An overview of oxy-combustion

In response to a global outcry over environmental concerns, the cement industry is setting out plans to reduce CO2 emissions as part of an international drive to cut global greenhouse gas emissions to net zero by 2050. At the recent COP27, many nations renewed their commitments and highlighted both their short and long-term steps towards realising their future air-emission targets. The progress on reducing the use of fossil fuels, though, has been less promising and a cause of concern for many ‘green’ campaigners. The use of fossil fuels may still be justifiable provided carbon capture, utilisation, and storage (CCUS) is effectively rolled out, possibly with some assistance from governments, i.e. tax breaks and or soft loans. Indeed, as far as the cement and lime industries are concerned, CCUS is the only solution, perceived at present, that can help producers meet their decarbonisation targets, but due to the high costs of CCUS, there is not yet a commercial application in operation either in the cement or lime sectors. However, a recently compiled IEA report quotes, CCUS costs are gradually falling, with ample potential for further reductions over the next 10 years. As per THE Global CCS Institute’s 2022 Status Report, it is emphasised that without CCS, reaching the industry’s shared climate goal is practically impossible and states that the outlook for CCS has never been more positive, which is good news for meeting the CO₂ mitigation targets. Cement and lime plants have responded to the future CO₂ reduction needs and have initiated several research and demonstration CCUS projects worldwide.

CO₂ capture technologies

The following are some of the most promising CO₂ capture technologies either commercially available or in various stages of development. There are two main categories, the precombustion and post-combustion separation of CO₂, the former is more expensive and complex for a cement/lime plant implementation and therefore not included here.

• CO₂ capture via absorption
• Adsorption-based CO₂ capture
• CO₂ Capture through membranes
• Cryogenic separation of CO₂
• Calcium looping cycle
• Oxy-combustion

This article will focus on oxy-combustion as a potential pathway to carbon neutrality which would require minimal plant modifications to implement.

Oxy-combustion

In comparison to other CO₂ separation technologies, oxy-combustion is an attractive option for the cement industry, involving the enrichment of CO₂ via oxygen injection and/or use of H₂ as a fuel. This would require minimum alterations to the components of a plant, at least when oxy-combustion is applied to the calciner where over 80% of the CO₂ enrichment level can be achieved. In the case of full implementation involving a kiln, more than 95% CO₂ enrichment is possible, thereby skipping the CO₂ separation step and moving directly to filtering, compressing, and making subsequent use of the CO₂. When N2 from air is replaced during oxy-combustion, CO₂ recirculation is required in order to ensure similar kiln and calciner temperatures. This will require some clinker cooler modifications due to the reduction in gas flow rate and re-establishing the combustion and calcination zones so that the effect on calcination reactions under a higher partial pressure of CO₂ is taken care of. Therefore, the most critical component being the calciner, where the appropriate retention time for effective calcination reactions to be completed within conventional residence time, since CO₂ enrichment (gas stream with higher partial pressure of CO₂) inhibits the release of CO₂ from CaCO₃ (calcination). A consortium of European companies and the European Cement Research Academy (ECRA) have been studying this issue as part of its CCUS programme for the cement industry for over five years.2 The results have so far been encouraging and the programme is seeking funds for a CCUS demonstration cement plant. Currently, a multinational cement company, with the assistance of Scandinavian R&D institutes and Scandinavian industry, have initiated feasibility studies to look at the possibility of capturing over 50% of CO₂ at a Scandinavian cement plant. The project objectives are to examine the extent to which excess energy from cement production can be utilised for capturing CO₂. A new EU-financed project in Eastern Europe will be the first of its kind to capture all of its CO₂ and divert it through a pipeline system with offshore permanent storage under the Black Sea, it could start operating as early as 2028.

With the possibility of making use of H₂ as a fuel in oxy-combustion, the overall process may become more economical provided that the green power is available to produce both oxygen and hydrogen using dedicated electrolyser(s). Hydrogen firing or cofiring with biomass, may also be considered with oxy-combustion (oxygen and carbon dioxide enrichment), as green hydrogen in future will mostly be produced onsite, using large electrolysers powered by wind and/or solar power. This will open up additional R&D fields expressing safety and handling measures of H₂ and O₂, which at present have never been utilised at a cement/lime plant. Separately, oxy-combustion has also been actively studied and with the development of low-cost CO₂ sequestration technologies, mineral CO₂ emissions can also be harnessed. When these two separately-developed technologies are combined, a plant may operate one kiln on oxy-combustion and the other on a fuel switching concept, or use both technologies in a kiln or calciner. Future cement plants could use both O₂ and H₂ streams produced from an onsite electrolyser, achieving the zero CO₂ target with substantial reduction in H₂ and O₂ production costs – a plausible solution, in conjunction with CCUS, to work on for the cement plants of 2050 and beyond. Within the cement industry, due to both technical and financial constraints, CCUS has thus far not been demonstrated at the full scale. Apart from CO₂ storage costs, the main technical challenges relate to the effective use of the oxygen/carbon dioxide mixture, which substitutes the nitrogen in the combustion/cooling air, and the minimisation of air ingression so that a CO₂ enrichment in excess of 95% is assured. Two experimental studies, conducted within ECRA Phase IV results have shown no adverse effect on clinker quality, refractory life, or hot spots; however lower levels of calcination were observed due to the higher CO₂ concentrations inhibiting the calcination reaction.

For oxy-combustion, small-scale experiments for stationary particles reveal that an increase in the calcination temperature of about 80°K can be expected for an increase of the gas-phase CO₂ partial pressure of 80%. However, the effect on the gas residence time remains relatively unknown, as the particles/stones were static during the experiments, implying that at full scale, both the calciner exit temperature and meal LOI will be affected under CO₂ and O₂ enrichment conditions as well as due to flow stratification issues.

Enjoyed what you've read so far? Read the full article and the rest of the October Issue of World Cement by registering today for free!

Read the article online at: https://www.worldcement.com/special-reports/27102023/an-overview-of-oxy-combustion/