Influence Of CO₂on Char-steam Gasification Reaction Mechanism

For coping the increasing demands of natural gas, the technique, coal tosynthetic natural gas (SNG) has the prioriy development position as a newgeneration coal chemical project in China, because of its main advantages,high efficency of energy conversion, low consumption of energy and steam perunit product, convenient transportation and so on. Because of the relative highCH4concentration in crude gas, mature technology and low investment,pressurized fixed bed dry bottom gasifier (PFBDB) become the mostcompetitive coal gasification technology for the production of SNG. In orderto ensure the coal ash discharge in solid form, excessive steam is used tocontrol the gasifier work under the temperature of coal ash fusion temperature,resulting in a low steam decomposition rate and large amounts of phenoliccontained waste water. Moreover, the high concentration and high pressureCO2gas from the CO2removal process is easy to use, and direct vent cancause C resources waste and greenhouse gas emissions. Taking CO2asco-gasifying agent provides a proposal to solve the current issue.However, to the PFBDB process which has a particular clear boundarybetween pyrolysis and gasification stages, the introduced CO2will inevitablyinfluence the gas evolution behaviour and the formation of char structure which will change the coal gasification reaction mechanism. So, it is necessaryto clearly understand the influence of CO2on gas evolution and char structurein order to investigate the role that CO2play during char-CO2/H2O gasificationprocess. To find out the reaction path of coal char gasification with mixed H2Oand CO2is the key procedure to realize the co-gasification technology.Meanwhile, how does CO2influence the amount of CH4in gas and emissionamount of CO2will also be answered in this study.Therefore,2different rank coals, which have remarkable difference inmacerals, XM (Ximeng lignite from Inner Mongolia) and YN (Yiningbituminous coal from Xinjiang province) are used as raw materials in thiswork. TGA and a self-made pressurized fixed bed reactor are used as the mainexperimental apparatus to perform the pyrolysis and gasification experiments.GC, XRD, FT-IR, Raman, and other characterization methods are also adoptedto help analyzing the influence of CO2on char-steam gasification mechanism.Aspen Plus chemical process simulation software is used to appraisal the effectof CO2introducing on the PFBDB operation parameters at the opreationpressure of4.0MPa. The main research conclusions are as follows:(1) The influence of CO2as pyrolysis atmosphere on the gas evolution isinvestigated, the results show that: CO2can promote the removal of water andabsorbing gases during the drying and degasing processes and inhibit thevolatile release during the active decomposition process. Within0.5MPa, CO2 can promote the release of CH4in advance. But above0.5MPa, the initialevolution temperature of CH4shifts to high temperature range. The initialevolution temperature of CH4in CO2atmosphere is slow because of the higheradsorption ability of CO2in coal char. Below700°C, compared with inertatmosphere, CO2atmosphere leads to higher char yields by inhibiting therelease of volatile. After700°C, compared with inert atmosphere, CO2atmosphere leads to lower char yields due to gasification reaction with coalchar, and the CO content is obviously increased.(2) The influence of CO2as pyrolysis atmosphere on char structureformation is investigated, the results show that: Chars prepared under differentatmosphere have different chemical and physical structures. The charsprepared under CO2atmosphere (CO2chars) at700oC have more oxygen-containing functional groups, more content of olefin, cycloalkane and otheraliphatic structures. As the pressure increases, the influence of CO2atmosphereon the content of mentioned structures become more evident. Compared withchars from inert atmosphere (inert chars), the CO2chars exhibited a largeadsorption capacity and BET surface area because of the plenty of microporous structure. The BET surface and total pore volume of CO2charsdecreased monotonically with increasing pyrolysis pressure, while the BETsurface area changes are not sensitive to the change in the pyrolysis pressurefor the inert chars. (3) The relationship between the char structure and the gasificationreactivity is discussed, the results show that: At ambient pressure, inertatmosphere derived XM chars show a slightly enhanced reactivity over CO2derived chars. Through the Raman spectra research, it can be found that thevitrinite-rich XM chars prepared under CO2atmosphere have less3-5ringaromatic structure compared with the chars prepared under inert atmosphere,which indicates that the poly-condensation degree of CO2chars are higher thanthat of inert chars and the chemical structure of CO2chars is more stable. Forfusinite-rich YN coal pyrolysis within700°C, the CO2atmosphere derivedchars are richer in3-5ring aromatic structure compared with the inertatmosphere derived chars. At700°C, the amorphous structures with smallaromatic ring systems (3-5rings) in CO2chars are preferentially consumedthrough CO2gasification and larger ring system was then formed. Large andcondensed aromatic ring system is less reactive to gasification reaction and itis difficult to retain the catalytic species such as AAEM (alkali and alkali-earthmetals) in microspore structures.(4) The influence of CO2as gasifying agent on H2O-char gasification isstudied, the results show that: Although the reaction rates of both XM and YNcoal chars gasification with pure H2O are all2times higher than that of CO2gasification, an obvious synergistic effect between CO2and H2O gasification isobserved when CO2/H2O is used as gasifying agent. The actual reaction rate of co-gasification is higher than the theoretic value; the reaction rate of coal charwith mixed CO2/H2O is not weakened by the introduced CO2. The synergisticeffect is closely related to the mineral catalysis in coal, this study has revealedthe main mineral species that lead to the synergistic effect. When simulatingthe coal char-CO2/H2O reaction mechanism by shrinking un-reacted coremodel in the temperature of800-1100°C, it can be found that in differentgasification temperature range, the reaction pathway and mechanism ofdifferent char gasification with CO2/H2O are different.(5) The influence of CO2as gasifying agent on PFBDB process iscalculated. Aspen Plus simulation in the gasification temperature range of800°C and1100°C indicates that: With increasing CO2concentration in thegasifying agents, the H2concentration in the crude gas decreases, and theintroduced CO2only has small influence on CH4content in the crude gas.When partly replace steam using CO2as the gasification agent, a large amountsof steam can be saved. At1100°C, when the substitute amounts of CO2increases from0%to60%, the H2/CO ratio can be reduced from2.1to0.7,which brings convenient to flexibly adjust the H2/CO ratio for differentdownstream synthesis。...