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ECN publicatie:
Towards green iron and steel industry: opportunities for MILENA technology in Direct Reduction of Iron
Gepubliceerd door: Publicatie datum:
ECN Biomass & Energy Efficiency 28-6-2016
ECN publicatienummer: Publicatie type:
ECN-E--16-022 ECN rapport
Aantal pagina's: Volledige tekst:
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The iron and steel industry is an energy-intensive sector, and an important source of CO2 emissions. Some technological options for net energy saving and CO2 emission reduction include recovery of exhaust heat, process gas recovery, and use of non-fossil energy sources. In this sense, the use of biomass could contribute to the shift towards a greener iron and steel industry. However, biomass oxygen must be removed at least partially in order to ensure the suitability of biomass as reducing agent. The removal of oxygen can be applied to the biomass via slow pyrolysis for the production of solid charcoal or to by CO2 + H2O removal from syngas produced from biomass gasification. This report analyses the use of producer gas from biomass gasification (in particular, producer gas from MILENA indirect gasification) as reducing agent in DRI processes. Direct reduction of iron (DRI) consists of the conversion of iron ore to quality metallic iron in solid state in one step using a reducing gas. In natural gas based DR processes, oxygen is removed from the feed iron ore by syngas produced from natural gas reforming. Gasification can be an interesting option in those cases where there is a combination of expensive natural gas supply and abundant, low-cost coal or biomass resources. The implementation of biomass gasification in DRI processes could contribute to the shift towards a “greener” iron and steel industry. The large scale (~400 MWth) of gasification plants required for a typical DRI plant makes it unlikely that biomass can completely replace fossil fuel consumption in the short term, but given the size of planned biomass- and waste gasification plants as well as the recent developments in legislation over the need for reduction of CO2 emission, DRI could become a feasible application for biomass and waste gasification in the medium/long term. An alternative for the implementation of biomass and waste in the short term might be the partial replacement of coal or natural gas by biomass, either by implementing a dedicated biomass gasifier in a natural gas-based DRI process or by co-gasification of biomass and coal in case of a coal gasification-based process. The main quality requirements of reducing gas in DRI processes include a high (H2 + CO)/(CO2 + H2O) ratio, as well as removal of dust and sulphur. The main advantages of MILENA include the possibility of producing a N2-free syngas without the need for an ASU unit, and the relatively high content of gaseous hydrocarbons in the producer gas, which promote the carbon content in the product. The composition of producer gas from biomass gasification is not in principle the most suitable for DRI application, since it has a large concentration of CO2 and H2O. However, the excess oxygen in the producer gas can be removed by proper gas cleaning and upgrading (dust and tar removal, water removal, desulphurization, CO2 removal) using conventional technologies. An economic analysis has revealed that biomass gasification at current prices is not very attractive. In the case of a natural gas-based DRI plant with a 50 MWth biomass unit a positive NPV is reached at biomass cost of 4 USD/GJ or lower. This changes if subsidy schemes are implemented, which reduce investment cost and improve profitability. In such a situation biomass gasification can then economically replace part of natural gas at biomass cost below 9 USD/GJ (50 MWth gasification plant) and below 11 USD/GJ (100 MWth gasification plant). In a coal gasification based DRI plant, biomass gasification only makes economic sense under specific and unlikely conditions of low biomass cost and high gasification capacities (< 4 USD/GJ biomass price for a 100 MWth plant). This improves if coal and CO2 prices raise.

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