Environmental benefits of micro-scale digestion

MSD in 2020 Energy Strategy

Figure: Greenhouse gas emissions in sectors not included in the EU Emissions Trading System (ETS) under 2020 Energy Strategy . Source: Eurostat.

Under the Effort Sharing Decision (406/2009/EC), Member States have adopted binding annual targets for reducing their GHG emissions from the sectors not covered by the European Union Emissions Trading System, such as agriculture, housing, waste and transport. These national targets for 2020 (green) and emissions in 2012 (brown) are visualised.

Figure: Share of renewable energy in gross final energy consumption under 2020 Energy Strategy in comparison with binding 2020 targets. Source: European Environment Agency 

Greenhouse gas emissions in agriculture

The Intergovernmental Panel on Climate Change (IPCC) indicates three gases as the most influential greenhouse gases (GHG), namely: carbon dioxide (CO2), nitrous  oxide (N2O) and methane (CH4).  The common unit to express the impact, or potency, of the different GHG is CO2-equivalent. This equivalent is the amount of CO2 needed to produce the same impact. According to the UNFCCC, CH4 is 23 times as potent as CO2 and N2O as many as 310 times more potent. Methane emission is the second largest contributor to climate change.

As you can see in Figure below, agriculture is the largest single source of CH4 and N2O. Furthermore, in agriculture, emissions of these two gases are significantly higher than CO2 emissions. Therefore GHG reductions in this sector should focus mainly on CH4 and N2O, not only on CO2.

Figure: Greenhouse gas emissions by economic activity and by pollutant, EU-28, 2012, Source: Eurostat.

Figure: Contribution of agriculture to total greenhouse gas emissions, 2010, EU 27, without energy use, LULUCF CO2 eq. Source: Eurostat 2012.

CH4 is mainly emitted by cattle and cattle manure. So, micro-scale digester (MSD) with manure as the main substrate can significantly reduce CH4 emission, by transforming it into bioenergy.

In the context of all greenhouse gases, as shown in Figure 16, of the 10% share that agriculture has in total GHG emissions, 1.63% (16.7% of the agricultural share) is from manure management . The production of bioenergy by MSD results in the reduction of the demand for fossil-based electricity and heat, thus reducing GHG emissions.

Emission reduction potential

Based on the results of the BioEnergy Farm 1 project and calculations performed using the anaerobic digestion profit calculator developed in the framework of that project, the simulations of the typical micro biogas plant were calculated in order to determine yearly emissions of greenhouse gases (GHG) resulting from:

• methane not covered into energy (unburnt due to incomplete combustion of the biogas in the engine and leaking from installation),
• traditional manure management – avoided if manure is digested,
• consumed electricity and heat from fossil fuel – avoided if fossil fuel is replacement by biogas from micro-scale digester.

The values presented in this estimation should be understood as illustrative only. They are not resulting from extensive studies, but rather from simplified compilation of the data gathered in the surveys performed during the Bioenergy Farm 2 Project with existing methodologies for GHG emissions calculation.

The most important conclusion which should be drawn from the below presented results is related to the very high potential of GHG emission reduction resulting from production and consumption of energy generated in micro-scale biogas plants. In connection with additional impact of traditional manure management replacement, this environmental effect is much higher than usually considered for renewable energy sources.

Table: Emission reduction resulting from operation of a single biogas plant.

At this point it should be noted that the difference between total avoided emissions from manure management of the same maximum power, e.g. for France and Italy, results from differing average annual temperature in each country. Similarly, the difference between total avoided emissions from fossil fuels for the same maximum power results from different most commonly used heat and electricity sources for each country.

Based on the above results, an emission reduction for each partner country is designed, describing the CO2 equivalent GHG emissions from the generated energy (and utilized on the farm) by micro biogas plants. The calculated factors were compared with country-specific emission factors for consumed electricity from national grid used for the projects related to the reduction of electricity consumption.

Usage of the electrical energy from the grid has a negative environmental impact quantified typically in GHG emissions. At the same time, energy from biogas, when used, implies positive environmental impact. The graph below illustrates the estimations of the emission reduction related to the micro–scale digesters in each partner country.  The amount of GHG reduction depends mostly on the utilization of the generated electricity or electricity and heat jointly.

As shown in the figure below, energy generated in a micro scale biogas plant has a very positive effect on the environment. In case of electricity production based on biogas, additional environmental benefits from manure management outweigh the emission factor of the grid electricity. The benefits from manure digestion are therefore particularly high in countries with low carbon emission electricity production. However, even for the countries with lower potential for such investments it is still an efficient way for effective GHG emission reduction and contribution to the sustainable development strategy.

Figure: GHG emission saving potential by kWh of electricity from micro biogas plants compared with GHG emission by kWh of electricity consumed from the grid.

Figure: GHG emission savings obtained by traditional manure management avoidance and by replacement of fossil fuels by biogas.

Figure: Electricity recalculated as equivalent number of km travelled by passenger car for two scenarios of energy-mix with the same GHG emission level.

These estimations were based on average carbon dioxide emissions per km from a new passenger car, 132.2 g of CO2 per km, and average consumption of electricity per capita for each country separately .

These calculations were carried out for two scenarios: with 5% share of electricity from biogas from micro-scale digestion and without this share. Depending on the energy-mix in each partner country and the level of electricity consumption per capita, emission savings related to one person correspond to about 600 km to 1100 km savings per year from one passenger car. And, as previously, it should be stated that the values compared below do not include the utilization of the excess heat, only bio-electricity. Therefore, for biogas projects with heat utilization, the result would be more optimistic.

Digestate as fertilizer

The first of the above mentioned advantages, higher levels of inorganic nitrogen (N) released from digestate, accelerates nutrients intake of plants. The net mineral release as a percentage of total N is higher in case of digested manure both for pig and cattle manure.  In untreated manure, ammonium represents about 50% for cattle slurry and about 70% for pig slurry of the total nitrogen content, while in digestate it amounts to about 80% (for mixture of about 50% pig slurry, 25% cattle slurry and 25% organic waste).

The second mentioned benefit, odours reduction, results from a lower concentration of volatile fatty acids contained in the digested manure. Various studies confirm that the concentration of odour in the air is significantly lower when digestate, instead of untreated manure, is applied on the fields. Odours problem with untreated manure was one of the main factors the farmers pointed to during bioenergy scans carried out in the framework of BioEnergy Farm 1 project.