Study On Removal Of Carbon Dioxide From Simulated Biogas

As a kind of renewable and green energy, Biogas can ease the global energy crisis by replacing the fossil fuels. The original biogas has a low calorific value and its utilization efficiency is limited, because it contains a lot of CO2. So the purification of biogas is of great significance to improve its utilization efficiency.In this paper, two different removal methods of carbon dioxide from simulated biogas were studied, that is pressure swing adsorption(PSA) and chemical absorption. Adsorption/desorption of CO2from simulated biogas by PSA(use5A molecular sieve as adsorbent) was studied in an adsorption column(Φ68×10×600). The effect of adsorption/desorption was investigated under conditions that adsorption pressure range of0.2～0.8MPa, space velocity range of11.1～44.2m/(m2·h), vacuum desorption pressure-0.02MPa. Removal of carbon dioxide from simulated biogas by chemical absorption with aqueous containing diethanolamine (DEA) and inorganic base was studied in a packed column(Φ35×1000). The effect of CO2absorption was investigated under conditions that concentration of absorption solution range of0.5～3.0mol/L, solution composition(total concentration2.0mol/L), space velocity0.0236～0.0393m3/(m2·s), spray density range of0.849～4.246m3/(m2-h), temperature range of10～40℃.The results of PSA showed that, with the increase of processing time, CH4content in output gas showed a rise first followed by a decline, and there existed a highest point. With the increase of adsorption pressure, the highest CH4content in output gas and CO2removal rate also show such a trend as above. The best adsorption result appeared at the pressure of0.4MPa. When the space velocity increased, the adsorption rate got higher as the highest CH4content and CO2removal rate reduced. The best adsorption effect appeared when the space velocity was22.1m3/(m2·h). The highest CH4content and CO2removal rate can reach89.3%and80.9%under the optimal condition. As the regeneration capacity of saturated5A molecular sieve was limited (regeneration rate was only about43%), it was difficult to recycle.The results of chemical absorption experiment showed that the absorption of CO2can be promoted by adding Na2CO3/K2CO3in the aqueous solution of DEA. Inorganic alkali of lower concentration had a obvious stimulative effect. The mixture contained alkali of potassium had a better absorbing effect than that contained alkali of sodium. There exists certain synergy between DEA and alkali of potassium. Higher proportion of Na2CO3contributes to the regeneration of solution. With the increase of total absorbent concentration, CO2absorption capacity would be improved, but the rise rate would be slower. Total concentration of the optimal experiment condition of mixture was2.0mol/L, while DEA concentration was1.9mol/L. The increase of space velocity showed negative effect on CO2removal rate, but showed positive effect on mass transfer coefficient and absorption rate of CO2. Larger spray density and higher temperature are beneficial to the absorption of CO2. DEA/Na2CO3absorbent had a similar effect (CO2absorption, CO2removal rate and regeneration rate can reach1.969mol/L,82.45%and81.58%) compared with MDEA/DEA absorbent under the optimal experiment condition.