Pressurized Chemical Looping for Direct Reduced Iron Production: Economics of Carbon Neutral Process Configurations (2)
As would be expected, the carbon neutral DRI configurations have a higher capital cost than the base case DRI facility since extra equipment is required to remove, condition, cool, and compress the CO2 from the flue gas. The advantages of pressurization via reduced equipment size are reflected in the lower capital costs of PCL-DRI compared to the base case + PCC. Further capital cost reductions for PCL-DRI may be achieved by optimizing the inlet temperature and mechanical design of the main air compressor feeding the air reactor, and by optimizing the inlet conditions and mechanical design of the gas expander on the vitiated air. It should be noted that capital costs associated with auxiliary steam production equipment for the base case + PCC are not considered within the scope of this analysis. If steam is produced from the combustion of fossil fuels, then the capture of these emissions must also be considered and will result in an increase in the size and cost of the amine capture units.
There are limited publicly available data showing economics of Midrex DRI processes broken down into costs for individual pieces of equipment. Total capital investments of USD 606–673 M have been reported for the entire DRI plant, after adjusting costs for the year of construction and production scale. The TCI for the base case presented in this work is lower than this range, but considering the error margin of +50% on class 4 economic estimates, the predicted result is still near the expected range. The important learnings from this work are the relative cost comparisons between the cases, given that the same cost basis has been applied and that the shaft furnace cost, one of the most capitally expensive items, is the same in all cases.
An increase in operating costs is expected when CO2 capture is incorporated, at a minimum to provide the electricity to compress and dry the captured CO2. Additional measures can be taken to reduce the operating costs of PCL-DRI, which can be considered in future analysis. First, the electricity consumption may be partially offset by optimizing the operating conditions of the vitiated air gas expander to drive a larger portion of the main air compression duty. This may be achieved by increasing the inlet temperature of the gas expander, as investigated by Symonds et al., though this must be balanced with higher capital costs for more exotic materials of construction. Second, the co-production of steam using waste heat from the PCL reactor effluents may be considered. Process configurations for power and steam production via chemical looping have already been investigated by others. This steam could be used to either drive rotating equipment, instead of employing electric drives, or could be used elsewhere in the iron and steel mill to reduce the power import or steam production and fuel costs from non-carbon neutral process equipment.
The goal of this work was to present the incremental costs of the most significant operating variables associated with employing carbon capture technologies within the DRI production process. We have not calculated the complete cost of carbon management, as this requires a deeper analysis of CO2 transportation, storage, and/or utilization costs. In many parts of the world, these costs are not well understood because the infrastructure and policy framework do not yet exist. On a high level, other researchers have considered a generic cost of CO2 transportation and storage of USD 10/tonne CO2, though it has been calculated to reasonably range from USD 4 to 45/tonne CO2 depending on the distance, scale, country, geography, and monitoring requirements. A more detailed analysis will be site-specific and include knowledge of (i) the nearest storage or utilization site; (ii) the existing access to rail, pipeline or shipping corridors; (iii) other large emitters in the area whose participation may reduce transportation infrastructure costs; (iv) the time horizon for the installation of the transportation infrastructure; (v) a least-cost analysis of potential transportation routes including routing around urban areas, indigenous lands, wetlands, and protected species.
Oxygen carrier development has been an active area of research for chemical looping technologies. The sensitivity analysis presented in Figure 9 highlights the importance of keeping oxygen carrier costs low, both through choosing low-cost materials and minimizing the required makeup rate. Many researchers have done extensive work developing synthetic oxygen carriers that offer improved reaction kinetics and reduced attrition rates compared to natural ores; however, the production costs of these novel materials are uncertain. Newby et al. looked at the economics of the large-scale production of a number of oxygen carriers using two preparation methods: co-precipitation and mechanical mixing. Production was considered both at a large, centralized facility (1,000,000 tonne/yr capacity) and on-site at a chemical looping power plant (88,000 tonne/yr capacity). Co-precipitated oxygen carriers made of Fe2O3-Al2O3 or CuO-TiO2 resulted in costs ranging from USD 5.36–9.40/kg, which is within the competitive cost range identified in this work only when using high-attrition-resistant oxygen carriers. The costs for some oxygen carriers produced through the mechanical mixing of raw components, such as Fe2O3-alumina and taconite-CuO-alumina, were predicted to be USD 1.30/kg or less, and thus would be appropriate for PCL-DRI even if they are less resistant to attrition. Since chemical looping is not yet a commercial technology, the market demand for a particular oxygen carrier upon plant startup may not be large enough to warrant the production at a large, third-party-operated centralized facility. This would necessitate either the construction of an oxygen carrier production plant on site at the PCL-DRI facility, or the limitation of oxygen carrier selection to the cheaper natural ores. The use of iron-based ores may have further advantages when PCL is employed at an iron and steel mill, as there may be the opportunity to recycle the spent oxygen carrier in the steelmaking process, reducing waste transportation, disposal costs, and raw material costs for steelmaking.
The basis of the analysis in this work was an improvement to a standard Midrex DRI plant to eliminate CO2 emissions. These results and conclusions can be extended to other DRI configurations as the technology continues to be advanced and upgraded. A detailed analysis of the Energiron zero reformer configuration could not be completed due to the lack of sufficient operating data in the open literature; however, the flue gas properties from the fired process gas heater are expected to be similar to the flue gas from the Midrex reformer. Thus, PCL-DRI could be employed to replace the fired heater in much the same way as it replaces the reformer in this work, with a similar cost advantage compared to applying amine PCC to reduce those emissions. PCL-DRI could also be employed in conjunction with biofuel utilization to obtain carbon negative operations, which could offset the emissions from other harder-to-abate process units within the steelmaking process.
3. Conclusions
Two potential carbon neutral DRI process configurations based on modifications to a standard Midrex DRI plant were investigated. While carbon capture does increase the capital and operating costs compared to the base case DRI plant, the analysis in this work did not consider carbon taxes or any sort of economic incentives to produce green steel. With these policies and frameworks in place, the carbon neutral options could become economically attractive to steel producers. Of the two carbon neutral options, PCL-DRI incurred both lower capital costs and lower variable operating costs than post-combustion capture via amine absorption. This gap could be further widened if the steam for the regeneration of the amine solution requires the expansion of the steel mill’s existing steam production capacity, which would increase the capital and operating costs by increasing the amount of new equipment that must be purchased, and increasing the scale of the amine plant to capture the additional CO2 produced from steam production.
The incremental variable operating costs for CO2 capture were USD 103 and USD 44 per tonne of CO2 produced for amine post combustion capture and PCL-DRI, respectively. Sensitivity analyses showed that ±30% variation in the cost of steam or electricity will not affect the ranking or general conclusions about the economics of the process configurations studied. The cost of the makeup oxygen carrier to the PCL-DRI plant has the largest potential impact on study conclusions. If a high-cost synthetic oxygen carrier is used, there is the potential for the variable operating costs of PCL-DRI to exceed those of amine capture. For this reason, low-cost natural ores are recommended for this application, with the added potential for the recycling of the spent oxygen carrier directly within the steelmaking process. Based on this analysis, PCL-DRI is an economically competitive carbon neutral method of producing DRI compared to the current commercially available amine carbon capture technologies.
Due to the limitation of space, "Methods" and "Discussion" are omitted. For details, please refer to:
Bond, N. K., Symonds, R. T., & Hughes, R. W. Pressurized Chemical Looping for Direct Reduced Iron Production: Economics of Carbon Neutral Process Configurations. The Energies, 17 (3), 545. https://doi.org/10.3390/en17030545
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