Calculating the Renewable Fraction of Energy from Waste
In the U.K. every year nearly 35 million metric tons of municipal solid waste (MSW) is generated. And up to 60 percent of MSW can be made up of renewable materials that could be used to generate energy.
In the U.K., generators of renewable energy can receive Renewable Obligation Certificates, which can then be sold on to electricity suppliers. ROCs are issued for every megawatt hour (MWh) of renewable electricity generated and each of these ROCs currently sells for around £50 ($81.13).
When you convert mixed waste streams into electricity, however, it can be difficult to determine how much of this energy is renewable. Many approaches have been considered (see table on page 51) but up until now, only one energy-from-waste (EfW) power station in the U.K. has received any ROCs.
The Energos plant on the Isle of Wight has been receiving ROCs since the start of this year, but its approach is to measure the biomass content of the fuel and the gross calorific value of the flue gas. This can be an unrepresentative and time-consuming affair.
Now, the British electricity and gas market regulator, Ofgem, which issues ROCs, has approved the use of a new method known as the carbon 14 technique to help calculate the renewable content of EfW so installations can benefit financially from the renewable energy they produce.
“Independent reports concluded that the carbon 14 technique is based on mature and well-understood technology,” says Richard Bellingham, biomass, waste and cofiring manager at Ofgem.
“We are therefore prepared to consider fuel measurement and sampling (FMS) procedures that propose to use the carbon 14 technique. As always, the FMS proposals will be considered on a case-by-case basis, taking into account the different feedstock used and the nature of each generating station.”
The approach takes advantage of the fact that as living organisms die and are converted to fossil fuel, the proportion of carbon 14 isotopes they contain will decrease, due to radioactive decay. So fossil fuels contain far fewer carbon 14 isotopes than recently living biomass. This can be measured in the smokestack (or flue gas) of power plants so they can claim ROCs.
Double or Nothing
EfW installations can receive double ROCs if they can prove the renewable content of their feedstock. The double ROCs banding is the highest incentive available under the U.K. Renewables Obligation system and is designed to support investment in emerging technologies, such as advanced gasification and pyrolysis.
Finding a technique to simply and accurately determine the renewable energy produced by EfW installations has been a major stumbling block for the industry, and has until now presented a missed opportunity for energy generators who could be claiming ROCs.
“Waste-derived fuels are a vital indigenous source of renewable energy for electricity, heat and ultimately transport fuels,” said Chris Manson-Whitton of clean energy project development company Progressive Energy Ltd. “We are very pleased that Ofgem has adopted this technique, which will facilitate industry in unlocking this valuable resource.”
“This has been the culmination of a four-year story to establish this exciting technique as a method for measuring the renewable content of waste-derived energy. We, along with other advocates like the National Non-Food Crops Centre, New Earth Energy and the Renewable Energy Association welcome Ofgem’s decision and are pleased that our efforts have opened the door for generators to use this pioneering approach,” he adds.
While the technology is new to the U.K., it is already in use in countries such as Belgium and the Netherlands and has proven highly successful in measuring the renewable carbon in their flue gases. Now several U.K. EfW installations have decided to start using the technology.
The U.K.’s National Centre for Biorenewable Energy, Fuels and Materials—the NNFCC—were amongst the first to propose carbon 14 as a simple and accurate method for measuring the renewable carbon of flue gas.
“The carbon 14 method is enabling easy and accurate differentiation between carbon dioxide emissions created from fossil sources and those from renewables,” says John Williams, head of materials for energy and industry at the NNFCC. “As a long-term advocate, the NNFCC hopes this trusted and established analysis technology will significantly increase the number of power stations making renewable energy from waste biomass.”
The carbon 14 technique starts with a sample of carbon dioxide taken from the flue gas of a facility. This can be taken in small increments every 30 minutes or so, then a composite sample can be made to accurately represent the whole month. The carbon dioxide is then sent to a lab for analysis, where it is burned, reduced to graphite and tested for isotopic abundance using a process known as accelerator mass spectrometry. From this, the renewable content of an EfW power station can be determined.
The method itself is an established technique operated under international standards ASTM 6866 and CEN 15591/15747. In 2010, the NNFCC was invited to be the U.K. representative on the European Committee for Standardisation panel for adopting a standard measurement of renewable content. This led to the creation of a new U.K. standards committee, designed to look at the wider implications of developing a biobased standard, which helped influence Ofgem’s decision.
But the carbon 14 method has some critics, including Richard Black, operations director of the U.K. Resource Efficiency Knowledge Transfer Network, who says “The advantage of this process is that gas analysis is very representative of the actual feedstock input, but the major disadvantage of the method is that it's retrospective and expensive.”
Similarly Tony Grimshaw, technical director of Energos, says. “This is the first step that Ofgem accepts it is an alternative,” he says. “But some trials need to take place and it needs to be demonstrated on the ground. Cost is the other issue. In my view, it is probably a year away before it all beds down.”
EfW could play an important role in contributing toward European Union targets to produce 20 percent of its energy from renewable sources by 2020. However, the problem until now has been how to measure the renewable content of waste.
But all things have a built-in time clock to differentiate, between renewable carbon (carbon 14) and fossil carbon (carbon 12). By measuring this ratio in a power station smokestack, we can determine the renewable carbon content of any source material using a single physical measurement, independent of secondary information. This will allow simple regulation of certification schemes in the future.
Author: Matthew Aylott
Staff Writer, U.K. National Non-Food Crops Centre