Danish researchers have shown it’s possible to remove most of the carbon dioxide (CO2) from waste incinerator emissions.

A key objective of net zero is to develop and exploit decarbonisation technologies such as carbon capture and storage (CCS) and carbon capture, utilisation and storage (CCUS).

A pilot plant is now operational in Copenhagen using a novel gas monitoring technology to optimise plant efficiency.

Researchers from the Technical University of Denmark (DTU), are working on a process which captures carbon dioxide (CO2) from emissions, using advanced gas analysers from industrial measurements firm Vaisala to measure carbon capture efficiency and CCUS viability.

The pilot plant is based at the Amager Bakke Waste-to-Energy Plant, one of the largest combined heat and power (CHP) plants in northern Europe, with the capacity to treat 560,000 tonnes of waste annually. (It features several innovations including a rooftop artificial ski slope).

The pilot plant was developed to capture CO2 from the emissions of processes such as wastewater treatment, biogas production, anaerobic digestion and waste incineration. However, the researchers are also investigating ways in which CO2 can be both captured and utilised. Prior to its installation at Amager Bakke, the pilot carbon capture plant was operated at a wastewater treatment plant. “The technology itself is not new,” explains DTU researcherJens Jørsboe. “However, the focus of our work has been to lower the cost of carbon capture, so that it can become economically feasible.”

How it works

Exhaust gas from the Amager Bakke incinerator is passed through an electrostatic precipitator (ESP) to remove particulates, NOx compounds are removed by selective catalytic reduction (SCR) and a scrubber removes oxides of sulfur. High levels of CO2 remain in the flue gas and the main purpose of the pilot carbon capture plant is to investigate the feasibility of its capture. To achieve this, the gas is passed upwards through a column packed with beads and a monoethanolamine (MEA) solvent which scrubs the CO2 from the gas. The solvent is then passed to a desorber which removes the CO2, now almost pure, and regenerates the MEA for re-use. As a research project the produced CO2 is currently still vented to air, but on a commercial basis there are many different industrial applications in which CO2 can be utilised. For example, CO2 can be reacted with hydrogen in the Sabatier process to produce methane (a gas fuel) and water, at elevated temperature and pressure, in the presence of a nickel catalyst. This can be a green method for manufacturing fuel if the hydrogen is generated by electrolysis using renewable energy – from solar, biogas or wind power for example.

CO2 is also used in a wide variety of other industries including food and beverages, refrigeration, medical, horticulture, firefighting, welding etc., so a variety of potential markets are available if CO2 can be produced on a commercial quality and scale.

Monitoring carbon capture efficiency

The optimisation of the carbon capture process can only be achieved if CO2 concentrations is continuously monitored before and after the carbon capture process. The world’s first inline CO2, humidity and methane monitor was developed by Vaisala in Finland prior to the pilot plant construction.

Exhaust gases from incinerators can be corrosive and potentially explosive, so in the past it has not been possible to conduct in-line monitoring. Until recently, the only solution was to extract samples for analysis outside of the process, but this method is not suitable for process control and optimisation, and has a number of inherent flaws, such as the need to remove humidity from the sample line and a requirement for frequent re-calibration.

The development of Vaisala’s multi-gas probe, the MGP261, resolved these monitoring challenges, especially when it was followed by sister product the MGP262, which was adapted for measuring high concentrations of CO2 and therefore ideal for the continuous inline monitoring of almost pure CO2 after the pilot plant’s desorber.

The pilot plant employs three Vaisala probes in total, with the MGP261 monitoring incoming incinerator exhaust gas, and the MGP262 measuring the purity of the extracted CO2. The third probe is a Vaisala CARBOCAP® CO2 probe, the GMP251, which checks the levels of CO2 (after carbon capture) in the pilot plant’s exhaust gas.

Monitoring technology

All three monitoring probes contain CARBOCAP® technology which utilises an electrically tunable Fabry-Pérot Interferometer (FPI) filter. In addition to measuring the target species, the micromechanical FPI filter enables a reference measurement at a wavelength where no absorption occurs. When taking the reference measurement, the FPI filter is electrically adjusted to switch the bandpass band from the absorption wavelength to a non-absorption wavelength. This reference measurement compensates for any potential changes in the light source intensity, as well as for contamination in the optical path, which means that the sensor is highly stable over time.

Within the MGP261 and the MGP262, humidity and CO2 are measured with the same optical filter, and a second optical channel measures methane. In many ways, this combines the analytical power of a laboratory spectrometer with the simple, rugged design of an industrial process control instrument.

Jens says: “We have been delighted with the accuracy and reliability of the multigas probes; not least because they have enabled us to learn a great deal about the management of flue gas from waste incineration. Much is known about the emissions from fossil fuel combustion, but less information is available on the emissions from waste incineration.

“The technology employed by the Vaisala probes is also helping to minimise operational costs because by effectively calibrating themselves the probes’ service requirements have been minimal and downtime is avoided.”

Carbon capture

With the benefit of continuous inline monitoring, the researchers have been able to optimise carbon capture performance following an evaluation of 12 different pilot plant configurations. Having proven the viability of the carbon capture process, the next step was to evaluate the relative advantages of carbon storage and utilisation. Jens adds: “At the moment, utilisation of CO2 is the more expensive option because of costs associated with the required further refinement of the CO2, so the owners of the Amager Bakke plant are planning to apply for 1.5 billion DKK ($230 million USD) for a CCS plant capable of capturing 500,000 tonnes of CO2 per year – if the right regulatory framework and sufficient funding is provided by the Danish state. This plant would employ the same amine scrubbing process that has been proven by the pilot carbon capture plant.”

The incineration of 1 tonne of municipal waste (MSW) is associated with the release of between 0.7 and 1.7 tonnes of CO2, depending on the content of the waste. Consequently, energy generation from waste incineration is more carbon intensive than the burning of fossil gas, so carbon capture offers an opportunity to manage the growing requirement for municipal waste treatment without generating unacceptably high levels of GHGs.

Jens believes this technology could be applied at every waste incinerator in the world, around 2,500 WtE plants, with a disposal capacity of around 400 million tonnes of waste per year. It should also be possible to harvest residual heat, which could be transferred to local industry or a district heating network.