CARBON DIOXIDE REMOVAL USING AMMONIA IN BIOGAS UPGRADING AND PURIFICATION

Abstract

An alternative source of energy like biogas has become of interest to reduce the dependence on depleting fossil fuels for the source of energy. It is environmentally friendly and can be generated from various biomass wastes. It consists of CH₄ (55%- 65%) and CO₂ (35%- 45%) with a calorific value of 22 000- 25 000 kJ/m³ when raw and after CO₂ is removed, the methane gas has a calorific value up to 39 000 kJ/ m³ and is referred to as biomethane. Ammonia is used as an absorbent in chemical scrubbing to remove CO₂ from biogas. A continuous system consisting of 1L digester was used for biogas production which was bubbled through an absorbent in 500mL gas washing bottle at a constant temperature in a water bath. The obtained biomethane potential was found to be 0.387 m³ CH₄/ kg VS which simply means that more methane gas can be obtained when using ammonia for absorption. An increase in the gas flow rate leads to an increase in the mass transfer coefficient resulting in an increase in the rate of absorption. The initial CO₂ concentration affects the removal efficiency because more work needs to be done for biogas with a high initial concentration of CO₂. NH₃ has better absorption capacity because higher biogas purity was achieved at lower NH₃ concentration. The removal efficiency for NH₃ increased from 69%-79% on average with CH₄ concentration reaching over 85% vol. This is equivalent to a calorific value ranging from 25- 33.5 MJ/Nm³ which is promising in terms of the gas ability to run in an automobile engine.