## 4 Discussion
### Cost  
The real (or perceived) cost of producing large amounts of oxygen carriers is often sensitive information that companies are not eager to disclose. In many circumstances, production costs are unknown and difficult to estimate, potentially leaving the selling price largely disconnected from the production cost. Instead, the selling price is determined by traditional price mechanisms, which means that it is an opaque function of supply, demand, and competition with alternative products. The answer one gets when inquiring also close collaboration partners about production costs and expected price of a potential product is that \"the price will be what the customer is prepared to pay\". To be fair, in many instances it is likely true that the real production cost of oxygen carrier particles cannot be accurately calculated. LD-slag is a by-product for which there is limited demand, and can be used as an example; Should parts of the cost for building and operating the steel mill be attributed to the oxygen carrier cost? Or can excess LD-slag be assumed a waste material that can be obtained for free? Both options are reasonable but will result in quite different estimations for production cost. Particulate slag-based products (based on copper smelter slag and blast furnace slag) sell on the consumer market in Sweden for as little as 5-10 \(\upmu\)/ton. Available products typically have too large particle sizes to use in CFB boilers and may contain unacceptable amounts of fine material. An already much costlier variant of this family of products (LD-slag 0/2 sieved to have particle size below 2 mm) was used for the BFB campaign at Savenas mentioned in Table 1. As has been explained above, this commercial fraction was difficult to pneumatically transport at the site, likely due to the high content of fines. While just an anecdotal observation, it seems reasonable to believe that even more extensive processing (e.g., sieving) would be required for use in boilers, which would inevitably affect the price. The same would be true for extracting limited amounts of materials from other large industrial plants e.g., for processing of ilmenite described above. As long as only a small fraction of the production is extracted for use as an oxygen carrier it may be reasonable to neglect capital costs for equipment, assuming that the rest of the production carries these costs. But if one were to build a facility dedicated to the production of large amounts of oxygen carriers, the oxygen carriers would have to carry the capital costs on their own. If mineral ores are considered as bed material, the raw material price varies greatly with quality and form, and also with the current demand for commodities. Surveying trading sites, one can conclude that the approximate raw material costs in autumn 2020 are 40-300 \(\up\up\)/ton for iron ore, 100-400 \(\up\)/ton for ilmenite concentrate, and 100-500 \(\up\)/ton for manganese ore. This does not consider any treatment. If extensive processing of raw materials is needed, this is set to increase the ultimate sales price significantly. This can be compared with the cost for silica sand for Swedish boilers which is in the order of 50 \(\up\)/ton. Table 5 summarizes the offered price for the preparation of three large batches of oxygen carrier particles, two of which were ultimately ordered and received. Most of the cost items are likely to be significantly reduced for truly large-scale operations due to economics of scale. The numbers are interesting nonetheless, if only as indications for the cost of e.g., a large CLC demonstration unit. The data in Table 5, shows that the raw material cost is dwarfed by the processing costs. The high costs offered for drying and calcination are noteworthy. The cost per ton oxygen carrier received ultimately was in the order of 10-15 times higher than for ilmenite concentrate.