Benefit on emission reduction of greenhouse gas in biogas digester on scale swine farm based on clean development mechanism

Abstract: Methane digesters in livestock production can serve as a renewable source of electricity, and reduce greenhouse gas emissions from electricity generation. However, most of methane digester systems in China were unprofitable. Newly established China's domestic carbon offset market could provide the possibility to improve the profitability of methane digester systems. To discuss this possibility, three different sizes of pig farms located in Jiangsu province were selected to analyze the carbon emission reduction and the cost-benefit of three methane digester systems. The livestock inventory of farm A was 20 000 pigs, 50 000 pigs were sold that year. The capacity of its methane digester was 2 200 m3 of manure. The electric generator capacity was 200 kW. The biogas production was 657 000 m3 and the generated electricity was 592 000 kWh. The livestock inventory of farm B was 15 000 pigs; 35 000 pigs were sold that year; The capacity of its methane digester was 2 200 m3 of manure; The electric generator capacity was 280 kW; The biogas production was 321 200 m3 and the generated electricity was 369 000 kWh. The inventory of farm C was 5 000 pigs; 10 000 pigs were sold that year; The capacity of its methane digester was 800 m3; The capacity of its generator was 24 kW; The biogas production was 116 800 m3 and the generated electricity was 105 120 kWh. This study calculated the carbon emission reduction by Methodology for Small Scale Projects (AMS) III.D, and the net present value (NVP) by cost-benefit analysis with electric sales, carbon credits, and sewage charges savings through several scenarios to give insights into the profitability of the methane digester systems in China. The results showed that the methane digester systems had a significant effect on carbon emissions reduction. The baseline carbon emissions for farms A, B, and C were 9 434.37, 7 075.77, and 2 358.59 t, respectively. The carbon emissions with the methane digester systems are 4197.42, 3 058.82, and 1 024.66 t in farm A, B and C, respectively. The reduction in carbon emissions was 5 236.95, 4 016.95 and 1 333.93 t, respectively. For all three farms, the reduction rate was above 55%. However, the digester profitability of three pig farms was negative at current situation. If the carbon emission reduction can be traded in the carbon offset markets at a high enough price, for instance at 50 RMB per ton, then the system will be profitable, and thus can be sustainable. So far, except quite a few large-scale pig farms which have made contracts with foreign companies or organizations as CDM （clean development mechanism）programs, most of the pig farms with methane digesters could not access the international carbon offset markets. However, China's newly established domestic carbon offset market can provide that possibility. More government support is needed for the market development. There are obvious scale economies in the methane digesters. The larger pig farms are more profitable methane digesters due to their larger output of methane. Even at the renewable electricity price of 0.67 RMB/kWh and with government subsidy for the methane digester systems, only the NPV of farm A, which is the largest one of the three farms, is positive in the case of the carbon price of 10 RMB/t. For small-scale farms, the construction of centralized digesters (collectively or by a contractor) will be a good option. Farms B and C are comparatively small-scale operations and their methane slurry is used in their own agricultural production.