Msc: Sustainable Engineering : Chemical and Process Engineering
Greenhouse Gas Emissions
Manure is indeed a valuable resource for farmers, but it also poses a risk of environmental pollution if not managed properly. This page delves into the CO2 emissions from typical farm practices, comparing various manure management scenarios with the current methods used in the UK.
The data on the percentages of manure management practices used in the UK was obtained from the Department for Environment, Food & Rural Affairs (DEFRA) (2022), Final Poultry Statistics Notice. The estimated CO2 emissions per storage method were calculated using the Cool Farm Tool calculator. This assumed a scenario of a dairy farm with 2,000 cows producing 39,420 tonnes of milk with a 3.9% fat content and an average cow weight of 509 kg. Based on these assumptions, the daily cow manure production amounts to approximately 23,400 kg. The following section presents the results of this scenario.
Manure Management
Greenhouse Gas Emissions
The graph depicts the estimated CO2 emissions associated with each manure management practice using the outlined scenario. These emissions solely account for the manure management process itself, except for anaerobic digestion AD.For AD plants with biogas upgrading, the CO2 emissions include both the manure management and the energy used during the process. AD itself requires 58 kWh, while upgrading to biomethane needs an additional 74.56 kWh, totaling 132.56 kWh per hour.
Biogas upgrading offers a significant environmental advantage by capturing and storing a highly concentrated stream of biogenic CO2. The stripping tower within the upgrading system can capture approximately 63.1 kg/hr of CO2 with a high purity of 88%. This translates to a potential capture of 504 tonnes of CO2 annually, equivalent to taking roughly 315 cars off the road.
Despite the additional energy consumption required for upgrading, AD plants with biogas upgrading still boast a lower overall CO2 footprint compared to AD alone and other manure management methods. Manure processing and energy use in AD plant with biogas upgrading emit an estimated 550 tonnes of CO2 per year. However, the captured CO2 from the upgrading process (504 tonnes) significantly reduce these emissions. This results in a net CO2 emission of only 46 tonnes, demonstrating the clear environmental benefit of biogas upgrading.
A key advantage of anaerobic digestion lies in its ability to generate energy without producing additional greenhouse gas emissions. Furthermore, the Scottish Government's "Negative Emissions Technologies (NETs) Feasibility Study: Final Report" classifies anaerobic digestion with biogas upgrading as a form of Bioenergy with Carbon Capture and Storage (BECCS).
BECCS is a Negative Emission Technology (NET) that tackles climate change by achieving negative carbon emissions. It works by capturing and permanently storing the CO2 released during the conversion of biomass (organic matter) into biofuels or during direct energy generation through combustion. This capture and storage process effectively removes CO2 from the overall cycle, contributing to a net reduction in atmospheric greenhouse gases.