## modeling of energy systemsI. Simeonov, N. Pan, H. Wang, Y. Tian, E. Chorukova, N. Christov. Energy Yields Comparative Study for One-Stage and Two-Stage Anaerobic Digestion Processes

Key Words: Anaerobic digestion; one-stage anaerobic digestion processes; two-stage anaerobic digestion processes; mathematical modeling; static characteristics; biogas yields; energy yield.

Abstract. Compared to traditional one-stage anaerobic digestion processes (OSAD) with biomethane production, this paper focuses on the study of energy yield of the two-stage anaerobic digestion processes (TSAD), which are able to produce simultaneously biohydrogen and biomethane. In TSAD, relatively fast growing acidogens and H 2-producing bacteria are developed in the first-stage hydrogenic bioreactor and are involved in the production of volatile fatty acides (VFA) and H2. On the other hand, the slow growing acetogens and methanogens are developed in the second-stage methanogenic bioreactor, in which the produced VFA are further converted to CH4 and CO2. This separation allows to optimize physico-chemical parameters for both groups of microorganisms which are not the same. Using mathematical models (mass balance type) developed by our team, theoretical comparative analysis of the energy yield from one-stage and two-stage anaerobic digestion systems is performed. Transforming the differential equations of these balance models, some algebraic equations called static characteristics for both bioreactors were obtained. They represent dependencies of the main process variables from the control variable (dilution rate). On the basis of theese results, the theoretical maximal values of the corresponding enery carriers (hydrogen and methane) yields can be found for different values of the inlet organics (perturbation). The possible maximal biohydrogen and biomethane yields and the overall energy production are calculated by the static characteristics and extremum points of both systems. From the performed analytical and simulation studies, it can be seen that the energy obtained with TSAD is from 32% to 48% greater compared to OSAD under similar conditions (depending on the concentration of the incoming organics).