A technical and economic comparison for single and two-stage biogas plants

The threat of fossil fuel depletion and the increasing demand for energy have highlighted the importance of renewable energy sources. Utilizing biomass for energy production addresses this issue, and among renewable energy sources, biogas systems stand out. Biogas, which combines waste management and energy production, is an environmentally friendly and carbon-neutral energy source during operation. Single-stage and two-stage biogas plants differ in the way their biochemical processes are carried out. In single-stage biogas plants, all biochemical processes (hydrolysis, acidogenesis, and methanogenesis) occur within a single reactor. In contrast, two-stage plants separate these processes into different reactors to enhance process efficiency and gas production capacity. This study compares single-stage and two-stage biogas systems from technical and economic perspectives. Three scenarios were analyzed for biogas production, electricity potential, installed capacity, cost, and internal profitability based on a daily input of 200 tons of waste. The scenarios include waste from cattle manure only, cattle and poultry manure, and a mix of cattle, poultry manure, and fruit and vegetable waste. The biogas production potentials of single-stage and two-stage plants were calculated as follows: for cattle manure only, 2,730,744 and 3,003,819 m³/year; for cattle and poultry manure, 4,044,152 and 4,448,567 m³/year; and for cattle, poultry manure, and fruit and vegetable waste, 8,568,650 and 9,425,515 m³/year, respectively. Corresponding electricity production from this biogas was determined as 6,922, 7,615, 9,863, 10,849, 19,860 and 21,846 MWh/year, respectively. The total installed capacities of the plants were calculated as 0.8 and 0.9; 1.2 and 1.3; and 2.4 and 2.7 MWh for single-stage and two-stage systems, respectively. Economic analysis showed that the capital expenditures (CAPEX) of the plants were 2.3 and 2.5; 3.3 and 3.6; and 6.6 and 7.3 Million Euros for single-stage and two-stage plants, respectively. The average annual revenues were calculated as 0.9, 0.1; 1.3, 1.4; and 2.4 and 2.6 Million Euros, respectively. Annual expenses were determined as 0.16, 0.18, and 0.20 Million Euros. The average cash flows were 0.78, 0.84; 1.09, 1.19; and 2.18 and 2.42 Million Euros, respectively. The internal profitability ratios were 22.6%, 22.22%, 22.22%, 21.89%, 21.97%, and 22.29%, with all plants having a payback period of 4 years. The results indicate that the two-stage plant utilizing cattle, poultry, and fruit and vegetable waste achieved the highest efficiency and economic benefit. Despite its higher investment cost, this plant has the same payback period as other scenarios and makes the most significant contribution to energy production and sustainable energy goals. A comparison of single-stage and two-stage plants using the same type of feedstock reveals that two-stage systems are more efficient. Biogas plants would significantly contribute to the city's energy consumption.