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Renewable natural gas, also known as sustainable natural gas, is a biogas which has been upgraded to a quality similar to fossil natural gas. A biogas is a gas methane obtained from biomass. By upgrading the quality to that of natural gas, it becomes possible to distribute the gas to customers via the existing gas grid, within existing appliances. Renewable natural gas is a subset of synthetic natural gas or substitute natural gas (SNG).
Renewable natural gas can be produced economically, and distributed via the existing gas grid, making it an attractive means of supplying existing premises with renewable heat and renewable gas energy, while requiring no extra capital outlay of the customer. Renewable natural gas can be converted in to liquefied natural gas (LNG) for direct use as fuel in transport sector. LNG would fetch good price equivalent to gasoline or diesel as it can replace these fuels in transport sector.
The existing gas network allows distribution of gas energy over vast distances at a minimal cost in energy. Existing networks would allow biogas to be sourced from remote markets that are rich in low-cost biomass (Russia or Scandinavia for example).
The UK National Grid believes that at least 15% of all gas consumed could be made from sewage slurry, old sandwiches and other food thrown away by supermarkets, as well as organic waste created by businesses such as breweries.
A biomass to SNG efficiency of 70% can be achieved. Costs are minimized by maximising production scale, and by locating plant next to transport links (e.g. a port or river) for the chosen source of biomass. The existing gas storage infrastructure would allow the plant to continue to manufacture gas at the full utilisation rate even during periods of weak demand, helping minimise manufacturing capital costs per unit of gas produced.
Renewable gas can be produced through three main processes; anaerobic digestion of organic (normally moist) material, thermal gasification of organic (normally dry) material and produced through the Sabatier reaction. In these cases the gas from primary production has to be upgraded in a secondary step to produce gas that is suitable for injection into the gas grid.
Göteborg Energi and E.ON are hoping to be among the first to develop a commercial scale bioSNG plant in Gothenburg, Sweden. The Energy Research Centre of the Netherlands has conducted extensive research on large-scale SNG production from woody biomass, based on the importation of feedstocks from abroad. SNG is of particular interest in countries with extensive natural gas distribution networks. Core advantages of SNG include compatibility with existing natural gas infrastructure, higher efficiency that Fisher-Tropsch fuels production and smaller-production scale than other second generation biofuel production systems.
Renewable natural gas plants based on wood can be categorized into two main categories, one being allothermal, which has the energy provided by a source outside of the gasifier. One example is the double-chambered fluidized bed gasifiers consisting of a separate combustion and gasification chambers. Autothermal systems generate the heat within the gasifier, but require the use of pure oxygen to avoid nitrogen dilution.
In the UK, the National Non-Food Crops Centre found that any UK bioSNG plant built by 2020 would be highly likely to use ‘clean woody feedstocks' and that there are several regions with good availability of that source.
In the UK, using anaerobic digestion is growing as a means of producing renewable biogas, with nearly 50 sites built across the country. Ecotricity has announced plans to supply green gas to UK consumers via the national grid. Centrica has also announced that it will soon begin injecting gas, manufactured from sewage, into the gas grid. In Canada, FortisBC, a gas provider in British Columbia, has begun injecting limited amounts of renewably created natural gas into its existing gas distribution system to begin to offer customers renewable gas options.
Sustainable SNG is produced by high temperature oxygen blown slagging co-gasification at 70 to 75 bar pressure of liquid and solid contaminated and wood, biomass, negative cost hazardous and non-hazardous wastes, coal and natural gas. This uses coal to SNG technology developed from the end of WW2 onwards, and successfully demonstrated at SVZ Schwarze Pumpe. The same technology can be transferred from the low grade lignite to fertiliser industry, where it is currently being successfully developed in China, to the renewable energy industry.
The advantage of a wide range of feedstocks is that much larger quantities of renewable SNG can be produced compared with biogas, with fewer supply chain limitations. A wide range of fuels with an overall biogenic carbon content of 50 to 55% is technically and financially viable. Hydrogen is added to the fuel mix during the gasification process, and carbon dioxide is removed by capture from the purge gas 'slip stream' syngas clean-up and catalytic methanation stages.
Large scale sustainable SNG will enable the UK gas and electricity grids to be substantially de-carbonised in parallel at source, while maintaining the existing operational and economic relationship between the gas and electricity grids. Carbon capture and sequestration can be added at little additional cost, thereby progressively achieving deeper de-carbonisation of the existing gas and electricity grids at low cost and operational risk. Cost benefit studies indicate that large scale 50% biogenic carbon content sustainable SNG can be injected into the high pressure gas transmission grid at a cost of around 65p/therm. At this cost, it is possible to re-process fossil natural gas, used as an energy input into the gasification process, into 5 to 10 times greater quantity of sustainable SNG. Large scale sustainable SNG, combined with continuing natural gas production from UK continental shelf and unconventional gas, will potentially enable the cost of UK peak electricity to be de-coupled from international oil denominated 'take or pay' gas supply contracts.