Flushing for fuel
10 Jan 2011
As they cook their meals and heat their homes, residents in some 200 Oxfordshire households must not think too hard about their source of fuel: they are the first in the UK to be supplied from a grid with biomethane gas produced from human waste.
This outcome follows from the recent addition of a biomethane plant at Didcot sewage works in a £2.5-million project managed by a joint venture between Thames Water, British Gas and Scotia Gas Networks (SGN).
Sewage arrives at the Didcot works from some of Thames Water’s 14 million customers to be treated at the plant. The entire process - from flushing a toilet to gas being piped to people’s homes - is estimated to take just 20 days.
The residents’ ’first’ aside, the pilot project could have a greater significance by heralding the arrival of biomethane-to-grid in the UK.
According to Centrica, the UK’s 62.5 million population could, in theory, generate enough renewable gas to meet the annual demand of 200,000 homes, up to 1% of the UK’s population.
More realistically, National Grid projects that biomethane from all sources could account for at least 15% of the domestic gas market by 2020.
Changing requirements
Exploiting this energy resource, however, will require the UK regulatory authorities to change their current requirements regarding the injection of biomethane into the gas network.
Unlike in many other countries, including Austria, Canada, Germany, The Netherlands, Sweden and Switzerland, biomethane injection does not, as yet, take place in the UK. This has, in part, been due to the lack of incentives for investment in these facilities, but also because of the technical specifications governing the quality of gas entering the network - barriers that could all soon come down.
Industry is now set to benefit from a feed-in tariff as part of the Renewable Heat Incentive (RHI), which is set to commence in April 2011. Full details of the incentives scheme are to be announced by the UK government by the end of the year.
The importance of the RHI scheme is underlined by another Didcot project partner, CNG Services. The project management and consultancy firm is working on a further 20 biomethane-to-grid projects which it expects to go ahead “providing the RHI is introduced in 2011 at a level that matches the return from generating electricity”.
Biomethane injection also currently falls foul of the UK’s stricter limit on the oxygen content of gas entering the national grid: this is set at 0.2%, compared with a level of 1% and higher in many other EU countries.
Tough choice
Tougher still, when it comes to measuring the calorific value of biomethane, UK energy industry regulator Ofgem approves only one device: a gas analyser which is designed for monitoring hydrocarbons from the North Sea, and that costs more than £100,000.
This requirement prevents the use of lower-cost options widely used in the EU, which the UK industry argues are more than capable of monitoring biomethane.
Industry is currently hopeful that Ofgem and the Health and Safety Executive will soon resolve both technical issues regarding oxygen content and gas measurement, said Stephen McCulloch, UK business development manager, Chesterfield Biogas, which supplied the gas-cleaning system for the Didcot project.
According to McCulloch, Ofgem is closely monitoring the results of the Didcot project, as it realises that the need to review the specifications in this potentially huge area of the energy sector - to replace the current approach of special exemptions.
“With around a dozen biomethane-to-grid projects in the pipeline, Ofgem needs to establish new standards to allow the use of biomethane in the grid at [oxygen] levels above the current 0.2% regulatory maximum,” he commented.
Ofgem is also reviewing its current requirement to use only the one approved gas analyser, said McCulloch. He expects Ofgem to soon allow the use of less expensive bespoke devices that can detect the limited number of molecules encountered in the production of biomethane.
At the Didcot site, engineers are learning to deal with the new process in which sewage sludge is treated in warmed-up anaerobic digestion (AD) vats - sealed tanks, which were already in place at Didcot - where bacteria break down biodegradable material, yielding biogas. The AD process leaves a sludge that can be dried and used as fertiliser.
Challenges involved with the AD operation include keeping output up, especially in winter when the air is cold, said an SGN spokesman. The gas scrubber, which removes impurities from the raw biogas to yield biomethane, also requires careful set-up at the plant to deal with the variable gas volumes involved.
The biogas production process typically generates a mixture of 65% methane and 35% carbon dioxide. For effective biomethane fuel use, it is desirable to increase the methane content and remove as much carbon dioxide as possible.
The biogas produced at Didcot, therefore, undergoes a clean-up and upgrading process using Chesterfield Biogas’s water-wash technology. This removes moisture, CO2 and H2S to produce a clean, dry gas composed of more than 97% biomethane.
Before the gas can be injected into the grid, a small amount of propane must be added so the calorific value of the biomethane matches that of the gas in the SGN grid. The system at Didcot has also been designed to operate at the low flow rates encountered with the process.
Another key piece of equipment at the Didcot site is a biomethane-to-grid skid, which has been specially designed to deal with the particular characteristics of the gas. The skid includes a meter to measure the energy flowing into the network, an analyser that ensures the biomethane produced meets gas safety regulations, an odorising plant to provide the characteristic smell that is added to all gas entering the network, and pressure-control equipment.
Flows into the network and outputs from the analyser are monitored in the site’s system control room. Telemetry is used to ensure that any changes can be monitored in real time and the information is used for metering purposes, and also fed back to the biomethane producer.
Apart from the £2.5-million capital investment, there are no additional costs involved in producing gas via the new plant compared to a conventional process, said the SGN spokesman. “The fact that the digesters were already in situ meant that no additional work was required. The capital investment figure includes all feasibility, design and construction of plant and control systems,” he explained.
The economics of future projects will, therefore, depend on the facilities in place at sewage-treatment works, the spokesman adding: “For example, existing sewage-treatment plants may have existing anaerobic digesters that can be used, whereas new plants would require them to be constructed and costs would therefore need to be assessed.”
