Powerstep

I want to make use of excess energy at my WWTP

Energy can be produced in various forms at a WWTP, e.g. biogas, electricity, or heat. Some parts of this energy may not be fully utilised, such as biogas which is flared, or waste heat that is not used. Making use of this excess energy can improve the energetic profile of the WWTP.

Energy is mainly produced in form of biogas from anaerobic sludge digestion, which can then be converted on-site into electricity and/or heat. However, most WWTPs do not fully utilise their full energy potential, burning biogas in a flare or wasting excess heat that is produced in a CHP plant. Making use of excess energy at a WWTP can improve the energetic profile of the plant and generate new revenues for the operator. Biogas can be converted on-site into electricity and heat with CHP plants or microgas turbines. It can also be upgraded to biomethane by separating CO2 and impurities, making it ready for direct injection into the public gas grid. “Power-to-gas” approaches can further improve the economics of biogas valorisation, converting CO2 from the biogas into biomethane using available low-priced renewable electricity from the grid. This biomethane can finally be injected into the grid after cleaning, providing stable revenues and virtually unlimited capacity of the market. The “heat-to-power” approach converts excess heat (e.g. from a CHP plant) into electricity, using innovative technologies such as Steam or Organic Rankine Cycle units or thermoelectric generators.

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Heat to power

Heat-to-power

At a WWTP, heat is mainly required to heat the digestor which operate at temperatures between 35-38°C, but also for heating of buildings or specific processes such as ammonia stripping. Heat is also produced on-site in CHP plants or gas boilers, which are usually operated with the biogas produced in anaerobic digestion. The overall heat balance of a typical WWTP is often positive, meaning that excess heat is blown into the air because it exceeds the demand on-site, and heat sale to external customers is not feasible.

This excess heat can be partially valorised with processes that turn heat into electricity (“heat-to-power”), drawing on excess or low-grade heat of the WWTP and increasing electricity production at the plant. Available technologies for heat-to-power include steam or organic rankine cycle units, or thermoelectric generators. The economic feasibility of these units also depends on the actual seasonal heat profile of the WWTP with varying amount of excess heat in summer and winter operation.

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Technologies

Steam Rankine Cycle/Organic Rankine Cycle

When operating a CHP plant at a WWTP, heat is available in the cooling water and exhaust gas. While a large part of this heat is used for internal purpose (e.g. heating of digestor), surplus heat is wasted as excess to the atmosphere via exhaust gas or plate coolers. This heat can be valorised by heat-to-power technologies such as steam rankine cycle (SRC) and organic rankine cycle (ORC).

These technologies convert low-grade heat into electricity using a thermal working fluid as heat carrier, which is steam in SRC and an organic fluid of high molecular weight in ORC. This working fluid is pumped to a boiler where it is evaporated, passed through an expansion device (turbine or other expander) to drive a generator, and then through a condenser heat exchanger where it is finally re-condensed. The choice of the working fluid highly depends on the temperature level and operational characteristics at which the process works to gain maximum efficiency. SRC is suitable to utilise heat at high temperature (e.g. exhaust gas), while ORC can also be adapted to use heat at lower temperature (e.g. cooling cycle). Usually, a SRC or ORC process can convert excess heat to power with an overall efficiency of 12-20% depending on working fluid and size of the system. Economic feasibility of SRC/ORC systems depends on the capital cost of the system and annual operating time, i.e. seasonal availability of excess heat.

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Thermoelectric generators

Thermoelectric conversion is a heat-to-power technology under development that can be applied to improve the electrical performance of a cogeneration plant (CHP). The thermoelectric generator (TEG) is integrated into the exhaust line of the CHP engine, replacing the conventional exhaust gas heat exchanger. It partially converts exhaust gas heat into electricity by utilising the physics of the Seebeck effect. Remaining heat energy is recovered like in a conventional heat exchange system.

Due to the absence of any movable parts, the maintenance requirements for TEG are very low. However today, the electricity production is still considerably lower in comparison with other commercially available heat-to-power technologies (e.g. based on Organic-Rankine-Cycle). Once the thermoelectric conversion technology is leveraged by high volume production like e.g. for automotive or consumer markets, a TEG system in a CHP is expected to amortise in about 5 years.

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Power to gas

Power-to-gas

Biogas produced in anaerobic digestion contains around 60% methane and 40% CO2. While methane can be readily utilised as fuel for CHP plants or directly injected into the gas grid, CO2 cannot be valorised and is usually emitted to the atmosphere. Using a power-to-gas (P2G) technology, this CO2 could also be converted into valuable biomethane by methanation.

This process describes the conversion of CO2 and hydrogen into methane, which can be realised by catalytic or biological processes. Hydrogen can be produced with an electrolyser operated during times of low electricity prices, producing H2 and O2 from water. While H2 is used for methanation, O2 and excess heat of the electrolyser can be used on-site to cover the internal demand of oxygen for the activated sludge system or to provide heat to the WWTP. The use of a P2G plant enables the integration of WWTP infrastructure into the dynamic energy market of the future and optimises economic revenues from biogas production.

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Technologies

Biological methanation

Biological methanation uses specific microbial metabolisms to generate biomethane out of CO2 and hydrogen. Archaea bacteria are capable to realise this conversion, using CO2 from biogas upgrading and H2 from water electrolysis. The technology has been realised in pilot and full-scale plants up to 1MW of electrical power in electrolysis. The reactor design is crucial for an efficient process to overcome the physical challenge of dissolving H2 into water. The process is scalable and can be operated at full or part-load with short start-up times, operating either on biogas directly or on CO2 from biogas upgrading.

The produced biomethane can be injected to the gas grid for large-scale and long-term storage or used locally, e.g. as fuel for transportation. Besides the energy storage aspect, biological methanation contributes to CO2 recycling, and electrolysis generates valuable by-products such as oxygen and heat at 65oC for direct use at the WWTP.

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Catalytical methanation

Biogas produced in anaerobic digestion of sewage sludge contains 60-65% methane and 30-35% CO2. If CO2 is separated from biomethane in an upgrading step, it can be used in a Power-to-Gas approach to be converted to biomethane. Utilising hydrogen which is produced in an electrolyser, the process of methanation converts H2 and CO2 into CH4 (Sabatier reaction).

Methanation can be reached in a physico-chemical process with catalytic methanation, or using microbial activity in bio-methanation. Catalytic methanation uses a metal catalyst to enable the chemical reaction, but requires high temperature (350-550°C) and pressure (1-100 bar) and is affected negatively by gas impurities. Currently, catalytic methanation is limited to a CO2 conversion of ca. 80%. Full-scale application of catalytic methanation is still under development, and different reactor configurations and operating modes are tested.

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Gas scrubbing

Biogas from anaerobic digestion of sewage sludge contains around 60-65% of methane, 30-35% CO¬2, and other impurities such as N2, H2S, and siloxanes. An alternative to valorisation of this biogas onsite (e.g. in a CHP plant) is the direct injection of gas into the public grid as biomethane. However, gas quality of the injected biomethane has to comply with defined standards of the gas grid operator. A high heating value requires a concentration of >95 Vol-% of methane in the injected gas. This can be achieved by gas scrubbing, which describes the separation of methane and CO2 in a physico-chemical process.

Gas scrubbing can be realised with adsorption using water, amine or other organic scrubbers or pressure-swing adsorption on activated carbon or zeolite. More recently, membrane-based scrubbing or cryogenic scrubbing have been developed. Important features of scrubbers are working pressure and related energy demand, methane content in upgraded gas and methane losses, and heat requirement. Before gas injection, further cleaning can be necessary to remove impurities such as H2S, siloxanes, and other hydrocarbons.

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