Small-Scale Biogas Upgrading System Modeling Tool Development

Abstract: The potential of biogas to decarbonize society depends partially on the success of small-scale systems. Two specific locations where biogas units can be implemented are considered in this study: small farms and small isolated populations. The access to energy sources, either traditional or renewable for these is often restricted and typically costly. Clean raw biogas can be the energy source to satisfy power generation, cooking, and heating needs. Upgrading widens the options to transport fuel and energy storage helping positively in the unlinking between production and demand. Water upgrading has three essential and exclusive benefits that make it a highly feasible solution for these isolated locations or small agricultural units. It is in general available in these places; it has a very low environmental impact if leakage or malfunctioning of the system, and has no toxicity per se. To aid the development of the biogas industry focused on small-scale systems a fast, easy to use, low cost, customizable tool is needed to help the design process of the high-pressure water upgrading units. The present study covers the development of such a tool. In the present report, the basis of the model to solve the mass balance of the system and to calculate the dimensions of the scrubber are described. The scrubber model is an implementation of the NTU-HTU model proposed by Billet and Schultes in two major publications (Billet, 1995) and (Billet & Schultes, 1999). The strategies used to solve the set of closed loop equations, and iterations are presented in a block diagram fashion. The tool was developed in visual basic for applications using Excel as the hosting application. The results of the tool are compared against those obtained from the same model ran in Aspen Plus. To perform such a comparison, 540 cases were used. The cases are the result of running three nominal raw biogas flows, using three different packing materials, varying the raw biogas and water flows, varying pressure, temperature, and height of the scrubber, and varying the pressure of the flash tank. Three sensitivity analyses are performed to check the influence of some variables in the model. One is designed to check the influence of the exponent choice for dimensionless numbers in the calculation of the volumetric mass transfer coefficient as an example of the various points where the Billet model is adjusted to follow the behavior of packed columns. Another analysis consists of comparing the results of height using different packing materials to see how the six packing constants affect the results of the calculations. The third analysis is performed to check the influence of the methane absorption model where three different approaches were used. The results show that the tool behaves coherently, and a validation step can be implemented via real experimental tests comparison. The tool has several points where adjustments can be made, like the mentioned exponents for the dimensionless numbers, or the constants used in the interfacial area calculation and the correction of the