A Procedure for the Development of a Low-Cost Treatment Technology of Raw Biogas for Application in a Gas Engine. A Case of Sizing a Carbon Dioxide Removal Unit

A test laboratory (lab) for carbon dioxide (CO₂) adsorption from raw biogas onto a novel adsorbent was used to size a CO₂ removal unit in the development of a low-cost biogas treatment technology. The novel adsorbent was made out of clay and burnt maize cob particles, impregnated with hot natural alkaline solution of pH 10 ± 0.5, degassed, and then activated at a temperature of 250°C, thereby making it low cost. The activated absorbents were spherical balls of average diameter 17 mm, density 410 kg/m³, and surface area 128 m²/g, and contained exchangeable ions due to the presence of clay and increased pore sizes due to impregnation, degassing, and activation. The effect of pressure drops on CO₂ removal, the breakthrough curve, and the absorption isotherm were studied. As a result, reduced pressure drops enhanced CO₂ removal and 102 Pa/m was the suitable pressure drop; pressure drops less than 102 Pa/m were impractical because the biogas did not exit. The breakthrough curve was in typical s-shape and thus satisfied its use for determining the adsorption rate constant (k₁) to be 0.001952 l/mg s and the maximum percent of CO₂ removal to be 87.8% at 102 Pa/m pressure drop and temperatures ranging from 20 to 28°C. The isotherm was found to closely conform to the definition of the Freundlich equation with the Freundlich coefficient of 0.01809 (l/g)ⁿ, where n = 1.37 at the same temperature range. Therefore, the determined k₁ and fitted Freundlich isotherm can be used to size the CO₂ adsorption unit under these conditions.