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

Abstract

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.