| Exploitation and application of new energy sources have become the global focus of attention because of the increasingly serious energy shortages and environmental problems.Landfill gas is a kind of biomass energy,which is mainly composed of CH4 and CO2.Upgrading CH4 from landfill gas to utilize can not only effectively alleviate the energy crisis but also control the greenhouse effect.There are many methods to separate CO2/CH4 gas mixtures.Adsorption has been widely used because it has some advantages such as small interference with impurities,convenient regeneration of adsorbents and low operation cost.In this work,carbonaceous adsorbents were chosen as starting materials and acids and metal ions were used to modify the raw material.N2 adsorption/desorption isotherms,FT-IR and SEM were adopted to describe the physical and chemical properties of the raw and the modified samples.Adsorption behaviours of CO2 and CH4 on the samples were measured and the adsorption mechanism was analyzed.The CO2/CH4 gas mixtures of simulated landfill gas were treated by pressure swing adsorption and the regeneration performance of the adsorbent was tested.The main conclusions obtained in this study are as follows:?Adsorption and separation of CO2 and CH4 on eight kinds of carbonaceous adsorbents were compared,finding that the results on the coconut shell activated carbon were the best which was determined as the raw material.Among the Langmuir,Sips and Toth models,the Sips model could fit the adsorption isotherms well.Thus,the Sips-IAST model was adopted to analyze the adsorption behavior of CO2/CH4 gas mixtures on the adsorbent.It was found that the adsorption process was competitive,during which the adsorption of CO2 was dominant.At the same condition,the selectivity factor of CO2/CH4 mixtures on the coconut shell activated carbon first decreased and then increased with the elevating pressure and decreased with the rising temperature.The adsorbed amounts of both CO2 and CH4 were closely related to the specific surface area and the micropore volume of the adsorbents and also correlated with the total pore volume.? Several different acid solutions were utilized to modify the coconut shell activated carbon and the physical and chemical properties of the modified samples were measured.Meanwhile,the adsorption and separation effects of CO2 and CH4 on different samples were compared.It was found that the sample treated by organic acids showed better performance than that by mineral acids and the sample modified by acetic acid was the best.After that,the adsorption capacity and gas separation effect of the activated carbons modified by different concentration of acetic acid solutions and that under different temperature and time were analyzed.Then the optimal modification parameters were obtained,that was 15%.vol acetic acid solution with temperature of 25 ? and time of 24 h.The saturation adsorbed amount of CO2 on the best sample calculated by the Sips model was 12.88 mmol/L,increasing about 29.58%compared with the raw material.And the separation factor of gas mixtures with different volume ratio of 60/40,50/50 and 40/60 on the sample analyzed by the Sips-IAST model were 5.04,4.92 and 4.79,respectively.?Different metal ion solutions were utilized to modify the coconut shell activated carbon.The results showed that the overall improvement degree of CO2/CH4 mixture separation effect on the modified samples by different kinds of metal ions were as follows:alkali metal>alkaline earth>heavy metal and the sample treated by potassium nitrate was the best.Then,the adsorption capacity and the gas separation effect of the samples modified by different concentration of potassium ion,modified temperature and time were compared.The best sample was obtained,which was modified by 0.1 mol/L potassium nitrate solution,namely potassium ion to carbon ratio of 0.01 mol/10 g,at 55 ? for 12 h.The saturation adsorbed amount of CO2 at 25 ? on this sample computed by the Sips model reached up to 12.25 mmol/g,increasing about 23.24%than the raw material.Besides,compared to the raw sample the separation factor on the best sample analyzed by the Sips-IAST increased more than 40%.?The two-step modified activated carbon was first treated by acetic acid and then by potassium nitrate,the pore structure of which varied a little,the surface morphology of which combined the characteristics of both the sample modified by acetic acid and the sample treated by potassium nitrate,and the functional groups and physical phase structure of which change little compared with the raw sample.The adsorption and separation effects of CO2 and CH4 on the raw material and the modified adsorbents were analyzed.The results showed that CO2 saturation adsorbed amount on the two-step modified sample calculated by the Sips model was the maximum,with its value of 13.36 mmol/g increasing about 34.41%compared with the raw one.The adsorption process of CO2/CH4 gas mixtures on the modified activated carbons were also competitive with the adsorption of CO2 dominant.The increasement of adsorption capacity of CO2 on the modified sample was larger than that of CH4.?The amount of the surface oxygen-functional groups of the activated carbon modified by acetic acid and the adsorption capacity of CO2 and CH4 on the sample were measured.On the basis that the adsorption was caused by both physical structure and chemical properties of the adsorbent,the total adsorbed amount on the samples before and after the modification by acetic acid was divide into two sections,which was the adsorbed amount within the pores and the amount onto the oxygen-containing groups.Then the contribution of physical structure and surface oxygen-functional groups to the total adsorption capacity were studied separately.And a semi-empirical relationship between the variation of the surface oxygen-functional groups and the variation of the adsorbed amount was established.Then it came out that there was a significant correlation between the saturation adsorption capacity of CO2 and the amount of oxygen-containing functional groups,especially carboxyl and hydroxyl.Besides,the adsorption of CO2 caused by oxygen-containing functional groups was exothermic and physisorption.?Adsorption thermodynamics were analyzed and the results were as follows.The adsorption of CO2 and CH4 on the activated carbons were physical and spontaneous with the adsorption spontaneity degree of CO2 higher.The adsorption system tended to be ordered.The standard enthalpy change of the modified activated carbons became larger,indicating that the interaction between the adsorbate and the adsorbent increased.The decreasing isosteric adsorption heats with the increasing adsorbed amount suggested the high degree of heterogeneity of the adsorbent surface.The absolute values of Gibbs free energy and entropy change respectively decreased and increased with the adsorbed amount,implying that the spontaneity degree of adsorption became smaller and the adsorption tended to reach equilibrium.Adsorption kinetics were also analyzed and the conclusions were listed as follows.The adsorption equilibrium time shortened with the increasing temperature.The gas adsorption rate became quicker as the temperature rose and the diffusion rate of CH4 was faster than that of CO2.The gas diffused faster in the modified samples.The adsorption breakthrough curves showed that CH4 penetrated through the adsorption column first and the breakthrough time of CO2 in raw activated carbon was the shortest and that in two-step modified sample was the longest.When at the same condition,the separation factor on the sample modified by acetic acid,potassium nitrate and two-step method were 3.62%?22.85%and 21.27%larger than that on the raw sample,respectively.By analyzing and contrasting thermogravimetric results and infrared spectrograms of the samples before and after the modification,it can be known that the adsorption processes of CO2 on the raw and the modified activated carbons were physisorption,which was consistent with the conclusions obtained by the adsorption thermodynamics.?The adsorption and separation effects of CO2/CH4 gas mixtures(40/60%.vol)in typical landfill gas on two-step modified activated carbon were measured by pressure swing adsorption technology.The results showed that as the inlet flow increased,the breakthrough time became shorter and the adsorbed amount decreased.As the adsorption pressure rose,the breakthrough time delayed,the adsorbed amount increased and the separation factor first decreased and then increased.Besides,with the moisture content increasing the adsorbed amount declined and the separation factor increased slightly.Therefore,on the premise of low operation cost,it was recommended to reduce the intake flow and increase the adsorption pressure in order to enlarge the use efficiency and separation effect of the activated carbon.The use efficiency of the activated carbon could be reduced when it adsorbed some water,but the separation effect was influenced little.And the regeneration experiments showed that this modified activated carbon possessed good regenerability. |
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