We believe that Anaerobic Digestion has the potential to deliver substantial environmental benefit by “closing the loop” in returning nutrients to the soil and recovering the energy from materials that would otherwise go to waste. This view is now widely supported, for example by:
It is often difficult to achieve all of the potential benefits of AD, as there may not be ready markets for the heat (60% of the energy recovered) or digestate. What makes the Halstead site so attractive is that there are good markets for all of the products. It is not possible to calculate precisely the environmental benefit of the project as this is subject to various uncertainties and external assumptions and some of the longer term benefits to soil structure are inherently difficult to quantify. What we can do, is to give a transparent calculation of the reduction in greenhouse gas emissions as expressed in carbon dioxide equivalents (CDEs) using methodology suggested by the Intergovenmental Panel on Climate Change (IPCC).
Our calculations show that the project will supply about 5% of Halstead’s total primary energy consumption, thus supporting the town’s contribution towards the UK target of sourcing 15% of energy from renewable sources by 2020. In terms of greenhouse gases, the total impact will be around 52,000 tpa of carbon dioxide equivalents. Hence the project will have a bigger impact on climate change than if all of Halstead’s residents gave up car use entirely. Going green need not compromise our quality of life.
Environmental Calculations
- The main impacts that the project will have on the environment are:
- Reduced CO2 emissions from fossil fuel burning
- Reduced methane emissions from decomposition of waste in landfill
- Reduced fossil fuel use to make artificial fertilizers
- Trace nutrient return to the soil
We have not attempted to quantify the trace nutrient benefit, instead quantifying just the greenhouse gas reductions. The site will also lead to a small reduction in vehicle use, as the food waste currently travels to more remote landfill or composting plants. However, this too is not possible to quantify.
Electricity export: The project will export 13,000 MWh pa of electricity. Based on a UK average CO2 intensity for grid electricity of 0.537t/MWh (Source: Carbon Trust), this gives a CO2 saving of 7,000t
Heat export: The project will be able to export up to 12,000 MWh pa of heat. The alternative energy source would be gasoil. Based on a CO2 intensity for gasoil of 0.265 t/MWh, this gives a maximum CO2 saving of 3,200t
Fertilizer substitution: Soil conditioner will contain a wide range of nutrients, of which the nitrogen would take the most fossil fuel to produce. The plant will produce around 235t pa of nitrogen equivalent in the soil conditioner. Using a CO2 intensity for ammonia production of 2.0 (Source: International Fertilizer Association), this gives a saving of just 500t of CO2.
Methane from landfill: The Halstead Renewable Power Project is intended to divert food waste from landfill. In a landfill the waste will naturally decompose to produce methane, which is approximately 25 times as active a greenhouse gas as CO2 (Source: IPCC 4th Assessment Report). Some of this methane will be captured once the landfill has been closed, or even before. How much is captured varies very widely. We assume that 50% of the methane is captured, meaning that some 1780t pa will escape to the atmosphere, with a greenhouse impact equivalent to 44,500t of CO2. This is mitigated by the 3500t CO2 saving assuming that the captured methane is used to generate electricity. Thus diverting food waste from landfill will potentially save nearly 41,000t of CDEs.
Halstead energy consumption: Total UK primary energy consumption in 2007 was 226m tonnes of oil equivalent (Source: UK Digest of Energy Statistics table 1.1) or 3.77toe or 43.6MWh per head. Assuming Halstead’s consumption for its popuation of 11,000 to be in line with the average, gives a consumption of 483,000 MWh pa. The Halstead Renewable Power Project will be capable of supplying 25,000 MWh of heat and electricity net of its own consumption. This represents 5% of the total. From the same source we see that domestic electricity consumption in 2007 was 9.893m toe, equivalent to 1.98MWh per head. This gives a figue of 21800 MWh for Halstead as a whole, of which the power plant will supply 60%.
CO2 emissions of Halstead’s cars: In 2002 the UK’s passenger cars were estimated to have created 62.8m t of CO2 emissions (Source: Transport report for Office for National Statistics). This equates to 1.06t per person. Scaling that figure to Halstead’s population gives a CO2 emission for the town’s cars of 11,600t