Exergy and environmental analysis of black liquor upgraded gasification in an integrated kraft pulp and ammonia production plant

Some byproducts of the Kraft pulp mills, such as the black liquor (1.4 t/t ADpulp, 15% solids), still contain more than half of the exergy content of the total wood fed to the digester. This residue represents a key supply of renewable energy to the pulp and paper plant, accounting for a power generation potential of about 670 TWh/y. In the customary practice, black liquor is concentrated in a multiple effect evaporator and burnt in waste heat recovery boilers in order to satisfy the combined power and steam demands. Moreover, as half of the steam generated is only used for evaporating the black liquor moisture, the efficiency of the electricity generation is relatively low. Additionally, the conventional process of concentration and combustion of the black liquor is responsible for a large amount of atmospheric emissions. Consequently, there is an interesting opportunity to improve the performance of the overall energy conversion process in the pulp and paper industry. Among the various upgrade routes for this byproduct, thermochemical conversion processes, such as improved drying approaches along with a pressurized gasification system, can be readily integrated to produce syngas from the concentrated black liquor. The produced syngas can be used for producing value-added products such as ammonia and CO2, thus improving the environmental and exergy performance of the cogeneration plant and the entire mill, at the expense of a small capital investment. Accordingly, in this work, the conventional scenario of the black liquor use is compared with other enhanced energy conversion routes in terms of the economics, exergy efficiency and environmental impact. The combined energy integration and exergy analysis is used to identify the potential improvements that may remain hidden to the energy analysis alone, namely the mitigation of the process irreversibility.