Integrated production of urea and nitric acid in the pulp industry: techno-economic and environmental analysis of black liquor gasification-based processes

The pursuit of alternative energy sources for the nitrogen fertilizers sector has recently earned renewed interest due to increasing concerns regarding the world dependence on non-renewable energy resources and also motivated by the more stringent environmental regulations. The decarbonization of this sector, responsible for 2% of the global energy consumption, might help not only improving its carbon footprint, but also reducing its dependence on international market prices of natural gas. In order to palliate the environmental impacts that nitrogen fertilizers production is responsible for, several efforts have been addressed to incentivize the partial or total decarbonization of the ammonia supply chain. The thermochemical conversion routes of biomass can play an important role in this decarbonization process and also is an interesting opportunity to capitalize on the underexploited biomass potential in tropical countries. Thus, in this work, two production plants, nitric acid and urea, are integrated to a kraft pulp mill via black liquor gasification. The proposed systems are evaluated in light of thermodynamic and environmental aspects. The choice of the utility system has been driven by the interrelation of the market conditions, as well as the extent of the energy integration of the chemical plants. As a result, the exergy efficiency of the integrated plants remains competitive against the conventional kraft pulp mill thanks on the optimal selection of the operating conditions of the utility system. Furthermore, the results for the overall CO2 emissions balance achieved negative values for some scenarios, pointing out the potential for CO2 depletion with the integrated processes proposed. Finally, the indirect emissions of the biomass supply chain demonstrated to have a great impact in the CO2 balance.