Performance Analysis Of The Power Generation System Using Solar Energy Integrated Low-rank Coal Pre-drying

In recent years,the high-quality bituminous and anturacite in central and eastern China decreased sharply and coal mining became more and more difficult.In this case,more and more coal-fired power plant began to burn or mixed low-rank coal.For a low-rank coal fired power plant,there normally is normally additional drying equipment for low-rank coal pre-drying,due to that low-rank coal is low in calorific value,with high water content and easy to weathering.It may be noted that,many of the sun-rich areas,such as Northwest China,are with large deposits of low rank coals and are highly depend on coal-based power generation.If the solar energy is integrated into the low rank coal pre-drying system,the efficiency low-rank coal pre-drying unit could improve and the local abundant solar energy resources could be effectively and economically utilized.This paper foucused on the the efficient utilization of the solar energy to pre-heat the low-rank coal.The solar collection,absorption heat pump and low-rank coal pre-drying unit are modeled,and the thermal performance of a low-rank coal fired power plant using solar energy fed pre-drying is based on the first and second law of thermodynamics.The changes of the coal mass flowrate and heat value with and without drying are analysed.The comprehensive thermal performance of the proposed system is assessed by the coal consumption,the solar-to-electric efficiency,overall energy and exergy efficiencies,and new system integration concepts using various coal types and different solar collection is proposed.Before the system design,sub-bituminous coal low-temperature drying experiment is taken,the effect of drying temperature on coal drying,re-absorption and crushing characteristics are investigated.The experiment show that,sub-bituminous coal is easy to be dry out than lignite,the low-temperature dry of 50?100? can make the most of its moisture evaporate at a rapid speed,as long as reasonable process design,residence time extended appropriately.The average particle size of pulverized coal decreased slightly and the re-absorption effect of pulverized coal is not good after pre-heat.The medium-low solar energy fully meet the demand of the low-rank coal drying,and the characteristics of slight re-absorption of low-rank coal benefits the night store after solar drying.As technology of vacuum-tube solar collector is mature and northwest china is rich in solar energy and low-rank coal resources,a low-rank coal fired pre-drying power plant using vacuum-tube solar collector as heat source is proposed.Taking a typical 600 MW supercritical unit as a case,the effect of proposed system on the thermal performance is tanalyzed based on thermodynamics,heat transfer,technical economics and other related theories.The results show that:the coal consumption rate decreases remarkably by using low-grade solar pre-dry low-rank coal.As the moisture of low-rank coal reduces from 25.0%to 10.0%,the coal consumption rate decreases by 8.9g/kWh.At the same time,the solar-to-electric efficiency reaches 25.3%and the static investment payback period is only 4.3 years.Parabolic trough based solar power has higher collecting temperature,integrated by absorption heat pump and absorb the low-grade waste heat of the steam cycle,achieving larger amount of heat with suitable temperature for coal's moisture removal prior to the furnace and improving the efficiency of solar energy utilization greatly.Relevant calculation shows:for the case unit,if the net output power is constant,low-rank coal fired power plant could reduce the raw coal consumption by 4.6kg/s with overall energy and exergy efficiencies improvement of 1.2 and 1.8 percentage points,respectively,as 73.OMW solar thermal energy was introduced.As for the perspective of solar-to-electric conversion,low-rank coal fired power plant could generate 0.44MW electric while 1MW solar energy is input.The cost of the solar generated electric power could be as low as ?0.275/kWh,which is much lower than the current parabolic trough SOPG,and the parabolic trough SAPG system,which is much lower than the existing parabolic trough solar-only power generation and solar aided coal-fired power generation,meaning the economic benefit of the system is remarkable.The research result of this paper can provide an improved concept with excellent thermal performance and economical technology to further advance the solar energy conversion and utilisation in solar-hybrid coal-fired power generation.