Preparation And Performance Of Shell Spherical Heat Carrier Based On Ash Waste Heat Recovery

The use of waste heat is an important way for companies to reduce fuel costs in order to achieve reduced consumption and increased efficiency,and also an important measure to achieve the goal of " peak carbon dioxide emissions ".With environmental pollution and energy scarcity becoming increasingly serious,biomass power generation technology is developing rapidly,and the proportion of biomass power generation in thermal power generation is increasing year on year.However,in the process of power generation,a large amount of thermal energy released during the combustion of biomass is lost with the discharge of flue gas and ash from the furnace,resulting in a serious waste of resources.While high temperature flue gas waste heat can already be effectively recovered in existing technology through coal economizers and air preheaters,high temperature ash is subject to corrosion,deposition and other factors that limit the form of waste heat recovery.Therefore,the development of biomass power plant ash waste heat recovery is of great significance in promoting the consumption of biomass energy and achieving green production.Currently,most domestic biomass power plants use the water quenching method to treat slag heat,but the water quenching method has extremely low thermal efficiency and is highly polluting.In order to achieve efficient and clean recovery of waste heat from ash,a method for direct contact recovery of waste heat from ash using metal spheres encapsulated with high heat storage density materials was provided in this work,and the thermal properties of heat storage materials,the thermal performance of heat spheres and the heat exchange effect of heat spheres with ash were analyzed.The main conclusions are as follows.(1)The thermal properties of sand,brown soil,stearic acid and paraffin wax were investigated and the results showed that the thermal stability of sand was better than that of brown soil,with a weight loss of 2.72% at 800 °C and a specific heat capacity of 0.84-1.24 J·g-1°C-1 at 60 – 800 °C under the "sapphire" test.The thermal stability of stearic acid and paraffin was similar,but stearic acid had a higher phase change temperature(68°C)and latent heat of phase change(200 J/g),with specific heat capacities of 1.53-2.07 J·g-1°C-1 at 30 – 61 °C and 2.19-2.60 J·g-1°C-1 at 96 – 200 °C.(2)The heat storage / release performance of the heat storage material before and after metal encapsulation was studied and the results showed that the thermal stability of sand in the sensible thermal storage material was better than that of brown soil;the thermal stability of stearic acid and paraffin wax in the phase change storage material was similar,but stearic acid had a higher phase change temperature and latent heat of phase change.The encapsulation of metal spheres could effectively increase the heating rate of the thermal storage material,improve the heat release capacity of the sensible thermal storage material and reduce the overall phase change time of the phase change material.The heat-up rates of sand,brown soil and paraffin spherical heat carriers were increased by 35.7%,10.5% and 113.0%,respectively,from 300-1200 s,and the time taken to cool down from 180 °C to 60 °C was reduced by 28.0%,17.6% and 23.4% respectively,to compared with that before encapsulation.The stearic acid spherical heat carriers were improved by 100.0% compared to pre-encapsulation and the time taken was reduced to 2160 s.(3)The waste heat of biomass ash was divided into two stages(high temperature waste heat: ash temperature > 300 ℃;medium temperature waste heat: ash temperature ≤ 300℃),using sand spherical heat carriers to recover high temperature waste heat,stearic acid spherical heat carriers to recover medium temperature waste heat.The influence of process conditions on the heat transfer effect of heat carrier ball and biomass ash was studied and results showed that heat source temperature,mixing mass ratio and filling amount were important factors affecting the heat exchange effect between spherical heat carriers and ash.After optimization of response surface experiments,the optimal conditions for heat exchange between sand spherical heat carriers and high temperature biomass ash were heat source temperature of 600 °C,mixing mass ratio of 1.5,filling amount of 60%,corresponding to heat exchanger thermal efficiency of 71.9% and waste heat recovery thermal efficiency of 43%.The optimal conditions for heat exchange between stearic acid spherical heat carriers and medium temperature biomass ash were heat source temperature of 250 ℃,mixing mass ratio of 1.5 and filling volume of 60%,corresponding to heat exchanger thermal efficiency and waste heat recovery thermal efficiency of 67.5% and 46.0% respectively.(4)The waste heat recovery process of biomass ash with spherical heat carriers as heat transfer medium was optimized.The most suitable overall process design was derived as follows: biomass ash at a temperature of 600 °C was selected as the heat source,the mass ratio of heat carrier balls to biomass ash mixture was 1.5,and the gravel heat carrier balls and stearic acid heat carrier balls were arranged in a mass ratio of 18:17.Under these conditions,the overall heat exchanger thermal efficiency was 66.7% and the waste heat recovery thermal efficiency was 59.1%.This was an improvement of 7.8% over the waste heat recovery thermal efficiency of the earlier process using the pure steel spherical heat carrier.The heat storage time of the obtained sand spherical heat carrier was 1.08 times longer than that of the pure steel spherical heat carrier and the heat storage time of the stearic acid spherical heat carrier was 5.44 times longer than that of the pure steel spherical heat carrier.In summary,the use of metal spherical shells to encapsulate high heat storage density materials to prepare spherical heat carriers for direct contact recovery of ash waste heat not only strengthens the heat transfer effect,but also effectively avoids ash pollution to the material.The sand and stearic acid are selected as the core materials and realize the reasonable matching with the heat source temperature can effectively improve the heat storage time of the heat carrier ball while recovering the waste heat of the ash at a low cost and high efficiency,and provide the possibility for the long-distance transportation and utilization of heat.The research results can provide reference for the development of new ash waste heat recovery technology and the application of phase change materials in ash waste heat recovery.