TECHNOLOGY TRENDS Inhibiting the Carbon Deposition from the Reverse Boudouard Reaction in Refractories Submitted to CO–H2 Atmosphere J. Poirier, J. Kadok, N. Bost, A. Coulon, M. R. Ammar, S. Brassamin, C. Genevois The catalytic decomposition of carbon monoxide via the reverse Boudouard reaction in the presence of 2 a mixture of CO–H2 gas produces a solid sp carbon deposit, which has deleterious effects on refractory materials. The catalytic effects of iron oxides and the structural evolution of carbon during the reverse Boudouard reaction in CO–H2 atmosphere is investigated at a nano and micro-scale. Sulphur compounds inhibit the carbon monoxide dissociation on contact with iron and iron oxides. The samples were characterised by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy. A mechanism that governs the inhibition of the reaction is proposed and validated at lab scale, in which the formation of a very thin protective FeS layer (0,5–1 nm) is involved. This research is applied to develop ew CO/H2 resist- ant refractories. particles are present in the refractories as From reactions (1) and (2), the water-gas 1 Introduction secondary mineral phases in raw materials shift reaction (WGSR) describes the re action The mechanism of carbon monoxide de- and as impurities due to crushing and mix- of carbon monoxide and water vapour to composition, called the reversed Boudouard ing while blending the refractory formula- form carbon dioxide and hydrogen reaction: tion. CO + H2O = CO2 + H2 2CO(g) → CO2(g) + C(s) results in the depos- The CO resistance is usually improved by DH° = –41,166 kJ/mol 2 ition of sp solid carbon. This C deposition, the selection of raw materials with a low K3 = P CO2 · P H2 /P CO · P H2O (3) inside the porosity of the refractory, causes content of iron particles and by increasing premature degradation of linings, which the refractory sintering temperature (Fe ions In direct reduction processes, the decom - operate under reducing atmosphere [1]. The are adsorbed in the lattice or bonded in the pos ition of methane into carbon also occurs destruction of refractory materials in CO at- glassy phase). However, these solutions are CH4 = C + 2 H2 mosphere is well explained in the literature not always very effective for industrial appli- DH° = 92,676 kJ/mol. 2 [2]. cations (such as low-CO blast furnace pro- K4 = P H2 · aC/P CH4 (4) The refractory damage process includes four cess, biomass gasification reactors, steam steps: methane reformers, direct reduction shafts, • diffusion of CO gas diffusion into the por- reactors in the petrochemical industry, etc.) J. Poirier, J. Kadok, N. Bost, A. Coulon, osity of the refractory material; where refractories are subjected to CO and M. R. Ammar, S. Brassamin, C. Genevois • reduction of iron oxides into iron then H2 extremely reducing atmospheres. CNRS, CEMHTI UPR 3079 carburation into Fe3C, which then acts as According to the literature, two reactions University of Orléans a catalyst for C-decomposition according explain the carbon deposition phenom- Orléans, France Fe3C → 3 Fe + C; ena in a temperature range between 400– • nucleation and growth of the carbon dep- 1050 °C: Corresponding author: J. Poirier osition, which generate thermomechanic - • the reverse Boudouard reaction E-mail: [email protected] al stress inside the refractory; 2 CO = C + CO2 • formation of cracks, damage then de- DH° = –172,4 kJ/mol Keywords: Boudouard reaction, struction of the refractory. K1 = P CO2 · aC /P CO2 (1) carbon deposition, refractory damage, This reaction occurs at temperature rang- cementite, sulphur ing between 400–900 °C with a maximum • the reverse water gas reaction intensity around 600 °C and is highly fa- CO + H2 = C + H2O The paper was awarded with voured by the presence of catalytic particles DH° = –131,3 kJ/mol the 1st Prize for Posters at ICR 2018 such as iron and iron oxides. FexOy and Fe K2 = P H2O · aC /P CO · PH2 (2) refractories WORLDFORUM 11 (2019) [2] 83 TECHNOLOGY TRENDS rate of C-deposition occurs from ~530 °C gas, was similar to the gas mixture used in for 0,8 % H2 to 630 °C for 19,9 % H2. In- low-CO blast furnace industrial processes. creasing the hydrogen content increases the Solid sulphur and sulphur dioxide are used amount of carbon formed. as inhibitors of the Boudouard reaction This paper reports studies of the carbon (Tab. 1). formation by the Boudouard reaction using Laboratory experiments were also carried FexOy catalysts, and a CO + H2 atmosphere, out with Fe2O3 powders mixed with solid at 600 °C. The reaction of the carbon for- sulphur and with SO2(g). SO2(g) was mixed mation was investigated, in these extreme continuously with the reducing CO/H2 gas. conditions, using thermogravimetric meas- Measurements of Fe2O3 mass variation urements and in situ Raman spectroscopy. were performed using a thermobalance After cooling, the samples were character- from NETZSCH STA 409C/CD coupled ised using various methods: X-Ray Diffrac- with a QMS 403/5 Skimmer. Fe2O3 sam- tion (XRD) and Scanning and Transmission ples of approximately 30 mg were placed Electron Microscopy (SEM and TEM). in an alumina crucible and were heated Fig. 1 Carbon deposition on a Fe O 2 3 The aims of this research are: to 600 °C at a rate of 10 C · min–1 under sample after exposure to a gas mixture • to understand the effect of iron oxides on 100 ml · min–1 of an argon inert gas prior consisting of 71,0 % CO, 3,0 % CO , 2 the carbon formation and to characterise to being exposed to a reducing CO/H gas 11,0 % H and 15,0 % N , for 5 h, 2 2 2 -1 at 600 °C the structural organisation of carbon ob- (100 ml · min ). tained by the reverse Boudouard reaction The crystalline phases were analysed with In reactions (1–4), K1, K2, K3, K4 are the in CO and H2 atmosphere; powder X-ray diffraction using a Bruker 2 equilibrium constants of reactions, P CO, P CO2, • to investigate the inhibition of the sp car- D8 A25 diffractometer (Bragg-Brentano P H2, P H2O, P CH4 the partial pressures of CO, bon formation in refractories from CO dis- geometry θ-θ) equipped with a LynxEye CO2, H2, H2O, CH4 and aC the carbon activity. proportionation catalysed by iron oxides XE detector and using the Cu Kα radi ation The effect of H2 in CO increases significantly under the most favourable conditions of (λ = 1,5418 Å). Raman spectroscopy was the reaction of carbon deposition. For ex- the reaction, i.e., in the presence of H2 at performed using a T64000 Jobin-Yvon ample, Walker, et al. [3], report a deposition 600 °C; multi channel spectrometer equipped with of 10 g of solid carbon after ≈350 min over • to describe in detail the mechanisms of an Ar–Kr laser source, aBX41 Olympus 0,10 g of carbonyl iron in 9 %-H2–91 % CO carbon inhibition. The effect of sulphur on micro scope and a liquid nitrogen-cooled gas mixture at 602 °C. the Boudouard reaction, catalysed by iron CCD detector. Fig. 1 illustrates this reaction. Few grams oxides in presence of H2 gas, was investi- The spectra were collected in a backscatter- of Fe2O3 were submitted to 71 % CO, 3 % gated under laboratory conditions; ing geometry under a microscope (×20 long CO2, 11 % H2, 15 % N2 gas mixture during • to find an efficient solution to inhibit the working distance objective) using the 5 h at 600 °C under atmospheric pressure. carbon deposit and to limit the industrial 514,5 nm wavelength (2,41 eV; 3 mW of A large amount of carbon formation was degradations of refractories. laser power), a 600 groves · mm–1 grating observed similar to a “popcorn” formation. giving a spectral resolution of 3 cm–1 in However, this phenomenon is not described 2 Materials and methods the 1000–2000 cm–1 wavenumber range. in CO + H2 atmosphere and the mechanism Various iron oxide particles were used as Heating the sample was achieved using a is not really understood. the catalyst of the Boudouard reaction. The TS1500 Linkam device (Tadworth/GB). The hydrogen content in the CO has an effect reducing gas mixture (71 % CO, 3 % CO2, The same temperature program was used on the temperature at which the maximum 11 % H2, 15 % N2), referred to as CO/H2 for Raman spectroscopy and for TGA (10 °C · min–1 until 600 °C). After the ex- periments, the samples were also charac- Tab. 1 Origin and purity of the raw materials terised by scanning electron microscopy Materials Origin Purity Particle Size Specific Surface (Hitachi S4500 FEG) and high resolution 2 [mass-%]) [µm] Area [m /g] transmission electron microscopy (Philips Catalysts Fe Sigma Aldrich >99 2–5 2 CM20 microscope). FeO Sigma Aldrich 99,9 2–5 6 Scanning transmission electron micros- Fe3O4 LKAB >99 2–5 2 a copy high angle annular dark field (STEM- Fe2O3 Sigma Aldrich >99 2–5 8.0 Sigma Aldrich >99 0,0035 a 42 HAADF) images were acquired in the Carajas >98 160–250 10,5 angular range of 50–180 mrad with an Carajas >98 1000–2000 3,7 8 cm camera length and a 0,1 nm probe Reducing gas CO/H Air Liquide >99.9 2 size. Elemental composition line scans Sulphur dioxide SO2 (g) Air Liquide >99,9 were performed by STEM-EDS using a b Solid sulphur S Alfa Aesar >99,5 ≤10 JEOL EDS system and a 0,13 nm probe a Data as provided by the Sigma-Aldrich Company, b Prepared by grinding in an agate mortar size.