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The paper below was written by a highly regarded independent consultant, to look at the carbon footprint of the Biocentre process compared to alternatives. The conclusions remain very valid today, although certain comparables, such as the grid mix of generation have changes significantly. It remains the case that the Biocentre process delivers very considerably better carbon footprint (global warming gasses) than any other waste treatment.
Introduction
Set out within this technical paper is an independent third party justification and analysis of the environmental performance of the Biocentre process compared to other solid waste treatment technologies. The paper introduces the reader to the concept of the ‘carbon footprint’, how it is calculated and how it can inform the decision making process when choosing a waste treatment technology. Finally the paper presents the carbon footprint and discusses the implications for a range of waste treatment technologies including the Biocentre technology.
What is a carbon footprint
‘Carbon footprint’ is a term used to describe the amount of greenhouse gas (GHG) emissions caused by a particular activity or entity, and thus a way for organisations and individuals to assess their contribution to climate change[1]. In waste management the carbon footprint is used as a tool to measure and compare the greenhouse gas emissions of different waste management approaches and technologies.
Greenhouse gases refers to those gaseous compounds that are known to contribute to the warming of the atmosphere, the so called ‘global warming’ effect. The most common greenhouse gas is carbon dioxide (CO2) however other species, primarily methane (CH4) and nitrous oxide (N2O) are equally important in waste management.
The latter species should not be confused with nitric oxide and nitrogen dioxide, both commonly referred to as NOx, and which play no part in global warming but, instead, are powerful contributors to acid rain.
Nitrous oxide is formed by the biological breakdown of nitrogen containing material and is therefore closely associated with composting processes. To a lesser extent nitrous oxide may also be formed in combustion processes. Methane is formed by the biological reaction of carbon under anaerobic conditions, and is most commonly associated with landfill gas emissions.
The degree to which a greenhouse gas contributes to global warming is measured by its Global Warming Potential (GWP). This is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by definition 1)[2]. A GWP is calculated over a specific time interval and the value of this must be stated whenever a GWP is quoted or else the value is meaningless. Commonly, a 100 year lifespan is used for carbon footprinting purposes.
The most potent greenhouse gases are the hydrofluorocarbons with GWP factors in excess of 10,000; many of these species are banned or their commercial use severely restricted because of the environmental damage which they can cause. Emissions of these species, which are generally used as refrigerants and coolants, are rare in the waste management industry.
Water vapour is also recognised as a powerful greenhouse gas because of its ability to absorb infra-red radiation. However, the concentration of water vapour in the atmosphere mainly depends on air temperature and as there is no possibility to directly influence atmospheric water vapour concentration, the GWP level for water vapour is excluded from carbon footprint calculations.
A carbon foot print is expressed in the form of mass carbon dioxide equivalency (CO2e or CO2eq), a concept that describes, for a given mixture and amount of greenhouse gas, the amount of CO2 that would have the same global warming potential (GWP), when measured over a specified timescale. The carbon dioxide equivalency for a gas is obtained by multiplying together the mass and the GWP of the gas
Put simply, from a life cycle perspective the process of turning waste into energy generates ‘direct emissions’ associated with conversion of carbon in the waste into carbon dioxide; ‘indirect burdens’ associated with the supply of chemicals to control air pollution and ‘avoided burdens’ from displacing the equivalent quantum of energy from conventional energy sources.
The patterns exhibited in carbon footprinting generally follow the convention exhibited in the ‘waste hierarchy’ that is recycling performs better than energy recovery which in turn performs better than disposal to landfill. Preference may alter slightly depending on the material, for example recovery of energy from timber is generally shown to have a lower carbon footprint than recycling of timber. Such anomalies are rare and therefore the aforementioned pattern normally holds true.
A final important convention adopted in carbon footprinting is the management of biogenic carbon emissions, that is carbon sourced from natural renewable materials such as food waste and paper. LCA convention determines that biogenic carbon is treated as having no impact on climate change, as it is part of the natural carbon cycle[1].This exclusion is only true for carbon dioxide as it is only this species that can be absorbed by growing plants and therefore contribute to the natural carbon cycle. The distinction is particularly important for landfill where carbon dioxide released from the gas engines will be excluded from the carbon footprint whilst fugitive emissions of landfill gas (methane) will be included
The carbon footprint for the Biocentre process has been calculated using the Environment Agency’s life cycle assessment tool, ‘Waste and Resource Assessment Tool for the Environment’ (WRATE), which has been specifically developed to compare the environmental performance of municipal waste management systems on a life cycle basis[1].WRATE includes default databases which model and describe the various inputs and outputs for a number of waste treatment technologies. It also includes the ability for users to create their own ‘user-defined’ databases for bespoke waste management processes
In developing the carbon footprint for Biocentre and comparing it to other waste management technologies a combination of default and user defined databases have been used.
Best option for different materials
Whilst WRATE provides a holistic assessment of the carbon footprint for a given waste composition it is possible to determine the least carbon approach by considering individual materials.
Table 1 provides comparative results for recycling, landfilling and energy recovery for individual materials. It clearly indicates that processes that incorporate efficient recovery processes for plastics, aluminium and steel will exhibit a better carbon footprint than those employing landfill or energy recovery.
Table 1: Carbon impact of managing individual waste components
Possibly, of greatest significance is plastic where recycling exhibits an environmental saving and recovery exhibits a lower carbon footprint than landfill. The latter is referred to as the ‘carbon sink’ effect whereby plastic deposited in a landfill leads to carbon being stored indefinitely rather than emitted to the atmosphere.
To understand the first result it is important to consider the carbon content of the material, which will be released as carbon dioxide when it is converted to energy, and the relative carbon benefit of displacing a unit of electricity from conventional sources.
Plastic bottles, are hydrocarbons and as such have a very high carbon content; 82% by mass for HDPE bottles. As a hydrocarbon, plastic has an energy content not dissimilar to fossil fuels such as coal and gas and in fact similar carbon contents too. Thus, if plastic was converted to electrical energy which in turn was used to displace fossil fuel derived energy the net carbon impact would be close to zero.
However carbon footprinting rules set down by the UK government convention dictate that when determining the impact of energy generating projects the carbon benefit must be based on the future marginal energy generating mix, which for the UK is a combination of combined cycle gas turbine (CCGT), nuclear and renewables.
Carbon balance for the Biocentre process
The carbon footprint for the Biocentre process has been determined using WRATE and a number of user-defined processes. Off-takes for the fuel products are assumed to be a dedicated biomass to CHP facility for the refuse derived humus (RDH) and a dedicated coal fired power station for the refuse derived fuel (RDF). The process also benefits from the recovery of recyclable materials primarily ferrous metals, non-ferrous metals and plastics; the benefit of which has also been included in the carbon balance.
Figure 1: Carbon Footprint for the Biocentre Process
The carbon balance indicates overall a net negative carbon burden; that is the avoided burdens of displacing primary materials processing and energy generation outweigh the positive burdens associated with emissions from the process and downstream management of product outputs.
The following interpretations may be drawn from the analysis:
· Recovery of recyclables displaces extraction and processing of virgin material resources with a resultant net negative carbon impact;
· The RDH is almost entirely biogenic in its carbon content; when processed through a biomass energy plant the energy displaces conventional fuels used for electricity generation and heating (CHP). The net carbon impact of this process is negative;
· Combustion of SRF, still containing non-recoverable plastics, generates a positive carbon impact;
· However, by directly displacing coal in a power station, combustion of the SRF delivers carbon savings (avoided burdens).
The balance of direct, indirect and avoided burdens results in a net negative carbon impact; that is the conversion of waste into useful materials and energy through the Biocentre process results in an overall benefit to the environment through a reduction in global carbon emissions.
Biocentre compared to other technologies
The performance of the Biocentre ART process compared to other waste management technologies is illustrated in Figure 2.
Figure 2: Comparative performance of the Biocentre (ART) Process
Biocentre compared to landfill As the dominant means of residual waste management it is common practice to compare carbon emissions against landfill. Despite the uncontrolled release of methane in the form of landfill gas, landfill performs remarkably well from a carbon perspective for a number of reasons: · Landfill acts as a carbon sink and in doing so captures fossil carbon which could otherwise be released to atmosphere through combustion processes; · The resultant methane that is captured is converted to electrical energy and as an entirely biogenic form of energy generation yields environmental savings. However, overall because landfill prevents recovery of recyclable materials, emits methane to the atmosphere and fails to maximise energy recovery potential the overall benefit comparing the two technologies is in favour of the Biocentre process.
Biocentre compared to Energy from Waste Energy from Waste performs marginally better than landfill because it increases the recovery of energy and provides beneficial outputs that displace virgin material extraction and processing. However because of problems with the combustion process which in turn limits energy efficiency and because of the release of fossil carbon, EfW fails to perform as well as the Biocentre process. Moreover, the carbon benefit of EfW is limited by concentrating on energy recovery rather than recycling. Consequently, the Biocentre process exhibits a better carbon footprint than EfW.
Biocentre compared to Autoclaving Like Biocentre, autoclaving offers the potential to produce high quality outputs. It does this by heating the waste to moderate temperatures with steam or hot air; in doing so the waste is cleaned and sanitised and the organic components are broken down into a cellulose fibre which in turn aids more efficient separation of the waste into constituent components. This separation process can be achieved so successfully that certain technology providers can provide systems that blend the segregated components to produce fuels with specific energy contents and physical attributes. However, from a carbon perspective the design element which renders the autoclave process so efficient, moderate heating, severely reduces the net carbon benefit because of the consumption of fossil fuels. In summary Biocentre exhibits a better carbon footprint than autoclaving due to much lower energy consumption.
Biocentre compared to Aerobic MBT Processes There are a range of MBT processes available, all of which comprise a combination of biological and mechanical processing with recovery of recyclables and a dried product that can be used either as a replacement fuel or consigned to landfill. The carbon assessment presents the more common MBT process; one which subjects the waste to biological drying and mechanical separation to produce an alternative fuel (MBT to RDF). MBT processes allow recovery of recyclable materials, primarily metals and plastics and in doing so deliver similar carbon benefits for these elements to the Biocentre process. However by producing a less refined product with less market potential to be used as a fuel the resultant carbon benefits are somewhat lower for MBT than the Biocentre process.
Biocentre compared to Anaerobic MBT processes The use of anaerobic rather than aerobic conditions to process residual waste is practised by a handful of technology suppliers. In such situations water is used to aid separation of the waste into constituent materials with the degradable organic proportion reacted in an anaerobic environment to produce a biogas, which can be converted to electricity and a solid organic residue which is either used a compost like product or consigned to landfill. In general these processes do not maximise energy recovery and generate an organic output which may undergo further biodegradation; as such anaerobic MBT treatment exhibits a lower carbon benefit than Biocentre.
Summary
Comparison of carbon footprints for waste management technologies clearly illustrates the environmental benefit of Biocentre’s process. The excellent environmental performance of the process is due to a number of factors: · Maximising the recovery of recyclable materials such as metals and rigid plastics; · Converting organic elements of the waste into biomass type fuel (RDH) that can be combusted in a dedicated biomass to energy and heat plant; · Converting other combustible elements of the waste into a high calorific value fuel (RDF) that can be used as a direct replacement for fossil fuels; · Selecting energy facilities that exhibit high energy efficiency technologies so that maximum energy can be extracted from the fuel; · Minimising the energy demand of the process; · Minimising the amount of waste sent to landfill.
In summary, by following the principles of the waste hierarchy, promoting recycling over recovery over landfill; and by minimising the process energy demand, the Biocentre process can be considered to offer optimal environmental performance for the management of solid waste.
Prepared by Dr Tony Yates, Principal, SLR Consulting July 2009 Note the paper above has been edited only to replace the old name for the process (ART) with the new name Biocentre