| China has abundant reserves of low-rank coal,which is characterized by high water content,high volatiles.Direct combustion of low-rank coal wastes a large number of highly active hydrogen-rich components and pollutes the environment.Coal pyrolysis-based staged conversion poly-generation technology can extract high-value added components from coal,which is one of the important technologies to realize clean and efficient utilization of coal.In order to solve the problems of large proportion of crushed coal in mechanized mining of low-rank coal,low yield and high dust content of tar in existing pyrolysis process,Zhejiang University has proposed a pyrolysis-based staged conversion technology for low-rank coal in dual fluidized beds using hot char as heat carrier.By the organic combination of fluidized bed coal pyrolyzer and circulating fluidized bed char heating furnace,large-scale pyrolysis staged conversion of crushed coal particles can be realized to obtain high yield tar and high-quality coal gas.Coal staged conversion products have wide-range in application.Tar can be made into light liquid fuel through hydrogenation process,which helps to alleviate the energy risk of China's increasing dependence on foreign petroleum.Char can be used in the mixed combustion of large-capacity pulverized coal boilers or in the production of briquette for heating.It can also be used to produce syngas through the gasification technology of water-char slurry,realizing the high efficiency and low pollution utilization of char.High quality coal gas can be used to produce synthetic natural gas(SNG)or as raw material for chemical synthesis,such as methanol,dimethyl ether and so on.Through the flexible combination of different utilization schemes,the cascade utilization of low rank coal resources is realized.At present,the awareness of the influence of char heat carrier on the fluidized bed pyrolysis product distribution is not sufficient,at the same time the process simulation and techno-economic analysis of the coal staged conversion poly-generation system in dual fluidized bed using char as hot carrier is short of advance and reliability.Relying on the National Key R&D Program of China,experimental study and process simulation and techno-economic analysis of the coal staged conversion poly-generation system in dual fluidized bed with char heat carrier was carried out in this thesis,aimed to provide reliable reference and data to the large-scale industrial application of the coal staged conversion poly-generation technology.First of all,with the typical low-rank coal-Xinjiang Runbei coal as raw material,pyrolysis experiments under different pyrolysis temperatures(500-800oC),different heat carrier condition(quartz sand,heat carrier to raw coal ratio=1:1,2:1,3:1 and 5:1)as well as the coal gas atmosphere was carried out in a bubbling fluidized bed reactor.The influence of temperature,heat carrier types and ratio of heat carrier to coal,and the atmosphere on the characteristics and distribution of the fluidized bed coal pyrolysis products was obtained.Results showed that using char as heat carrier in fluidized bed pyrolysis process could affect the heating process of coal particles,promote the initial reaction of pyrolysis and facilitate the release of phenols,cracking of carboxyl groups and decomposition of heavy components in tar,thus improve the tar and gas production rate,reduce the char yield.The promoting effect of hot char heat carrier was more significant under low temperature,while high-temperature cracking effect played a major role in higher temperature.The char under char heat carrier condition had more developed pore structure,but its combustion performance was worse.With the increase of the ratio of char heat carrier to coal,the gas yield increased gradually,while the tar production gave the peak under the ratio of2:1,and the yield of char decreased gradually.Under the condition of char heat carrier,the yield increase of CH4 comes from the cracking of aliphatic structure in coal,while the increase of H2and CO mainly comes from the decomposition of phenols and the condensation of macromolecules in tar.It was also found that char heat carrier had a certain sulfur fixation effect.When the ratio increases from 1:1 to 5:1,the content of heavy components in tar decreased by about 6 percentage points,the quality of tar was improved,and phenols in tar were decomposed into aromatic hydrocarbons.Under the atmosphere of pyrolysis gas,the catalytic effect of char heat carrier on CH4 and CO2 reforming and the promotion effect of H2 on pyrolysis made the yield of char lower than that of inert atmosphere,while the yield of tar and water was relatively higher.Secondly,pilot-scale experiments of two kinds of coal samples were carried out at different temperatures(580oC,630oC and 680oC)in a 1MWt dual fluidized bed device.It was found that both coal samples were suitable for dual fluidized bed pyrolysis technology,and the material circulation between pyrolyzer and circulating fluidized bed char heating furnace is normal and stable.The pyrolyzer operating temperature could be controlled by adjusting the material circulation between the two furnaces and the temperature of the heating furnace.The maximum tar yields of the two coal samples were 10.84%and 13.27%obtained at 630oC,respectively,which both exceeded 90%of the Gegin retorting analysis.High-quality coal gas was collected during the experiment,and the content of CH4 is abundant,with the volume proportion of about 35%-40%,and the volume share of CO and H2 is between 25%-35%.The asphaltenes accounted for40-50%of the tar components.Increasing the pyrolysis temperature could promote the cracking of asphaltenes and saturated hydrocarbons,into aromatic hydrocarbons and other non-hydrocarbon substances.The resistance of fly ash collected from the secondary cyclone of pyrolyzer meets the requirements of high temperature electrostatic dust removal operation.To obtain higher-quality oil and gas,high temperature electric dust removal can be arranged in practical industrial application.Within the temperature range of the experimental study,the quality control of pyrolysis products can be realized to some extent by adjusting the operating temperature of the two furnaces,which further verifies the feasibility of the dual fluidized bed coal pyrolysis-based staged conversion technology.Furthermore,the results from pilot-scale experiments can provide technical support and regulation experience for the design and operation of the demonstration device in the near future.Finally,simulation of dual fluidized bed coal pyrolysis-based staged conversion poly-generation system with char heat carrier coupling with a 2×660MW ultra-supercritical(USC)power plant was established using Aspen Plus.In the proposed system,char was fed into USC pulverized power plant for electricity.After phenols extraction from tar,naphtha and diesel were synthesized by heterogeneous suspended bed hydrogenation process.The H2 required for the tar hydrogenation unit was from the gas deep processing process,and the pyrolysis wastewater was sent into the phenol-ammonia recovery unit.Three different gas deep processing routes were designed according to the market condition and product characteristics.In Case A,the gas was first desulfurized and purified by the Selexol unit,and the purified gas was sent to the steam methane reforming(SMR)unit.After separation of the H2 required for tar hydrogenation by the pressure swing adsorption(PSA)unit,the remaining syngas was used for methanol synthesis.The heat required for SMR unit was provided by the exhaust gas from the methanol synthesis unit and the tar hydrogenation unit.In Case B,part of the purified coal gas was sent into the SMR unit,then mixing with the left non-reforming gas into the CO shift unit.The shifted gas was sent into the PSA unit for CO2 removal,extraction of CH4 as synthetic natural gas(SNG)and H2 for tar hydrogenation.SMR heat supply in this case was provided by combustion of tail gas from PSA unit and tar hydrogenation unit.For Case C,part of coal gas along with the tail gas from PSA unit and tar hydrogenation unit were burned to provide reforming heat.The remaining coal gas was treated successively by reforming,shifting and CO2 removal and H2 extraction.All of the H2 was product except the part for tar hydrogenation.Whole process system simulation and techno-economic analysis of the proposed three cases were carried out,and the thermodynamic and economic performance were compared with the USC power generation system with the same scale.Results showed that with the coal feed of 628t/h,all of the three poly-generation cases could realize the co-production of 10.33t/h crude phenols,24.80t/h naphtha and 31.46t/h diesel oil.Moreover,Case A co-produced 65.34t/h methanol and net power generation at 1314.48MW with exergy efficiency at 51.15%.The exergy efficiency for Case B was 51.98%with cogeneration of35699.12 Nm3/h SNG and electricity at 1445.86MW.Among them,Case C has the highest energy and exergy efficiency(56.33%and 53.99%respectively),which were 10.51%and 11.19%points higher than those of the USC power plant.The after-tax IRR of the three cases was 23.24%?21.83%and 29.52%,respectively.The difference of fixed capital investment(FCI)among the three cases was little,and the FCI of Case C(about 6.923 billion yuan)was slightly higher,which was about 2.1 billion yuan higher than that of the USC power plant.Case C had the shortest payback period,the static and dynamic payback period was 5.06 years and 5.74 years respectively,showing obvious advantages in economic benefits.From the angle of risk resistance,the main factors affecting the economy of the three poly-generation cases were the annual running time and the price of raw coal.The three cases remained financially robust during the year when operating hours were sharply reduced and coal prices rose.Among them,Case C had the least change in the IRR when the raw material price and annual running time changed,showing the strongest risk resistance.According to the simulation and calculation results,these three cases have obvious advantages in energy utilization efficiency,economy and anti-risk ability,good market prospects and flexible product scheme design. |
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