Low Rank Coal Moving Bed Dehydration, And Heat Transfer Characteristics During Low Temperature Pyrolysis Process

Due to the huge reserves and limited usage of low rank coal, it is necessary to explore a highly efficent and energy-saving approach to utilze. Dehydration and pyrolysis is a vital technical approach to use low rank coal. Substantial fundamental studies have been carried out on low rank coal dehydration and low temerature pyrolysis, which exploited various low temperature pyrolysis furnace by applying fixed and movig bed. However, there are more research reports about structural properties and operation effect with a little reports about heat-transfer character. Consequently, this paper develops a 500 mm wide low rank coal fixed bed and gas heat carrier moving bed furnaces. It also studies the dehydration, pyrolysis and partial heat-transfer character of coal on the basis of the above. The main results and conclusions are as follows:1.According to the heat transfer characteristics and distance, the temperature profile of low rank coal in coking chamber could be divided into three zones:the near (0-10 mm), middle (10-186.5 mm), and remote zones (186.5-250.0 mm). The distinguishing index between near and middle zones was a low-temperature-constant (80-100 ℃) stage. (?)T/(?)t2=0,(?)T/(?)t=n were the distinguishing indices between the middle and remote zones. When the combustion chamber temperature is 600℃, the coal in different zone of the coking chamber undergoes the different heating stages:In the near zone, there is a 3 h rapid temperature increase stage and a 3.7 h gradual temperature increase stage; In middle zone, a 1-8 hour Low Temperature Constant (LTC) stage and 30-38 hour medium-speed temperature increase stage; In remote zone, a 16.2 h LTC stage and a 29 h slow-heating-rate stage. The shorter the heat transfer distance from the coal coking furnace wall is, the larger the heating process is. The largest heating rate dT/dt is 154.0,64.5,32.0,19.5 and 20.0℃/h from S1 to S5 isothermal surface. Based on the characteristics of heating process, the start up times of coal in different zone in coking chamber are shown respectively: HM0% is more than 15 h, HM7% isless than 10 h, HM15% is less than 15 h, and HM30% is more than 15 h. With the increase of the surface moisture, the heating rate first increases and then decreases. As the heat transfer distance increased, the start up time increases, and the heating rate reduces, the heating rate of the raw coal slower than HM>0%, and the HM30% is theslowest.2.The total dehydration processof low rank coal includes the surface moisture (lower than 100℃) removal and the oxygen-containing functional groups and moisture (100 to 300℃) removal stages. As the whole, the higher the combustion chamber temperature is, the dehydration time of tSM, tFG and ttotal are shortened; With the increase of heat transfer distance and surface moisture content, ttotal,>o% increase. In the near zone, with the increase of the moisture on HM coal surface, the tSM of HM will be shortened and the removal time tSM of raw coal will be the longest, a higher dehydration load yield a shorter dehydration time. In the middle zone, with the increasing of the surface moisture and heat transfer distance Hi, the dehydration time tSM,all increase; a higher combustion chamber temperature yields a shorterdehydration time, the tSM,o% of raw coal is longer, and the tSM,7% is the shortest; in remote zone, the heat storage wall decreases the heat transfer and heating process, tSM,o% is the shortest, the tFG is longer and the ttotai,7% is shortest. A higher dehydration load yields a longer dehydration time, and tSM>o will be longer.3.The volatile components of chardecreases with the increase of the HM surface moisture, the residual moisture component of the char maintains between 1-2% and changes slightly; Fixed carbon maintain between 55-60%; Ash component increases slightly, achieving 22-24%. With the increase in the temperature, the moisture component of the char changes slightly. Thus, the Fixed Carbon and volatile component decrease slightly, and ash components increase slightly.4.With the decreases in Hi and dehydration load, the movement rate of 100℃ isothermal surface S100℃ is faster. The movement rate of the isothermal interface S100℃ between S0-S1 and S2-S3 reach 4.17 and 5.00 mm/h, the water vapor, generated by coal, make S100C,>0% mobile faster; More condensed water slows down the movement of the S100℃. The move rate of the S100℃,7%is quick and then slow, and achieve the minimum (9.09 mm/h) between S3-S4.5.The carbonization temperature of coking chamber, obtained by CFD simulation, the calculate temperature distribution followed the Chen-Clayton equation:T(t,z)= 563.85×et(?)+76.6714. The average root mean square error (RMSE) of simulation, calculated and measured temperature on S1 to S5 isothermal surface were 17.59 and 24.50. The residual analysis of simulated, calculated and measured temperature residuals satisfy 3 sigma principle; The Peason correlation coefficient of simulated and measured temperature respectively are more than 0.9632, for calculated temperature are more than 0.9549. The goodness-of-fit of Simulated and measured temperature are more than 0.9338; for calculated temperature:0.9161. The calculated and measured temperature are very low, The simulated and calculated temperatures were fitting degree and consistent with the measured temperature.6.Comprehensive coefficient of thermal conductivity, influenced by coal and char, is unimodal distribution, and the function equation is: (?),The coefficient function of thermal conductivity on different isothermal surface incoking chamber can be divided into three parts:the coal/coke hybrid thermal conductivity λhybrid stage, coal/coke comprehensive thermal conductivity λi,coal/ coke stage, and semi-coke thermal conductivity λcoke stage. The longer the Hi is, the later the peak time of λi,coal/coke appears; the longer residence time tv of devolatilization and higher final carbonization temperature are, the wider the peak width is, the peak width are as followed:λ2>λ3>λ4>λ5·λ5,closes to be constant among 0.40-0.80 W/m K, respectively. The thermal conductivity correction model, with internal heat source, is established, (?), corrected thermal conductivity Ki satisfies Ki(λuverage,Hi)=-1.77×Hi0.24-0.019×λaverage-0.895+1.717.The gas heat carrier low rank coal pyrolysis consecutive SPU device has been established, the design processing capacity is 6-9 kg/h, coal diameter 5-10.0 mm, reactor inner diameter 150 mm, superficial velocity ～0.48 Nm/S, air speed 5708 h-1, pyrolysis time 15-20 min, residence time 5-25 min, the reactor center temperature can reach 660℃. The SPU device can meet the experiment requirements.This device mainly includes four stages:formation of gas, carrier gas heating, coal pyrolysis and exhaust gas purification and treatment stage.8.Along with the pyrolysis temperature increasing from 400 to 400℃, the dehydration rate increases in the gas heatcarrier process, the dehydration time reduces, the tar yield increases, the char yield decreases, and the volatile component of char decreases. With the increase in pyrolysis time, the total water yield near to 6.37 wt%, the char yield decreases. The moisture and ash component within the scope of a small fluctuations.With the increase in final pyrolysis and residence time, the quality of char decreases. The char is performed without a binder to help in agglomerating at 460℃ 25 min for the first time, as the pyrolysis temperature and residence time increases, the agglomeration phenomenon increases.9.Along with the pyrolysis temperature increased from 400 to 400℃, the specific surface area of char, obtained by gas heat carrier, increased from 3.50 to 7.02 m/g, the average pore diameter reduced from 24.65 to 16.42 nm; with pyrolysis time increased from 5 min to 25 min at 640℃, specific surface area increases from 5.63 to 8.14 at first and then decreased to 7.01 m2/g, the average hole diameter increases from 8.02 nm to 16.42 nm.The higher the temperature is, the increase in mesopore near 10 nm.10.When the pyrolysis process withThe SPU device, the coking chamber is divided into three zones:the bottom (0-100 mm), middle (100-200 mm) and top (200-300 mm) area. Gas heat carrier process undergoes three stages:carrier gas rapid temperature-decline stage, coal temperature-rise stage and coal temperature constant stage. The turning point temperatureis a turning point between carrier gas rapid temperature-decline and coal temperature-rise stage. With the increase in the height of coal bed and decrease in carrier gas temperature,the turning point temperature decrease at first and then increase.