适宜增压流化床操作参数提高生物质热气化气合成甲烷效率

Improving methane production efficiency from biomass product gas via pressurized fluidized bed system

生物质热化学气化制取甲烷是人工获取代用天然气的重要方式之一,其中生物质热气化气合成甲烷是该技术的关键步骤之一。在自行设计的增压流化床反应系统上,开展生物质气化气合成甲烷的试验,分别研究了反应温度、反应压力、空速和氢碳比对甲烷生成速率和CO转化率的影响。结果表明,在增压流化床反应器上可高效的合成甲烷,最大甲烷生成速率超过3.2 mol/(L·h),CO转化率超过80%。提高反应温度有利于甲烷生成速率和CO转化率的提高,且当反应温度在350℃左右时达到最大值;反应压力对甲烷化过程有很大影响,提高反应压力有利于甲烷化过程;随着空速的增大,甲烷生成速率增加,但是CO转化率会下降;而甲烷生成速率和CO转化率则随着氢碳比的增大而增大。为获得较高的甲烷生成速率和CO转化率,适宜的反应温度在350℃左右,空速在10 000 h-1,氢碳比在3附近,反应压力可取在0.3 MPa左右。该研究结果将为进一步研究生物质热化学气化制取甲烷奠定基础。

Natural gas is one of the clean primary energy sources and high-quality chemical raw materials. Technology of methane production from biomass thermo-chemical gasification （biomass-to-SNG） is one of the most important pathways to produce synthetic natural gas （SNG） to substitute diminishing natural gas. In the biomass-to-SNG process, the biomass is first converted into product gas through biomass gasification. Then, the product gas full of CO and H2is synthesized into methane through the methanation processes after some proper cleaning and conditioning processes. Finally, the crude synthetic natural gas is upgraded with CO2 removal and gas dehydration. In the whole biomass-to-SNG process, the methanation process of product gas is a key step. A pressurized fluidized bed methanation reactor system was designed and constructed, which is mainly composed of a main reactor and auxiliary equipments. An experimental study of methane production from product gas was carried out on this methantion reactor system with the commercial methanation catalyst as bed material. The Energy Dispersive Spectrometer analysis indicates that the methanation catalyst contains high nickel content and was squashed into small particles for the study. Then, the effects of methanation temperature, pressure, space velocity, and ratio of H2 to CO on the performance indexes （i.e. methane formation rate and CO conversion rate） were investigated. The results show that methane is efficiently produced on this pressurized fluidized bed methanation reactor system and the typical methane formation rate is higher than 3.2 mol/（L＆#183;h） while the CO conversion rate is more than 80%. Higher methanation temperature is favored to the methanation process and the methane formation rate and CO conversion rate achieve the maximum values at the methanation temperature about 350℃. However, when the methanation temperature is higher than 350℃, the methane formation rate and CO conversion rate decline slowly since the methanation reactions are exothermic reactions and high temperatures are thus unfavorable to the methanation reactions and may also cause the catalyst to deactivate because of carbon deposition and sintering of catalyst. The methanation process is also benefited from higher methanation pressure since the methanation reactions are volume-contraction reactions. The methane formation rate and CO conversion rate increase with the rise of methanation pressure, especially when the methanation pressure is higher than 0.3 MPa. The methanation process is heavily affected by the space velocity, too. With the increase of space velocity, the methane formation rate increases while the CO conversion rate declines accordingly. The ratio of H2 to CO is another key influencing factor in the methanation process. With the rise of the ratio of H2 to CO, the methane formation rate increases accordingly, while the CO conversion rate rises and reaches the highest values when the ratio is about 3 and then the CO conversion rate maintains a high value. In short, to achieve higher methane formation rate and CO conversion rate, suitable methanation temperature is about 350℃, space velocity is around 10 000 h-1, the ratio of H2 to CO is around 3, while the methanation pressure is 0.3 MPa, taking the biomass utilization scale and investment costs into account. These results may lay a solid foundation for further studies of biomass-to-SNG process.