Research On Slurryability,Combustion And Gasification Properties Of Coal-oilfield Wastewater-slurry

Oilfield wastewater(OWW)is a kind of organic industrial wastewater with complex composition,difficult treatment and calorific value.Besides oil,it also contains lots of metal cations,suspended matter and organic material,and it is easy to cause water pollution and ecosystem destruction if untreated.In the past,most of the treatment methods for OWW were relatively high and the effect was not significant.Therefore,a new technology for the utilization of OWW with simple process and low cost was needed.Coal-water-slurry is a kind of relatively mature oil substitute fuel and gasification material,which has the characteristics of high combustion efficiency,small pollution discharge and good adaptability to pulping water.Therefore,this paper has used OWW to prepare coal-water-slurry,that is,coal-oilfield wastewater-slurry(COWS),not only reduced the wastewater discharge,but also saved clean water,and achieved the purpose of making use of difficult treatment of organic industrial wastewater.In this paper,the slurryability,combustion and gasification properties of COWS were studied in the laboratory,and the combustion experiment was carried out on a horizontal furnace in pilot-scale.And the paper hopes to show industrial use value of the new slurry fuel.In order to study the slurryability of COWS,three coal samples of different coal ranks,Ximeng Lignite,Xukuang Coal and Huangling Coal were used to prepare COWS and ordinary coal-water-slurry(CWS),and concentration and shear rate were measured.The concentration with fixed viscosity(SCmax)and viscosity-concentration curves of the two slurries were compared and analyzed.The results showed SCmax of COWS was slightly higher than CWS,and the chemical composition of OWW is not enough to maintain qualified slurryability,therefore it is still necessary to add additives,and selecting NDF could reach the best slurryability.The effects of coal moisture content,coal oxygen content and coal rank on the slurryability were studied.It was found that the higher the moisture content,the higher the oxygen content,the lower the coal rank,CWS would performance worse slurryability.The contact angles of Ximeng Lignite with OWW and deionized water were the smallest in the three coal samples,and the OWW would reduce the contact angle and improve wettability.The rheological properties of COWS and CWS were compared and it was found that OWW would not change the rheological properties,but also improved the pseudoplastic behavior of CWS,which would be beneficial to storage,transport and atomization.The stability of COWS and ordinary CWS was studied by water separation method.The result showed that the water separation rate of COWS was lower and the Zeta potential value of coal powders in OWW was larger,which indicated that the OWW could improve the stability of the slurry.The combustion characteristics of COWS and CWS were studied by thermogravimetric method.The combustion characteristic indexes were determined according to the experimental results,and the reaction kinetic parameters were obtained by the reaction series model(n = 3).The results showed that the combustion thermogravimetric curve(TG curve)of COWS and ordinary CWS would undergo water loss stage,mass stabilization stage,ignition and combustion stage as well as the burn-out stage.The combustion differential thermogravimetric curve(DTG curve)have three weight loss peaks,respectively representing the rapid water loss stage,volatile combustion stage and fixed carbon combustion stage,and the second and third peaks were partially overlapped.The ignition temperature,burnout temperature and combustion activated energy of COWS were all lower than those of CWS,as well as other combustion characteristic indexes,which indicated that the OWW could promote combustion characteristics by reducing ignition temperature and apparent activated energy.The gasification characteristics of COWS and ordinary CWS were studied by thermogravimetric method as well.A series of gasification characteristic indexes were determined by TG-DTG method.The results showed that the COWS and CWS would undergo rapid water loss stage,mass stabilization stage,volatile phase stage,fixed carbon reaction stage and gasification end stage.The DTG curves had three obviously different phases,respectively representing the rapid water loss stage,volatile phase stage and fixed carbon reaction stage.The initial temperature and the end temperature of gasification reaction of COWS were all lower than that of CWS,as well as other gasification characteristic indexes,which indicated that the OWW could also promote the gasification of coal-water-slurry.The effect of OWW on combustion and gasification was investigated by scanning electron microscopy in microcosmic aspect.It was found that COWS had considerable concentrations of Na and K ions,whereas CWS had very low concentrations of Na and K ions.The results showed that the promoting effects of OWW on the gasification of coal slurry were mainly due to alkaline ions.Na ions were distributed evenly on the surfaces of coal particles.By contrast,more K ions were distributed in the pores and voids than on the surfaces of coal particles;some clusters of K ions even appeared.The widely distributed Na and K ions provided active sites for combustion and gasification,and enhanced the combustion and gasification activity.The COWS and CWS were prepared by Xu-3 and Xu-6 respectively.The pilot-scale pulping equipment was used for dry pulping.The concentrations of COWS and CWS were 57%?61%,and the apparent viscosity was 750?1300mPa.s,in line with the requirements of industrial applications.Combustion experiments of COWS and CWS were operated in the pilot-scale horizontal furnace,and the combustion,burnout and slagging characteristics were studied.The results showed that all slurry samples in the furnace burned steadily at full load,the temperature of the main combustion zone in the furnace reached above 1300?,and the temperature of the main combustion zone was also above 1100? at down load.All slurries burned full,had high burnout rate,fly ash carbon content of 0.5%,and the combustion efficiency reached 99.6%.The results showed that slagging in the furnace was medium-to-heavy degree,and there was local melting phenomenon in the furnace especially at high temperature areas,and slagging are improved obviously at low temperature areas.The combustion temperature in the furnace was an important factor to affect the slagging for the same kind of coal.The calculation of slagging rate showed that the addition of OWW would slightly increase the slagging of coal-water-slurry.The pilot-scale experimental results showed that the application of COWS in boiler was feasible,and it also provided a reference value for the industrial application of OWW to prepare slurry fuel.