Although the Renewables Obligation Scheme is being phased out, it is still central to the UK renewable energy strategy.
But how does it work?
In this article we will explore the current state of renewables generation from different gas-based sources.
The Renewables Obligation (RO)
The Renewables Obligation is the main support mechanism for large-scale renewable energy projects across the UK. For smaller scale generators, the Feed-In-Tariff scheme has been the main source of support.
The RO first started in 2002 for England, Scotland, and Wales – closely followed by Northern Ireland in 2005. This scheme places an obligation on UK electricity suppliers to source an ever-increasing proportion of their electricity that they supply from renewable energy sources.
The scheme closed to all new generating capacity on the 31st of March 2017. Additionally to this there are also a number of early closures which are in force for specific technologies. Generators who are eligible can apply for a grace period can gain entry to the Renewables Obligation after these closures for a specified amount of time.
Renewables Obligation Certificates (ROCs)
The certificates issued to operators of accredited renewable generating stations for the renewable electricity they generate are called Renewables Obligation Certificates (ROCs). Operators are able to trade their certificated with other parties and are ultimately used by suppliers to demonstrate that they have met their obligations. ROCs are useful to suppliers who are incentivised to support the growth of renewable generation. The number of ROCs received for each MWh or electricity produced depends on the technology (wind, solar PV, anaerobic digestion, etc.) of the generation assets and when it was built. This process is called ROC banding and helps to promote newer technologies which will then receive more support with multiple ROCs per MWh.
Where suppliers do not present the minimum number of ROCs to meet their obligations in the reporting period (one year) they will be charged an equivalent amount to be paid into a buy-out fund. The administration cost of the scheme is then recovered from this fund and the rest distributed back to suppliers in proportion to the number of ROCs they produced in meeting their own obligations.
| April | May | June | July | August | Total | |
| Biomass 50kW DNC or less | 1,963 | 2,161 | 1,909 | 1,885 | 1,874 | 9,792 |
| Landfill Gas | 212,740 | 218,002 | 209,706 | 206, 223 | 202,863 | 1,049,534 |
| Sewage Gas | 53,530 | 53,436 | 48,753 | 46,622 | 42,361 | 244,702 |
Biomass Generation
Biomass is a general term for plant-based materials used as a form of fuel to produce heat or electricity. Common examples are wood or wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. As biomass can be used as a fuel directly in the form of wood logs for example, some people use the terms ‘biomass’ and ‘biofuel’ interchangeably.
Governments in the EU and US define biofuel as either a liquid or gaseous fuel used for transportation. The European Union’s Joint Research Centre use the concept of solid biofuel and define it as raw or processed organic matter of biological origin used for energy, like firewood, wood pellets or chips.
Around 86% of modern bioenergy is used for heating, a further 9% used for transport, and 5% for electricity. Interestingly, most of global bioenergy is produced from forest resources. Power plants that use biomass as fuel can consistently produce a stable power output, unlike other renewables (solar and wind).
In 2017, the International Energy Agency (IEA) described bioenergy as the most important source of renewable energy. They also stated that the current rate of bioenergy deployment is still well below the levels crucially needed to reach low-carbon scenarios, and that accelerated deployment is sorely needed. In the Net Zero by 2050 scenario, traditional bioenergy is actually fully phased out by 2030 and replaced by modern bioenergy, increasing from 6.6% to 13.1% in 2030, and 18.7% in 2050. In 2014, the International Renewable Energy Agency (IRENA) projected a doubling of energy produced from biomass in 2030, also including a small amount from traditional bioenergy. There is an argument from the Intergovernmental Panel on Climate Change that bioenergy has a significant potential to mitigate climate change if done right.
Landfill Gas
Over time, the organic mater that is buried in landfill undergoes a process called anaerobic decomposition. This simply means a breakdown of the organic matter without oxygen, and results in the generation of gases including methane. When this is released into the atmosphere, methane acts as a greenhouse gas that traps heat and contributes to global warming. It does so at a rate 25 times the rate that carbon dioxide does. Up to half of all waste in landfill is organic matter such as food, textiles, timber, and household waste.
The solution could possibly lie in landfill gas capture and processing to transform this gas into a valuable resource – energy. Gas is collected and processed, and is then burned to power many homes. Benefits of this type of generation are a reduction in greenhouse gas emissions, power generation for the grid, and less demand of fossil fuels. There is also the additional benefit of reducing health and safety issues such as odour emissions and the risk of landfill fires caused by gases.
Sewage Gas
There was once a sentiment among wastewater professionals to accept the high costs of operating wastewater treatment facilities as a consequence of meeting their discharge permit requirements. As the cost of energy rises and the emphasis placed on renewable energy increases, local authorities and governments are exploring novel solutions to save money and meet renewables requirements. Gas engines are a potential avenue offering combined heat and power technology that could result in long-term savings for wastewater treatment plants. Alternatively, biogas upgrading plants can convert biogas to biomethane for injection into the gas grid or even for vehicle fuel.
Benefits of CHP at Wastewater Treatment Plants
- Generation of renewable energy from waste material through cogeneration or CHP
- Reduction of carbon emissions especially when compared to aerobic sewage treatment
- Economical onsite electrical power production and reduced transmission losses
- Production of a low-carbon fertiliser or soil improver
- Cost effective and proven technology
Costs of Wastewater Treatment
Treating waste water includes energy-intensive operations like aeration and pumping. As a result, wastewater treatment plants can require significant energy to fuel their functioning. As electricity prises continue to increase, plant operators face spiralling energy costs in order to meet their discharge permit requirements. The second highest expense to plant owners is the cost of energy, falling only behind personnel. For plants who employ anaerobic digestion for biosolids treatment, the process of combusting digester gas to produce electricity and heat through cogeneration or CHP may provide a solution to rising energy costs.
A large proportion of the world’s sewage systems do not even attempt to recover value from the sewage in the form of electricity and heat. But, the renewable fuel that can be recovered from sewage gas can be converted to electricity and heat, offsetting as much as two-thirds of a plant’s electricity demand and eliminating the need to purchase fossil fuels.
How does conversion work?
- Preparation of the input material including the removal of physical contaminants
- Digestion and fermentation, consisting of hydrolysis, acetogenesis, acidogenesis and methanogenesis
- Conversion of the biogas to renewable electricity and useful heat through cogeneration or CHP
- Post-treatment of the digestate
Advantages of Sewage Gas CHP
- Seamless dual fuel mixing maximising renewable energy output and smoothens gas production fluctuations by supplementing with natural gas when required
- High electrical efficiencies as it can generate more electricity per unit of sewage gas used. Electrical efficiencies of up to 43% based upon the lower heating value of the gas
Niccolo Gas and Power
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