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Methane emission from livestock manure contribute significantly to the total climate footprint of agriculture. Acidification of manure is known to reduce methane emission. However, the mode of inhibition by acidification is not completely understood and may depend on the acid or acids used. In this work, we examined the effect of organic, inorganic, and mixed acids on greenhouse gas emission from pig slurry. Initially, pig slurry was incubated in glass vials and acidified to pH 5.5. Methane production was measured with a GC-TCD. After initial screening, a continuous gas monitoring setup was used to simulate conditions occurring in a pig house. Greenhouse gas emission, was continuously measured with a cavity-ring-down spectrometer, while different acid combinations were used to treat the pig slurry to pH 5.5 - 6.25. The effect of daily-added pig slurry, as well as weekly slurry removal, and acidification of the residual slurry was examined using this laboratory setup as well. In the initial screening, nitric acid reduced methane with > 99% (p = 2.0E-10) when acidified to pH 5.5. Combinations of acetic acid with sulfuric acid or nitric acid showed no further inhibition of methane production compared to nitric or sulfuric acid alone. In the continuous gas monitoring setup nitric acid reduced methane emission, but increased nitrous oxide emission, reducing the treatment effect on total CO2-e considerably. Treatment with HNO3 to pH 6.25 increased nitrous oxide emission by > 800% (p = 0.042). Treatment with different ratios of nitric acid and sulfuric acid to pH 5.5, was not better than pure sulfuric acid treatment, which resulted in 92% CO2-e reduction (p = 0.002). When daily addition of slurry was applied, the efficiency of acidification decreased significantly after one week, and CO2-e was not significantly different from the control (p = 0.085 for H2SO4 treatment).

The main purpose of the FutureGas project was to contribute to a green transition through interdisciplinary and coordinated research across the gas sector. The goal was to answer two main research questions: What could the role of natural gas, renewable gas and the gas infrastructurebe in a future climate-neutral Danish energy system? Which role could gas have in the sustainable energy transition towards aclimate neutral energy system?The project included the major stakeholders of the Danish gas industry as well as Danish and international universities (see below). The project was supported by the Danish Innovation Fund. The project analysed the gas chain from supply to regulation: efficient production and use of green gases including potential conditioning to natural gas quality, flexible use of gas also for transport, system integration, as well as the application of measures to ensure an economically efficient use of gas. The authors of this report carry individual responsibility for their respective chapters. All reports from the project can be found on the homepage www.futuregas.dk. Scientific articles can be acquired by mail to the authors.