On the Efficiency, Exergy Costs and CO2 Emission Cost Allocation for an Integrated Syngas and Ammonia Production Plant

This paper presents an exergy and environmental assessment of a 1000 metric t/day ammonia production plant based on the steam methane reforming (SMR) process, including the syngas production, purification (CO2 capture) and compression units, as well as the ammonia synthesis and purge gas treatment. An integrated heat recovery system produces power and steam at three pressure levels, besides exporting hot water, CO2 and fuel gas, with no additional heat or power consumption being required. Two configurations for ammonia refrigeration process (-20ºC) are compared in terms of power consumption. Exergy cost data for upstream processing stages of natural gas is used to calculate the extended exergy cost of the products of the plant, namely ammonia, CO2 and fuel gas. Moreover, an appropriated methodology is employed to properly allocate the renewable and non-renewable exergy costs, as well as the CO2 emissions of the reforming, shift and furnace stack among the products of the plant. By considering that the cost reduction of the combustion gases is a linear function of the exergy flow rate reduction in each component of the heat recovery system, an improved allocation of the CO2 emission cost along the convection train is performed. A breakdown of the total exergy destruction rate of the plant (136.5MW) shows that about 59% corresponds to the reforming process followed far behind by the ammonia synthesis and condensation (18.3%) and the gas purification units (13.2%). The overall exergy efficiency of the ammonia plant is calculated as 66.36%, which is enhanced by recovering the hydrogen-rich and fuel gases in the purge gas treatment process. The total and non-renewable exergy costs and CO2 emission cost of the ammonia produced are calculated as 1.7950 kJ/kJ and 0.0881 kgCO2/MJ, respectively. In addition, a rational exergy cost of 1.6370 kJ/kJ and CO2 emission cost of 0.0821 kgCO2/MJ are allocated to the CO2 gas, which can be supplied as feedstock to an associated chemical plant (urea, methanol, polymers, etc.).