Aerobic Composting of Poultry Manure and Wheat Straw-Kinetic and Reactor Model

Abstract

Laboratory-scale aerobic composting tests with mixture of poultry manure and wheat straw were conducted in closed thermally insulated column reactors with effective volumes of 1 L and 32 L. Two 14-day experiments with two different mixture ratios (manure to straw, 2.77:1 and 5.25:1 on dry weight, respectively) were performed in order to compare the performances of composting process in both reactors as well to obtain the parameter values in the kinetic model and to validate proposed kinetic and reactor model in this work. The maximum temperature of 64.6°C was reached after 2.1 days in the large reactor and 64.5°C after 1.3 days in the small reactor. The temperature in these reactors was maintained above 55°C for 2 days, which should be sufficient to maximize sanitation and to destroy pathogens. The greatest mass of carbon dioxide was generated during the first 3 days. The maximum consumption of oxygen was noticed in the exit gas mixtures from both reactors after the first day of the process (the small reactor 16.0 vol. %, the large reactor 11.7 vol. %). The maximum emission of ammonia was observed from the small reactor after first day and from large reactor after the third day. The organic matter content of the materials decreased in both reactors during the process, but this reduction was greater in the large reactor (47.60%) than in the small reactor (41.36%). The proposed kinetic and reactor model was described by a differential equation set with 12 dynamic state variables: mass of organic matter, mass of oxygen, mass of dissolved carbon dioxide, mass of dissolved ammonia, mass of water in the substrate, molar amount of O2 (gas phase), molar amount of CO2 (gas phase), molar amount of NH3 (gas phase), molar amount of H2O vapour (gas phase), molar amount of N2 (gas phase), temperature of gas phase, temperature of solid-liquid phase. In the proposed kinetic model, four kinetic parameters were calculated. The model was validated by the results of several experimentally measured dynamic state variables (temperature of substrate, conversion of organic matter conversion, concentration of carbon dioxide, concentration of oxygen). Comparisons of experimental and simulation results showed good agreement during the whole duration of the process in a reactor, except for ammonia (good agreement was achieved for the first four days and for the last three days of the process). Different simulation scenarios were performed with the model in order to study the effects of initial moisture content, airflow rates and air temperature on the substrate temperature and organic matter conversion. A sensitivity analysis showed that two of four kinetic parameters had a great influence on all three objective functions (maximum conversion of organic matter, maximum concentration of carbon dioxide and maximum substrate temperature).