Enriquecimento de biogás: modelagem da absorção de CO 2 utilizando água como solvente

Abstract

The production of biogas from animal waste has attracted attention because it is able to contribute positively in solving environmental pollution problems in rural areas. In addition, it can generate renewable energy. Among the available options for using biogas, we highlight two that are able to add more value to the feedstock: its use in stationary internal combustion engines coupled to electricity generators, and the production of vehicle fuel. In order not to compromise the useful life of mechanical components, in both cases, hydrogen sulfide (H 2 S) and water (H 2 O) present in biogas must be removed. Carbon dioxide (CO 2 ) must also be removed for the production of vehicle fuel. Depending on the amount of gas that will be processed; this removal is also advantageous for power generation, as it is able to increase motor efficiency. In a previous work, a detailed literature review on the methods and technologies available to purify and enrich biogas was presented. The following sequential order was selected: CO 2 absorption (and rough removal of H 2 S); H 2 S adsorption using iron oxide fixed bed; water condensation through gaseous stream cooling. The present work aimed to specify and size the CO 2 absorption process using water as a solvent, and obtain a simplified model for calculating the effective height of the column and flow of solvent. Thus, we proposed a mathematical model applied to the case of the CO 2 absorption present in biogas. This model considers the physical mechanism of absorption through macroscopic and microscopic approaches. In order to obtain an accurate sizing of the absorption column, the model considered the effects of several variables: operation pressure and temperature; inlet and outlet mole fraction of CO 2 present in biogas; biogas flow; column diameter; and Pall rings diameter (used as packing material). Simulations were performed to calculate the effective height of the column and water flow rate for different situations, based on experimental design techniques. Simplified empirical models for calculating the effective column height, as well as the solvent flow rate, functionally dependent on the variables mentioned above were adjusted. With these, the design of the absorption column used to remove CO 2 from biogas on a small/medium scale can be easily determined.