1. Introduction
Integrated steelworks are major contributors to today’s global CO
2-emissions. Review publications screening the steelmaking process around the globe revealed that approximately 27 to 30% of any industrial CO
2 emissions are directly linked to this sector [
1,
2]. With a world-wide crude steel production of 1869 million tonnes in 2019 and a 3.6% per annum average growth rate, the steel demand of our society is increasing strongly. These large amounts of steel are mainly required for building and infrastructure (~52%), mechanical equipment (~16%) and the automotive sector (~12%) [
3].
Figure 1 shows the most common route of steelmaking globally, which includes a blast furnace for the reduction of iron ore to hot metal and a converter or basic oxygen furnace for the batch-wise production of molten steel. The accumulating by-product gases, such as the blast furnace gas (BFG), basic oxygen furnace gas (BOFG) and coke oven gas (COG), have a very rich content of CO
2 and carbon monoxide (CO), among other gases (
Table 1). At the current stage, these by-product gases are buffered within the steelwork and utilised as an energy carrier internally. Nevertheless, additional fossil energy sources, such as natural gas, are needed to cover the whole energy demand for the power plant and auxiliary energy conversion. Prior to any further use, the product gases are cleaned in a two-stage process, including, for example, a dust collector for the separation of coarse dust, and a venturi scrubber for fine dust and water-soluble components.
With the challenging targets of the climate agreements being set, the steel industry sector logically seeks for possibilities to reduce their greenhouse gas (GHG) emissions, as well as to incorporate green energy sources in the steelmaking process itself. One way of reducing the GHG emissions, and simultaneously substituting the need for fossil fuels, is the implementation of synthesis processes, such as methanation. In this process, CO
2 and CO react with hydrogen (H
2), gained from green energy sources—for instance, renewable power driving water electrolysis—to create methane (CH
4) and steam (Equations (1) and (2)) [
5].
These two reactions are highly exothermic and are linked via the reverse water–gas shift reaction (Equation (3)).
Although these reactions are well-known, the behaviour with real, untreated steelworks gases, as the CO
x source, are yet to be investigated. The detailed fundamentals behind the methanation concept, possible reactor designs and available catalysts are documented by Rönsch et al., combining them with an up-to date overview on methanation projects and the state-of-the-art in the research [
6].
Current work on the methanation of steelworks gases primarily focuses on the usage of COG due to its favourable composition. The high amount of hydrogen (up to ~60%) makes it an attractive feedstock to produce synthetic natural gas (SNG), as it also works as an alternative hydrogen source compared to a solution involving an electrolysing unit (e.g., PEM). Müller et al. [
7] investigated the direct conversion of CO and CO
2 from synthetic COG into methane using nickel (Ni)-based catalysts in a fixed-bed reactor. It was shown that additional CO
2 from other sources (e.g., air, flue gas) is required to compensate for the high surplus of hydrogen in the COG to achieve a desirable methane yield. Razzaq et al. [
8] tested various Ni-based catalyst support materials (SiO
2, Al
2O
3, ZrO
2 and CeO
2) for the methanation of synthetic COG. The CO
x conversion rates and CH
4 selectivity in a fixed-bed reactor were evaluated. Results showed that ZrO
2-CeO
2-coated catalysts have the highest activity and selectivity for CO and CO
2 for synthetic gases with COG composition.
Medved et al. [
9] showed in their work that, although the gas from the coke oven seems to be the favourable by-product gas for methanation, it is already fully energetically integrated into the process chain of integrated steelworks. Due to its high calorific value of up to 19.000 kJ/Nm
3, it is used plant-internally at the blast furnace for firing processes, as well as for the power plant. Consequently, it is not readily available as an input for a methanation unit without significantly affecting the steelmaking process and disturbing the energy balance of the plant. The utilization of COG necessitates its substitution by external energy sources, such as electric power and natural gas, respectively. Therefore, the other two by-product gases, BFG and BOFG, with their very high amount of CO
2, CO and N
2, have been evaluated for their applicability as a feed gas for a methanation plant. Furthermore, the authors concluded that the enrichment of BFG and BOFG via methanation, without the necessity of nitrogen removal, as a lean product gas shows a utilisation potential in the integrated steel plant as a substitute for natural gas. Schöß et al. [
10] concluded that, although both gases (BFG and BOFG) are suitable carbon sources for the SNG process with the addition of hydrogen for reaction stoichiometry, the necessary specifications of the natural gas grid cannot be met, due to the high content of nitrogen and the resulting low calorific value. In addition, the significant amount of catalyst poisons present in the already cleaned by-product gases needs to be addressed. Lehner et al. [
11] added that, for converting steelworks gases to methane, a load flexible reactor setup is favourable to meet the fluctuations in the process gas and hydrogen availability.
Studies on methanation are mainly based on synthetic gas mixtures simulating real gas compositions. Nevertheless, Müller et al. [
12], for example, evaluated the direct CO
2 methanation of flue gases at a lignite power plant, showing that the same commercial Ni-based catalyst used in this work did not degrade during the time frame of the experiments. The real gases included the following catalyst poisons: 63 ppm SO
2, 36 ppm NO
2. The same authors further investigated the CO
2 methanation of flue gases emitted by conventional power plants [
13]. They used synthetic gases simulating the real gas composition, including contaminations of up to 100 ppm NO
2 and 80 ppm SO
2. The experiments showed a decrease in the conversion, yield and selectivity by 17% in 12.5 h, or 1.36% per hour. The authors also concluded that the deactivation caused by SO
2 is low in relation to a possible degradation caused by traces of H
2S. Rachow [
14] studied the influence of catalyst deactivation by sulphur compounds and NO
x components. The author confirmed through experiments with bottled flue gases from coal-fired power plants, as well as with real gases from the cement industry, that Ni-based catalysts strongly degrade in the range of hours when exposed to SO
2 and sulphur compounds in general. The degree of deactivation depends on the catalyst, its active surface area, the SO
2 concentration and the total volume flow rate. No deactivation was observed during experiments with NO
x contaminations. Méndez-Mateos et al. [
15] studied the CO
2 methanation over modified Ni catalysts, integrating promoters (transition metals, such as Mo, Fe, Co or Cr), which were added to the catalyst formulation in different portions. The target was to improve the catalyst’s resistance over sulphur, and H
2S in particular. The authors showed that the catalyst activity between 573 and 773 K at 10 bar increased when transition metals were added, except for Mo. In addition, it was possible to regenerate the Co-modified catalyst with oxygen, recovering to a 13% methane yield compared to the fresh catalyst. Calbry-Muzyka et al. [
16] reported on the technical challenges and recent progress made when using biogas as an input for direct methanation. Due to the varying composition of the biogas feedstock, no standard gas cleaning solution has been developed so far. Nevertheless, thorough H
2S and non-H
2S sulphur removal to the sub-ppm level is necessary in order to prevent catalyst deactivation. Witte et al. [
17] demonstrated the stable operation of a catalytic direct methanation with biogas in a fluidised bed reactor for over 1100 h. Only a slow deactivation by organic sulphur compounds was identified, which broke through the gas cleaning unit with concentrations between 0.5 and 3 ppmv. The installed unit contained a two-stage adsorption-based biogas cleaning system for deep desulphurization [
18]. Fitzharris et al. [
19] concluded in their work that Ni-based catalysts are highly sensitive to poisoning by sulphur due to geometric, and not electronic, effects. Concentrations of H
2S as low as 13 ppb in H
2 reduced the steady state methanation activity by more than two orders of magnitude. Bartholomew et al. [
20] showed that, under typical low-pressure reaction conditions for methanation (525 K, 1 atm) in a reaction mixture containing 10 ppm H
2S, Ni-based catalysts lost most of their activity within a period of 2 to 3 days. The rate of deactivation due to H
2S poisoning increased with an increasing H
2/CO ratio, as well as with an increasing reaction temperature.
Although the applicability of steelworks gases as feed for the synthesis of methane was addressed and confirmed by multiple authors [
7,
8,
9,
10,
11], experiments with real gases, including contaminations poisonous to catalysts, have not yet been performed. Consequently, the main aim of this work is to show the degree of degradation when using real BFG and BOFG (including nitrogen) as an input for a methanation unit in a composition, as given in
Table 1, as well as to present a working solution to overcome the problem of catalyst deactivation.
4. Conclusions and Outlook
In this work, methanation experiments with real gases from the steelworks industry have been performed. These experiments included real by-product gases from the blast furnace, as well as from the basic oxygen furnace (converter), that were directly bottled at an integrated steel plant during normal operation. No further treatment, such as a CO2 or N2 separation step, was performed prior to the filling procedure. Methanation without additional gas cleaning resulted in an instant, as well as steady, catalyst degradation due to the poisons present in the real gases. Over the evaluated periods, a drop in the COx conversion of ~30%, or 1.8% per hour, was detected for blast furnace gas. The usage of unfiltered real gases resulted in a 3.2 times higher amount of Ni transported out of the reactor setup with the condensate, compared to experiments with synthetic gases meeting the same composition and operating conditions.
As a working solution, an activated carbon filter coated with copper oxide was implemented, showing that a further pre-treatment of the already cleaned steelworks gases is essential prior to feeding them to a catalytic methanation plant. With the activated carbon filter in place, methanation experiments could be performed without any noticeable degradation until the 187 g of activated carbon adsorbent were fully loaded with catalyst poisons. The first signs of deactivation appeared after 7.2 h of operation with real BFG, by means of a drop in the reactor temperature measured at the bottom of the catalyst bulk. Over another period of 5.2 h, the product gas composition and overall conversion rate remained constant, after which, the methane yield started to drop. During this period of 12.4 h, on average, 15.1 g of adsorbent was consumed per hour of operation. The loading of catalyst poisons within the adsorbent bed stayed within a range of 0.01 to 0.73 wt.-% depending on the type of poison. While continuing methanation over another 6.7 h, the catalyst consumption increased from 0.4 to 4.8 gcat/mmolsulphur on average, and the relative activity of the catalyst decreased by ~45% compared to its starting performance. In the case of real BOFG, no signs of catalyst deactivation could be observed during the course of the experiments, which is a result of the far lower catalyst poisons present in this type of gas.
For a real application-based scenario in an integrated steelworks, with the target to substitute the demand of any externally sourced natural gas with a plant-internally produced SNG through methanation, the figures obtained through the experiments at lab-scale would result in the need of ~2.4 t of adsorbent and a deactivation of 88.1 kg of catalyst per hour of operation.
Future work will show the usability of the methanation setup for dynamic experiments as they occur in a steelworks plant, including frequent load changes of up to 25% of gas input power in the range of 5 to 45 min simulating a dynamically operated PEM electrolysing unit. The values obtained through the lab-scaled experiments will also assist in the technical design of a pilot plant for the steelworks industry, including a gas cleaning step based on activated carbon prior to feeding the real gases to the methanation units. Furthermore, the catalyst degradation due to poisons present in the real gases will be investigated in detail through Raman spectroscopy, as well as BET analysis.