Preliminary technical feasibility analysis of carbon dioxide absorption by ecological residual solvents rich in ammonia to be used in fertigation

Abstract

Carbon dioxide absorption using residual sludge rich in ammonia as absorbent is studied. Ammonia concentrations in residual sludge rich in ammonia are at low concentrations (<0.2 wt%) compared to ammonia concentrations industrially used for carbon dioxide absorption solvents (8–10 wt%). The proposed solution avoids emissions of ammonia and carbon dioxide at the same time. The resulting aqueous stream is useful for fertigation, and therefore, energy consumption for solvent recovery by absorption is not required. The stability of the output stream is checked. The relation between minimum required solvent flow rate, solvent concentration and C:N ratio is determined. Minimum flow rate of solvent is defined as the lowest solvent flowrate required to satisfy the separation problem, using infinite length absorption column. However, due to the low ammonia content required for a residual sludge with a C:N ratio suitable for algae growth (mass fractions from 0.000182 to 0.000063), a high flow rate of absorbent is still required (only from 10 to 20 % flow rate decreases compared with pure water). Nevertheless, C:N ratios around 1 lead to a decrease of solvent flow rate requirements of 85.3 % compared to pure water solvent. Although this ratio is not suitable for plant growth, terrestrial plants capture the carbon dioxide mainly from the atmosphere equilibrating the balance. An illustrative example is presented to show that irrigation water can become a suitable sink for carbon dioxide and ammonia, providing fertigation. Therefore, two residual streams (sludge and carbon dioxide) are converted into a useful fertigation stream which is of great importance because most of the water that humanity consumes is not only for drinking but for food production as well. Nowadays, ammonia studies on carbon dioxide absorption are controlled by the physical absorption (0 % NH3) or by the chemical absorption (8–10 % NH3), simplifying the modelling. The obtained results show the importance of the study and identification of the region where physical and chemical absorptions are both significant.