Basic Study On Thermal Extraction Of Coal And Its Products For The Preparation And Application Of Hot-pressed Coal Briquette

Coal is the dominant energy source in China.A large amount of pulverized coal is inevitably produced in the process of coal mining,transportation,and utilization.However,the utilization efficiency of pulverized coal is relatively low,and if not handled properly,it will form dust and pollute the air.The coal briquette technology is an important way to improve the utilization efficiency of pulverized coal,optimize the performance of raw coal,and reduce environmental pollution.As an important additive in coal briquette technology,the binder helps to form pulverized coal and improve its mechanical strength.However,traditional binders,such as biomass,bentonite,starch,asphalt,molasses,high molecular polymers,etc.,are difficult to be used as metallurgical raw materials due to high ash content,low mechanical strength,or poor thermal stability.Solvent thermal extraction is an important way to increase the added value of raw coal.It can not only obtain the extract with extremely low ash but also enable the extract to have some special properties,such as high calorific value,high reactivity,good thermoplastic properties,etc.The extract is a potential binder for the preparation of hot-pressed coal briquette(HPCB)due to its good thermoplastic properties,low viscosity,and a strong affinity for coal.HPCB is expected to have high mechanical strength and good thermal stability,and it will have huge application potential in the metallurgical field.First,the thermal extraction method was applied to extract different kinds of bituminous coals in a high-pressure reactor at 350℃ using N-methylpyrrolidone as solvent in this paper.The results show that the thermal extraction method had a high extraction yield for the bituminous coal with a high hydrogen-carbon ratio,and the highest extraction yield can reach 60-70wt%.The ash content was mainly transferred to the residue,and the ash content in the extract was reduced to less than 1wt%.Compared with the raw coal,and the volatile matter in the extract was increased by more than 8wt%.Swelling and pyrolysis had important effects on the thermal extraction process.The extract contained a large number of light components,while the residue contained a large number of heavy components.The carbon elements mainly existed in the form of C-C/C-H,followed by C-O-H/C-O,and C=O;oxygen elements mainly existed in the form of C-O,C=O,and O=C-O;nitrogen elements mainly existed in the form of pyridinic-N,pyrrolic-N,and quaternary-N;sulfur elements mainly existed in the form of thioether,thiophene,and sulfoxide/sulfone.The thermal extraction process had a strong ability to extract hydrocarbons,but a weak ability to extract compounds such as alcohols,phenols,ethers and carboxylic acids.To expand the source of binder and make use of energetic solid waste and hazardous waste,the thermal extraction method was applied to extract the coal direct liquefaction residue(DCLR)due to its high content of asphaltenes.Proximate analysis and elemental analysis show that the sulfur and ash were transferred to the residue after the thermal extraction,The ash content was decreased from 11.72%to 0.72%,and the sulfur content was decreased from 1.72%to 0.09%.The extraction yield reached about 70.80%,showing an excellent extraction effect.GCMS shows that 60.6%of the volatile substances in the extract were PAHs,while the residue had only 6.8%of PAHS.It is proven that the residue has been rendered harmless to humans and the environment.Since the residue has a high content of elements such as Fe,Ca,Si,Al,etc.,the two-stage sintering process was applied to prepare the ceramsite using DCLR extraction residue,quartz sand,and clay as raw materials.When the ratio of DCLR extraction residue:quartz sand:clay was 2:1:7,preheat at 450℃ for 30 min and sinter at 1100℃ for 30 min,the 1-hour water absorption rate of ceramsite was 10.5%,and the compressive strength reached 6.89 MPa.The extract was rich in hydrocarbons and had excellent thermoplastic properties,its bond index exceeded 100,and good fluidity was observed during the thermoplastic temperature range.Therefore,the hot-pressed coal briquette(HPCB)was prepared using the extract as a binder and the pulverized coal as a raw material.The effects of extract ratio and type,holding period,hot-pressing temperature,and coal type on the compressive strength of HPCBs were investigated.The results show that DCLR extract was more suitable for use as a binder compared with bituminous coal extract,and the binding effect of DCLR extract was improved by about 50%compared with DCLR.Bituminous coals with weak caking properties and anthracite powder were ideal raw materials for preparing HPCBs.The optimal conditions for the preparation of HPCBs were as follows:The compressive strength of HPCB can reach 2.5 MPa when the DCLR extract ratio was 15wt%,the holding period was 30 min,and the hot-pressing temperature was 350℃.In addition,the combustion characteristics and gasification characteristics of HPCBs were studied by using a simultaneous thermal analyzer.The results show that the flammability index C and the comprehensive combustion index S show a decreasing trend with the increase of the coalification degree.The combustion reaction of HPCBs was the first-order reaction,and the apparent activation energies were 55-75 kJ·mol-1.The gasification reaction of HPCBs made from bituminous coals was the 1st-order,while the gasification reaction of HPCBs made from anthracite was the 2st-order.With the increase of the coalification degree,the apparent activation energies of the gasification reaction show an increasing trend.The carburizing characteristics of raw coal,extract and HPCB were observed by a high-temperature laser scanning confocal microscopy.The results show that the carburizing ability of the extract was the strongest,followed by the raw coal,and HPCB.In addition,the thermal stability of HPCBs was investigated by simulating the rapid pyrolysis and gasification reactions when they were fed into COREX melter-gasifier by thermal experiments.The results show that the anti-destruction ability index TMS+8 of semi-coke was higher than 90%,the pulverization rate index TMS-2 was lower than 8%,and the CO2 reactivity index RI was less than 15%,and the post-reaction strength index RSI was greater than 90%.As a result,HPCBs prepared in this study fully meet the requirements of COREX melter-gasifier.Based on above experimental results,a new route for the clean and efficient utilization of coal is proposed in this paper.First,the thermal extraction method is applied to extract low-rank coal and inferior coal or solid waste containing hydrocarbons,and then the extract with excellent thermoplastic properties is used as the binder to prepare HPCBs with the pulverized coal through the hot-pressing process,which can replace the lump coal in COREX melter-gasifier or fixed bed gasifier;the extraction residue with high ash content is used as the raw material to prepare the ceramsite through the sintering process,which can produce construction materials.This route can not only increase the use-value of low-rank and inferior coal,improve the utilization rate of pulverized coal,but also reduce the excessive dependence on coking coal resources and lump coal resources.It is of great significance to the clean and efficient utilization of the coal industry,and the energy conservation and emission reduction of the iron and steel industry.