氧化催化转化器对甲醇压燃尾气中甲醛排放的影响

Effect of oxidation catalyst on formaldehyde emission from methanol compression ignition

在一台经过改装的压燃式发动机上，采用柴油／甲醇组合燃烧（DMCC）模式进行了台架实验，利用气相色谱分析技术检测了1800r·min^-1时发动机尾气中甲醛的排放特性，对比研究了负荷、甲醇掺烧比、排气温度、催化转化器对甲醛排放的影响．研究结果表明，在压燃式发动机上采用DMCC模式，低负荷运行时，尾气中甲醛含量随甲醇掺烧比的增加而增多，在甲醇掺烧比为46％时，甲醛排放量达到180×10^-6（体积分数），催化后可以减少25％～45％；在中、高负荷运行时，甲醛排放量随甲醇掺烧比的变化基本不变，高负荷比中负荷运行时甲醛排放稍高．但尾气经催化处理后，甲醛排放反而增加，最高增加1倍．催化转化器的转化效果与排气温度密切相关，当排气温度在大约300℃以下时，催化转化器可以降低甲醛排放量；当温度为300℃～410℃时，催化后尾气中的甲醛排放较催化前有所增加；当排气温度高于425℃时，催化后尾气中甲醛排放显著减少．

A diesel engine was operated in a diesel/methanol compound combustion mode（DMCC） in bench tests. Gas chromatography analysis was used to detect the formaldehyde emission from the engine while it was operating at 1800 r·min^-1. The influence of load, methanol ratio,oxidation catalytic converter and exhaust temperature on formaldehyde emission was investigated. The experimental results showed that the formaldehyde emission increased with the increasing of methanol proportion at low load,and its concentration reached 180 × 10^-6 when the methanol proportion was 46% in thermo-value in the fuel. The formaldehyde was cut down about 25% -44% when an oxidation catalyst was used. The formaldehyde emission did not changed much with the proportion of methanol in the fuel when the engine ran at the load higher than that medium ones, nevertheless, the formaldehyde emission was slightly higher at heavy load than that of medium one. However, under certain conditions, the formaldehyde emission was increased while using the oxidation catalyst. The transfer efficiency of the oxidation catalytic converter is dependent on the temperature of the exhaust. The formaldehyde emission from DMCC engine with an oxidation catalytic converter decreased at exhaust temperatures below 300℃ , however, it increased between 300℃ and 410℃. When the temperature was higher than 425℃ , the formaldehyde was greatlv reduced again.