Identification of an Environmentally friendly Symbiotic Process for the Reuse of Industrial Byproduct – an LCA Perspective

Abstract

There are various technologies that have been developed to reduce nitrogen oxide (NOx) gas emissions from nitric acid plants. The current study, however, focused on chemical absorption with the following key objectives. First, the paper assessed several reaction pathways that entail the use of combination of common industrial absorbents and oxidants for the absorption of NOx. Second, an analysis on the production of potassium nitrate (KNO3) fertilizer using the byproducts formed from chemical absorption was studied. In addition, a life cycle assessment (LCA) on the environmental implications of NOx absorption and byproducts utilisation for the production of fertiliser through industrial symbiosis was evaluated. This was performed to find the combination of absorbent and oxidant with the least environmental impacts for industrial symbiotic formation. The goal of the LCA was set as the production of 1 kg of potassium nitrate fertiliser. The analysis has shown that use of hydrogen peroxide in combination with calcium hydroxide had the least environmental impacts per kg KNO3 produced. These were a global warming potential (GWP) of 9 kg CO2 equivalent emissions (CO2-e), acidification potential (AP) of -0.073 kg sulphur dioxide equivalent (kg SO2 -e), eutrophication potential (EP) of -0.058 kg phosphate equivalent (kg PO4-- e), 0.1 kg of solid waste generated, and 15.7 Litres of water used. The most striking finding was that this combination reduced the overall GWP, AP and EP per kg KNO3 produced by 7.8 kg of CO2-e, 0.12 kg SO2 -e and 0.08 kg PO4-- e respectively in comparison to the production of conventional KNO3 fertilizer. The hotspots for all reaction pathways were water and energy use. Water use could not be reduced due to effects on reaction efficiencies for all pathways. However, a solution to reduce the carbon footprint from energy use is the substitution of energy generated from fossils with wind energy. This was evaluated to have the potential of reducing GHG emissions of ozone use by up to 93%, leading to overall GWP reduction of about 86% and 80% for calcium hydroxide and sodium hydroxide use with ozone respectively.