppm,the conversion degree of commercial CaO increased by about 37%,and the conversion degree of CaO calcined from Ca CO₃ increased by about 58%.Under certain reaction conditions,the conversion degree of CaO calcined from Ca CO₃ is greater than that of commercial CaO.For example,at 900oC and 2000 ppm SO₂,the conversion degree of the former was about 68%higher than that of the latter.The desulfurization process presented two reaction stages:chemical reaction controlled stage and product layer diffusion controlled stage.The unreacted shrinking core model can be better used to describe the relationship between the conversion degree of two CaO samples with time.The values of the activation energies?E_a?of commercial CaO and CaO calcined from Ca CO₃ in the chemical reacton controlled stage were 10.17 k J/mol and 6.43k J/mol,respectively,and the values of activation energies?E_p?in the product layer diffusion controlled stage were 136.77 k J/mol and 96.62 k J/mol,respectively.The activation energy of the product layer diffusion controlled stage is far greater than that of the chemical reaction controlled stage.For the prepared hydration sorbents,the effects of hydration time?2-12 h?,diatomite/CaO and rice straw ash/CaO ratio?0:5-5:5 g/g?and hydration temperature?45-85oC?on the specific surface area and desulfurization conversion of diatomite/CaO and rice straw ash/CaO hydration sorbents were examined.The hydration sorbent prepared from diatomaite/CaO with the hydration time of 4 h,diatomite/CaO ratio of1:5 g/g,hydration temperature of 65oC had the highest desulfurization conversion?0.61?,which was about twice of CaO.The hydration sorbent prepared from rice straw ash/CaO with hydration time of 4 h,rice straw ash/CaO ratio of 2:5 g/g,hydration temperature of 65oC had the highest desulfurization conversion?0.66?,which was about1.3 times higher than CaO.In the temperature range of 750-900oC,the activation energies of diatomite/CaO?D6?and rice straw ash/CaO?R4?hydration sorbent samples in the chemical reaction controlled stage were 6.74 k J/mol and 4.71 k J/mol,respectively,which were less than that of CaO?36.09 k J/mol?.Generally speaking,the greater the specific surface area,the greater the desulfurization conversion of the hydration sorbent,but the desulfurization conversion is not only related to the specific surface area,but also related to the pore volume and pore size distribution.This investigation can provide basic data and theoretical support for further research on the desulfurization performance of calcium-based desulfurization sorbents and the development of high-performance sorbents under circulating fluidized bed conditions.