How to make CCS feasible

A collaborative research project called CONvert has been looking at the techno-economic feasibility of installing carbon capture and storage at Danish biomass-fired CHP plants, with full heat integration, and utilisation of the subsurface for heat and CO2 storage.

The study, carried out by GEUS (Geological Survey of Denmark and Greenland), DTU (Technical University of Denmark), engineering company Rambøll, and the Norwegian research organisation SINTEF, with partial funding from the Danish government via EUDP (Energy Technology Development and Demonstration Programme), concludes that the integrated approach significantly improves the economics of CCS and makes it look much more feasible.

The full report from the project, covering modelling of geological carbon dioxide storage reservoirs, economics and energy system analysis (including the possibility of using geothermal energy and subsurface heat storage), as well as carbon capture process aspects and heat integration, will be published and openly available before the end of this year.

The CONvert study analyses a generic biomass fuelled power plant retrofitted with CO2 capture. The reference for the generic plant is the Avedøre 1 CHP plant (pictured below), located in the outskirts of Copenhagen, originally coal fired but now fully converted to biomass. The plant has an installed capacity of 640 MWt, with net electric power output of 219 MWe and district heating production of 352 MWt at full-load conditions. It operates with 100% wood pellets. For the purposes of CONvert, the Avedøre plant was considered representative of a state-of-the-art power plant that has been converted from operating with coal to biomass.

For modelling the plant is assumed to be retrofitted with a generic MEA (monoethanolamine) based capture system (3.7 GJ/t CO2 captured). In the integrated concept, steam from CHP plant is used to regenerate the amine employed to capture CO2.

The CO2 transport and injection case is based on capture from the Nordjyllandsværket CHP plant, in the Aalborg municipality, after conversion from coal (present fuel) to biomass-firing, which is currently under discussion. When leaving the capture plant, the CO2 is compressed to 110 bar, transported by 3 km pipeline, and injected into a storage reservoir 1300 m deep. 30 years injection of 1 million tonnes/ year of carbon dioxide is considered feasible via one injection well. There are several geological formations with suitable reservoir characteristics close to Aalborg.

The CONvert research has shown that it is possible to recover a considerable amount of heat from five individual heat sources in the CO2 capture plant and the CO2 compression train for use in the district heating system, significantly improving the thermal efficiency of the biomass fuelled plant and consequently reducing the cost of CO2 capture. The economic analysis, which included an assessment of capital and operating costs associated with the CO2 capture and compression process, estimated that capture costs are reduced by roughly 30%, going from about €77 to €52 per tonne of CO2 captured.

The techno-economic evaluation shows that the economic performance of the integrated power plant and CO2 capture process in terms of €/t CO2 captured is not strongly affected by the size of the capture plant and the boiler load in the investigated range (75-100 % boiler load).

The work has included detailed process modelling of the CHP facility and CO2 capture plant. Even though absorption-based CO2 capture is an end-of-pipe technology, it affects the CHP plant because of the large amount of steam needed for MEA capture. The CO2 capture plant, including compression system, was modelled with the process simulator Aspen HYSYS by SINTEF. The CHP plant was analysed using an existing model of the steam system by Rambøll. The two models were manually linked in close co-operation between SINTEF and Rambøll. Capture system heat streams evaluated in the study included: reboiler condensate (138°C); direct contact cooler; lean solvent cooler; stripper overhead condenser; and CO2 compressor intercoolers.

The integrated CHP and CO2 capture process investigated can be seen as an example of BECCS (bio-energy with CO2 capture and storage), which has the potential to be CO2 negative, given proper management of the whole supply chain from cultivation of biomass to the permanent geological storage of CO2. As of today, there are no economic incentives for CO2 negative technologies, but if they are introduced, the CO2 capture economics would be further improved.

The Danish context

The research of course has particular resonance for Denmark, which is well advanced in changing its heat and power production from coal-fired combined heat and power plants to renewables such as wind, solar and geothermal, supplemented with CO2 neutral bioenergy (biomass, biofuel and biogas), with around 60% of renewable energy consumption in Denmark being bioenergy-based.

So far, technology used for carbon capture and storage has been way too expensive, and previous attempts at implementing the technology at CHP plants have been abandoned, notes Rambøll, but “this study looks very promising so far, and the CCS technology assumed seems to be technically feasible for major CHP plants based on biomass, and with heat recovery for district heating, the economic feasibility has improved dramatically”, says Rambøll engineer Thomas Paarup Pedersen.

He observes, however, that installed wind capacity in Denmark is greater than average power consumption, and that wind+solar often exceed consumption at the weekends, so that “reduced electrical output of a CHP plant in Denmark resulting from the parasitic load of CCS plant is not as detrimental as for strictly power-producing plants elsewhere.”

Stefania Gardarsdottir of SINTEF also cautions that the results might “only be transferrable to  CHP plants operating in Scandinavia/north-west Europe, where large district heating systems are operated and there is a good business case for being a district heat producer.” In Denmark, around 65% of households are connected to district heating.

She notes that “there are a number of parameters that will affect the results, usually very dependent on the plant location and the regional energy system and market conditions” iand mentions, for example, that the study assumes a fixed CHP capacity factor of 96% (8400 operating hours per year).

In the context of BECCS and CHP plants, “it is also worth mentioning that waste incineration plants could prove to be good candidates”, she says, although this was not studied as part of the CONvert project. Waste can contain a significant proportion of biomass and incineration plants primarily produce heat in a stable manner throughout the year, which means that “the improved economics of CO2 capture arising from heat recovery is also applicable for these plants.”