低浓度煤层气液化技术及其应用

针对大庆庆深煤层气气源条件，提出采用液化精馏的方法处理含有大量氮氧的低浓度煤层气，生产LNG。定义了适用于低浓度煤层气液化系统的煤层气气源中甲烷摩尔分数为30%~87%；该方法中的净化单元采用MEA溶液化学吸收酸性气体，液化单元采用具有高效率的混合工质制冷剂液化流程，低温部分采用精馏塔脱除氮氧组分提高LNG中甲烷含量；运用大型数值分析软件对液化系统进行了模拟计算与比较分析，分析了混合制冷剂中CH₄、C₂H₆、C₄H₁₀及N₂等组分对液化单元换热器内部最小温差的影响。分析结果显示：合理的制冷工质配比能够有效地降低换热温差，减少不可逆损失，提高换热效率。LNG中的含氧量随着精馏塔理论塔板数的增加而降低，但精馏塔的高度相应地增加。因此，应适当增加理论板数以有效地提高LNG中甲烷的浓度。计算结果将有助于低浓度煤层气液化装置的国产化应用与推广。

Liquefaction Technology on Low-concentration CBM and Its Application

According to the characteristics of conditions in the Daqing Qingshen Gas Field, the paper presents a rectification and liquefaction method which is suitable for low concentration CBM containing plentiful nitrogen and oxygen. The effectiveness of low concentration CBM gas source in low concentration CBM liquefaction system is 30%-87% of CH4 by mole. In this method, the purification unit adopts chemical method to remove acid gas using the MEA solution. The liquefaction unit adopts high efficiency mixed refrigerant liquefaction cycle. The low temperature part adopts the rectification tower to remove the nitrogen and oxygen from LNG. The low concentration CBM liquefaction system is simulated and analyzed using large scale numerical analysis software. It analyzes the influence of mole fraction of different MR components, such as CH4, C2H6, C4H10 and N2, on the minimum temperature difference of the heat exchangers. The analysis result shows that a reasonable proportion of refrigerant is effective in lowering refrigeration temperature difference of heat exchangers to reduce the irreversible loss and to improve the efficiency of heat exchangers. With the increase of the distillation tray number, the oxygen content in LNG is reducing. However, the height of the distillation tower is increasing correspondingly. Therefore, the appropriate increase in the number of theoretical plates can be effective in raising the concentration of methane in LNG. The calculated result provides guidelines for the application and commissioning of low concentration CBM liquefaction plant in China.