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steel burner deflector temperature was seen to increase from 480°F when fir- ing natural gas to 1,300°F when firing 95% hydrogen and NOx levels increased from just over 40 ppm to nearly 70 ppm, respectively.11 These issues were shown to result in higher temperatures, longer heating resident times, and different heat distri- butions seen by refractory lining materi- als in service. Additionally, hydrogen produces more water compared to other hydrocarbon fuels and may result in water vapor being present in the furnace atmosphere, which can lead to increased refractory corrosion for certain refrac- tory compositions. All these factors are known to have possibly deleterious effect on refractory materials depending on the compositions employed.12 It also has been noted that changes may be seen within the boiler regarding where and how heat transfer occurs, along with increased furnace gas exit temperatures due to the higher flame temperatures. Such changes in boiler performance may require alternative strategies for type and location of refrac- tory materials used. Hydrogen has been used in combina- tion with natural gas for industrial heat treatment furnaces as well.13 Natural gas/hydrogen blends were used as alter- native fuel due to economic potential for decreasing carbon dioxide emis- sions. As noted previously, alterations were required to the heating system to account for the differing thermophysi- cal properties of the fuel blends and the corresponding changes to the flue gases (thermodynamic and chemical). In particular, increased NOx emissions were noted (increases of 10% for air-staged combustion and about 100% for flame- less combustion were measured at a 40% hydrogen content in comparison to pure natural gas firing). Again, refractory issues were not noted, but similar issues to those highlighted above are expected with the change in furnace conditions. In China, hydrogen-rich fuel was injected into a steel blast furnace in place of part of the coke loading to reduce carbon dioxide emissions and energy usage.14 The effect on refractory performance was not discussed, but the increased hydrogen content of the fur- nace atmosphere was found to change the thermodynamic and kinetic condi- tions of the furnace due to altered tem- peratures (increased flame temperatures) and gas flow (lower gas flow rates). The existence of more water in the furnace was also noted. All these factors lead to reduced efficiency of the blast furnace and would be expected to alter the per- formance of the furnace lining system. Additionally, the effects of using hydro- gen in place of coal will be compounded because the coal not only provides heat but also carbon monoxide and physical structure for the reactions occurring within the blast furnace.4 Also for steel production, Tenova S.p.A. introduced a new burner system (TSX Smartburner) for use in steel reheat furnaces in 2020.15 This megawatt-size flameless combustion system is capable of burning any mixture of natural gas and hydrogen (up to 100% hydrogen) using Tenova’s integrated advanced digital control solutions. NOx emissions are con- trolled by the flameless combustion tech- nology (releasing < 80 mg/Nm3 @ 5% of oxygen with furnace at 1,250°C). It also boasts optimal heat transfer uniformity within the furnace with full adaptation of the fuel mixture to balance the avail- able hydrogen stream through the burner control logic. This design is expected to address some of the problems noted pre- viously regarding uneven furnace heating leading to hot spots and to be flexible to varying hydrogen availability, therefore possibly reducing these effects on refrac- tory performance. Additionally, in Germany, multi- national steel producer ArcelorMittal received state funding to implement its plans to invest in a demonstration steel plant using hydrogen produced from renewable electricity.16 The proposed plant will be a direct reduced iron plant using green hydrogen to reduce iron ore in a carbon-free steelmaking process. Starting in 2025, they plan to produce all “green” steel using clean direct reduced iron (up to 100,000 tons) from a 50 MW electrolyser and melted scrap in a green powered electric arc furnace. The direct reduced iron process is much more amenable to the use of hydrogen as an alternative fuel because it tradition- ally uses natural gas and generally not coal, as is the case for blast furnaces. Still, changes to the chemistry and ther- modynamics of the furnace atmosphere are expected and therefore refractory issues should be a consideration. The use of hydrogen was also explored in glass melting. Since 1991, numerous container glass furnaces were converted from air-fuel to oxy-fuel firing, where pure oxygen is substituted for part of or all the air mixed with the combustion fuel. Recent advances in this technology have looked at substituting hydrogen in place of oxygen.17 Such a substitution is hoped to further reduce fuel requirements and emissions while also improving glass quality. It is noted that some batch modification to optimize the glass fining chemistry and control glass foaming may be required, along with further burner improvement. It is therefore expected that reevaluation of the furnace refractory structure may also be required, as was the case when the move to oxy-fuel firing was first undertaken.18 Relatedly, in September 2021, NSG Group announced that they success- fully manufactured architectural glass at their Greengate location in the United Kingdom using hydrogen in place of natural gas for all power production at the site.19,20 This demonstration was part of their “HyNet Industrial Fuel Switching” project to prove that hydrogen was as capable as natural gas in achieving excel- lent melting performance while also reducing carbon emissions by replacing natural gas in the float glass furnace, which accounts for most of the company’s overall carbon emissions. Although extended furnace performance was not monitored and therefore the effects on the refractory lining were not evaluated, in this initial short-term three-week trial, a “seamless transition” between fuels was noted. Examples of the use of hydrogen in cement production were not found, but it is estimated that 30% of high- temperature industrial heat is used in the cement industry and hydrogen should be well suited for use as an alternative fuel.4 Due to the large carbon footprint of this industry, decarbonization of the fuel source should be attractive. Refractory American Ceramic Society Bulletin, Vol. 101, No. 2 | www.ceramics.org 29PDF Image | hydrogen as an alternative fuel
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