Utilisation of industrial alkaline streams for capture of CO₂

Mitigating global climate change is probably the largest environmental challenge of our age. Although the problem has been known for decades, the use of fossil fuels continues to rise generating more CO2 emissions each year. Carbon capture and storage (CCS) processes have been considered one of the main technological means for curbing global climate change alongside energy efficiency and switching to use low-carbon energy sources. As large investments are required for enabling CCS, alternative, process-specific solutions are being investigated that could provide a partial reduction of CO2 emissions by utilising alkaline mineral waste materials. In this work, the potential to use alkaline streams, such as alkaline by-products, wastes and internal process streams, for capture, storage and/or utilisation of CO2 was assessed. The study was limited to Finnish industry. Only plants emitting more than 100,000 t CO2 were assessed. In addition, also metal refineries and the mining industry were included in the survey. The preliminary results from the study show that there is a significant theoretical CO2 capture capacity in the various alkaline material streams in Finnish industry. However, in all cases there are constraints to the utilisation of these streams for practical CO2 capture. For instance, the two steel mills in Raahe and Tornio produce about 800,000 t/a of steelmaking slag each, containing a significant share of calcium silicates and calcium oxides/hydroxides that could be converted into calcium carbonates. However, a large part of the slag produced is already being utilized for various purposes in civil and highway engineering, e.g. soil conditioners, road construction, and as a raw material in industrial process replacing natural minerals. Therefore, the practical utilization potential would be less than 100,000 t/a CO2 per facility. The pulp & paper industry in Finland is based on the Kraft process that process large amounts of sodium hydroxide: the ten largest pulp mills each process 100,000-230,000 t/a NaOH. Most of this is regenerated using calcium hydroxide, requiring only about 10% make-up chemicals. The NaOH use corresponds to a theoretical CO2 capture capacity of 110,000-250,000 t/a, or 7-17% of the total CO2 emissions of the plants. As both NaOH and Ca(OH)2 are needed as input for the Kraft process, utilising these streams for CO2 removal would require additional steps for regeneration or doubling the lime kiln capacity for calcination of produced CaCO3 (to regenerate hydroxide), not necessarily making CO2 capture any cheaper than conventional CO2 capture processes. The mining industry has by far the largest theoretical CO2 storage potential, with over 50 million tonnes of mineral waste being produced each year. While waste rock from mining is readily available, natural magnesium- and calcium silicate containing minerals require mechanical, thermal and/or chemical activation in order to speed up their reaction with CO2 to time scales that are of relevance for industrial processes. This work was carried out in the Carbon Capture and Storage Program (CCSP) research program coordinated by CLEEN Ltd. with funding from Finnish Funding Agency for Technology and Innovation (Tekes)