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Methane sequestered and waste heat put to work

Germany is testing whether a nitrate salt thermal storage system could aid the provision of process steam for industry while earning subsidies for capturing emissions of methane, a potent greenhouse gas, and combusting it for energy supply

A combined heat and power station in Germany is exploring the business viability of a nitrate salt thermal storage system that supports the provision of process steam to industry while earning subsidies for capturing emissions of methane, a potent greenhouse gas, and combusting it for energy supply

Industrial steam:
The temperature of steam used to drive turbines for electricity is neither high enough nor sufficiently stable for use in industrial processes that need super-hot steam in a continuous flow

Experiment:
Can a nitrate salt thermal energy storage system charged with waste heat from the combustion of methane replace one of two boilers that supply super-hot steam derived from burning natural gas

Deciding factor:
The real test will be whether the value of the system’s fuel savings and greenhouse gas reductions from burning less natural gas is greater than the cost of the thermal salt storage
A small combined heat and power (CHP) station at Wellesweiler in the German state of Saarland is testing whether a nitrate salt thermal storage system could end the need for a permanently running back-up system for steam supply to a neighbouring plastic foil production plant. The challenge is ensuring the temperature of the steam supply stays constant since if it drops below 285°C, the plastic foil sticks to the production rollers snarling up the system. The Wellesweiler power station is a small unit with an electrical generating capacity of five megawatts (MW) and steam take-off of 27 tonnes an hour. It runs mainly on mine gas and is one of around 200 cogeneration power stations using fossil and renewable fuels owned or operated by Steag New Energies. Construction of a high temperature heat store at Wellesweiler began in September 2018. In the power station, mine gas is combusted to heat water, with the resulting steam powering a turbine which drives an electricity generator. The high temperature waste heat from the process is used to create the steam needed in plastic foil production and for other customers of super-hot steam. Two separate natural gas fired back-up steam boilers cover customers’ higher steam needs in winter. In summer, the second boiler is maintained at minimum load ready to correct pressure levels and provide emergency back-up if needed. Although underground mining in Saarland has ceased, mine gas with a methane content of 30-80% continues to be released and collected for local use. In Germany, electricity generated from mine gas is supported in the same way as that from renewable sources because combusting it for electric generation with carbon dioxide (CO2) as a waste product is preferable to letting the methane escape into the atmosphere. The climate impact of methane is more than 25 times greater than CO2 over a 100-year period, says the US Environment Protection Agency.

Stringent steam supply

The nitrate salt thermal system, by eliminating the need for a back-up summer boiler, reduces costs and the emissions associated with running a natural gas plant on standby. Carbon dioxide emissions savings will depend on the weather and temperature, how efficiently the combined power station and storage system is running, and customer steam demand. The need for an exact and steady supply of steam at a constant temperature rules out the conventional industrial steam storage method of saturated liquid water in a so-called steam accumulator, since the temperature and pressure falls when heat is extracted. The choice fell, instead, on latent thermal storage where the input energy is stored through the change of one material state to another, usually from solid to liquid. Energy is released, when required, by allowing the material to return to a solid state. The important advantage of latent storage is that the temperature remains constant during the phase change from liquid to solid,” explains Andreas Dengel at Steag New Energies. Sodium nitrate was selected as the salt because it solidifies at 305°C, slightly above the steam temperature required for the plastic production process. The salt is melted using waste heat produced during the gas turbine operation. The eight-metre-high nitrate salt storage tower at Wellesweiler, with a 1.5 by 2.5 metre footprint, will store 1.5 MW hours of heat. Should steam supply from the waste heat boiler fail, the store will kick in, releasing heat as the salt transforms from a liquid to a solid state at a constant 305°C. The process is designed to take over steam supply within two minutes, maintaining it for the 15 to 20 minutes needed to bring one of the back-up steam boilers into full service. Melting the salt to recharge the thermal store will require 12 to 18 hours.

Costs and savings

Capital expenditure is estimated at €85 a kilowatt hour and €74 a kilowatt capacity, says a report from the International Energy Agency published in September 2018. Larger thermal stores would probably lower capital expenditure costs. Wellesweiler’s commercial business case may rely on saved fuel and CO2 emissions allowance costs and, potentially, higher electricity earnings. Germany’s renewable energy act guarantees €41.7 a megawatt hour (MWh) for mine gas generated electricity compared with €61.1/MWh for onshore wind, the average winning bid at a 1 February 2019 auction. The Wellesweiler gas turbine is therefore run whenever possible. Industrial companies with cogeneration plants that produce and use high temperature steam are interested in the nitrate salt thermal storage system. Latent thermal storage systems save fossil fuel, are quiet and can provide high energy density, low volume storage, an advantage where space constraints exist. The real test will be how the costs of the Wellesweiler project stack up against the cost of the fuel saved and the associated savings in CO2 penalties. Ultimately, the commercial viability of latent thermal storage will depend on the specific application, says DLR, the German Aerospace Centre, which designed the system with support from German government funds.

Writer: Sara Knight