ENERGY ANALYSIS OF BIO GAS PLANTS

Abstract

Energy balances treat all forms of energy as equivalent, without differentiating between the different grades of energy crossing the system boundary. Energy balances provide no information about the various internal losses of the system. As the energy conservation is a key goal of our economy, the most effective way to reduce the energy demand is to use energy most efficiently. A relatively new technique based on a concept of exergy which represents a universal measure of work potential or quality of different forms of energy in relation to a given environment. An exergy balance applied to a process or a whole plant tells that how much of usable work potential, or exergy, supplied as the input to the system under consideration has been consumed by the process. The - -loss of exergy called commonly as `irreversibility', provides a quantitative measure of a process inefficiency. This thesis presents a theoretical investigation on thermodynamic analysis of a biogas plant, using the concept of exergy. The relative values of irreversibilities and exergies at various locations in the plant demonstrate the degradation of energy sequentially. This is represented quite vividly through a Grassmann diagram. Theoretical exergy analysis of three bio-gas plants have been carried out in the present work. First, a fabricated laboratory model of bio-gas plant. Second, a BLAABJERG bio-gas plant with capacity of generating 6900 MWh electricity and 10,000 MWh heat. Third, a proposed bio-gas plant. The exergy losses in various components have also been derived. These expressions can be used for evaluation of exergy at various locations. In addition to the theoretical analysis, an experimental analysis is also carried out on the laboratory model of a bio-gas plant fabricated for this study. The exergy analysis carried out in the present study brings out that some components or processes in the various plants studied are most .inefficient from the energy utilization point of view, i.e., where irreversibilities are significantly high compared to other components in a system. In the bio-gas power plant, it is the gas turbine. The loss of exergy- due to the irreversibility in the gas turbine can be used and thus the efficiency of energy utilization can be increased. This is done by circulating cold water through gas turbine and hot water thus obtained is used in Milk Chilling plants. The irreversibilities in the rest sub-regions are relatively small. Fibre is obtained in the plant is used to generate heat and separated liquid goes to the field as fertilizer. The experimental exergy analysis of the laboratory model of. a bio-gas plant reveals that (i) the digester and (ii) the pump are major sources of irreversibility.