Powerstep

I want my WWTP to take part in a dynamic energy market

The integrated energy market of today has a dynamic price structure and multiple levels of engagement for market players. The rising share of renewable energy (e.g. wind, solar) leads to fluctuating energy prices and new challenges in grid load management, both providing opportunities for WWTP operators to reduce their energy costs and obtain maximum revenue from selling energy to the market.

Most WWTPs today operate on long-term contracts for energy purchase and sale, leading to stable energy prices but missing out on the opportunities of the integrated energy market of today. This market is highly dynamic and offers several options for WWTP operators to optimise their energy bill, i.e. reduce costs of energy purchase and increase revenues from energy sale. Due to available storage capacity of energy at the WWTP (e.g. biogas storage) and potentials to shift energy consumption and production patterns, WWTPs can access markets with flexible energy prices or offer capacity on the load balancing market. Load shifting can be realised by adjusting the operation time of CHP units or by turning of major energy-consuming aggregates for a certain time. This can either be used for optimising self-supply of energy (e.g. fully cover the internal demand of electricity during times of high electricity price), or for offering remote control of these units to the grid operator for load balancing. Another option of dynamic energy management is the “power-to-gas” approach, allowing the conversion of low-priced renewable electricity into valuable biomethane which can then be sold at stable prices to the gas grid. Overall, integrating WWTPs into the “smart grid” of the future provides a significant potential to reduce energy costs and optimise energy management. WWTP operators can either test and implement their own strategies, or seek help of professional energy traders offering contracts for dynamic energy management.

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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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Smart grid

Smart grid

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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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Load balancing market

The electricity market of today is more dynamic due to the increasing share of renewable energy (e.g. wind, solar) that increases or decreases depending on weather conditions, adding to instability of the electricity grid. Grid system operators have to operate or buy balancing power to level these changes and keep a constant frequency in the alternating current. Hence, large consumers or producers of electricity can offer capacity at the load balancing market and get revenues for both offering capacity reserves and actually using them.

Depending on reaction time and load capacity, primary, secondary and tertiary reserves can be offered with different models of revenue. WWTPs can also offer a certain capacity for load balancing if they are able to cut or increase loads on a remote signal from the grid operator. Minimum loads for the market can be reached by combining several WWTPs in a load cluster.

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Load shifting

The WWTP is a large consumer of electricity, and the energy demand profile is relatively dynamic over the day depending on the influent load of the plant. As the electricity market of today offers different energy prices at different times of the day and also for maximum peak demand, the time-related shifting of electricity loads can be an option to decrease operational costs for electricity consumption. Maximum peak demand of the WWTP can be reduced by stopping aggregates during times of high electricity prices: pumps, centrifuges, and even aeration systems can be operated dynamically to restrict peak demand and save on costs.

In addition, load shifting can be used to align electricity consumption and production profiles at the WWTP, so that the demand for external electricity from the grid is minimised. Altogether, load shifting can be fully automated depending on electricity consumption at the plant and target profiles for optimised energy costs, following a multi-step strategy to drop off loads of several aggregates in a row if peak demand is reached.

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Optimise electricity use

Electricity demand is a major cost driver of WWTP operation, and is also responsible for a high share of the environmental impacts associated with wastewater treatment. Hence, it is of high interest to optimise the use of electricity at a WWTP to minimise both operational costs and environmental impacts of the process.

Options to optimise the use of electricity on-site include the flexible and predictable operation of the plant in terms of electricity demand and production depending on the situation on the public electricity market, following a “smart grid” strategy. Today, market prices of electricity are dynamic over time due to a rising share of renewable sources (e.g. wind, solar), so that WWTP operation can be optimised to utilise this dynamic and minimise electricity costs and revenues. Predicting and adapting electricity demand and production at the WWTP to the hourly market price profile of the next day (“day-ahead market”) enables savings in electricity costs. Storage facilities for biogas or sludge at the WWTP enhance flexibility in operation and facilitate the optimisation of the profile of electricity use and production on-site.

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