The partial oxidation process (POX) produces a more stoichiometrically optimum synthesis gas that is fed to the methanol reactor. In this process, oxygen reacts with methane to produce two moles of hydrogen per mole of CO. The POX reactor is exothermic and does not require combustion with air. Therefore, NOx emissions from this process are negligible. Combining a POX plant with a steam reformer is a particularly advantageous process since the exothermic heat from the POX unit can be used as energy for the steam reformer. When available, adding CO2 can enhance the efficiency of methanol production.
Energy consumption data for steam reforming and POX were obtained from several studies. Natural gas reformers are fueled with process gas left over from the methanol synthesis. This gas is primarily composed of hydrogen with CO, methane, CO2, and methanol. Emission factors for natural gas were used to estimate NOx, CO, methane, and NMOG emissions. CO2 emissions are determined from a carbon balance. The difference between carbon in the natural gas feed and carbon in the natural gas product is carbon in the form of CO, hydrocarbon, or CO2 emissions. Over 99 percent of this carbon is emitted as CO2. POX process produces NOx emissions since combustion with air does not occur. A small amount of pollutants are emitted from flaring purge gas.
Methanol plants can be either importers or exporters of electricity. Power generation emissions associated with net electric power were included with the fuel production emissions. Electricity demand for the POX process includes required energy for an oxygen plant.
The energy input for methanol production depends largely upon the production technology. In some cases, waste CO2 (perhaps from an oil field), can be added to the feed stream to generate a CO/H2 mixture that has a higher methanol yield.
The energy input for methanol production only affects global CO2 emissions. The technology for methanol production facilities does not affect emissions in CA. Total CO2 and hydrocarbon emissions are presented in the Methanex annual report. These emissions combined with the amount of methanol produced could provide a comparison to other estimates of methanol production emissions.
4.6 Methanol Production from Landfill Gas
Methanol can be produced from landfill gas through a steam reforming or partial oxidation process similar to the synthesis from natural gas feedstocks. Emission estimates for a landfill based methanol production facility are shown in Table 4-20.