2022 Volume 62 Issue 12 Pages 2573-2577
A new carbonizing and pulverizing process of woody biomass has been proposed, which utilizes sensible heat of industrial flue gas using a heat storage material (HSM). In the present study, Fe–Mn–C alloy was examined as a candidate of HSM, however, it has rather poor high-temperature oxidation resistance and therefore its aluminizing treatment was attempted to solve this problem. The Al-rich layer growth and alumina formation behaviors on the Fe–Mn–C alloy during aluminizing treatment using the pack cementation method were studied. Alloy samples were aluminized at 700–900°C for 3–12 h. Change in the sample weight was measured using a TG at 1000°C for 24 h in air.
FeAl layer formed in the early stage of aluminizing, followed by Fe2Al5 layer formation. Acicular FeAl2 phase precipitates in the FeAl layer. The sample aluminized at 700°C for 12 h does not have sufficient oxidation resistance, whereas ones aluminized at 800°C for 12 h and 900°C for 3–12 h show superior oxidation resistance. Continuous Al2O3 layer is formed due to presence of FeAl2 or Fe2Al5 in Al-rich layer.
In the integrated steel works, coke and coal are used as the main energy sources and reducing agents, and therefore alternative materials are expected to reduce greenhouse gas emissions originated from fossil fuels. Woody biomass1) widely distributed in Japan and has carbon-neutral feature. Woody biomass is preliminarily carbonized to improve transportation efficiency and energy density.2) However, the carbonization is an endothermic process, hence energy loss occurs due to partial combustion of woody biomass.3) Although a large amount of high-temperature gases is exhausted in the integrated steel works, most of them are utilized.4) Un-used high temperature gases generally containing dusts and corrosive and intermittent substances.4)
The authors have proposed the process to carbonize and pulverize woody biomass by recovering sensible heat from high temperature gas using balls of the heat storage materials (HSM).5,6) Then, the HSM balls are fed to a rotary kiln type furnace together with woody biomass. The woody biomass is heated through heat exchange with the HSM balls and carbonized to be biomass char, while the char is pulverized by the collision with the HSM balls. The HSM balls are sent to the heat exchanger again to recover heat. Pyrolysis of woody biomass generates gas mainly composed of hydrocarbons, and it can be used to additionally increase in the gas temperature. The obtained biomass char is considered to substitute for coke used in the high temperature processes such as iron ore sintering.
It is important to improve the performance of the HSM such as heat storage capacity, high-temperature oxidation resistance and mechanical properties. Latent heat storage associated with phase transformation is effective to improve the heat storage capacity.7,8) Further it is possible to supply heat at the phase transformation temperature by combining latent and sensible heat storage. This process requires both the high temperature oxidation resistance and high mechanical strength above 600°C3) for HSM.
In this study, Fe-based Fe–Mn–C alloy is selected as HSM using solid phase transformation. The Fe–C alloy shows eutectoid transformation at 727°C. Benz et al.9) reported that the eutectoid point of Fe–Mn–C alloy decreases with increasing Mn addition. The alloy is expected to recover latent heat at different waste heat temperatures by varying Mn content. The effect of Mn concentration on the latent heat storage behavior of Fe–Mn–C alloy has investigated in the previous study with high heating and cooling rate.10) The result showed that the eutectoid transformation temperature of Fe–Mn–C alloy decreases with increase in Mn concentration.
Another problem for the Fe–Mn–C alloy is poor high temperature oxidation resistance. Although formation of protective oxide layer such as Al2O3 and Cr2O3 is effective, Cr and Al elements should be added into the Fe–Mn–C alloy. However, these elements affect to stabilize the crystalline phase. Therefore, the aluminizing by the pack cementation method has been proposed. Aluminizing is a method to generate Al-rich layer by diffusion and penetration of Al on the surface of HSM, and to generate continuous Al2O3 layer by the oxidation treatment.11) The aluminizing leads to add Al only to the surface of the Fe–Mn–C alloy, resulting both high temperature oxidation resistance and latent heat storage by phase transformation. In previous study, the aluminizing of Fe–Mn–C alloys has been investigated and found that the formation of continuous Al2O3 layer by the oxidation in the air after aluminizing.10) Here, some Fe–Al intermetallic compounds were formed during aluminizing. Identifying the intermetallic compounds to contribute the formation of the protective oxide later is important to obtain efficient aluminizing conditions. In this study, the growth of Al-rich layer and alumina formation behavior on the surface of Fe–Mn–C alloys during aluminizing using the pack cementation method was studied.
Powder mixture of “Mn: 2 mass%, C: 0.7 mass% and Fe: balance” was prepared using respective reagents and the alloy ingot was fabricated in a tri-arc furnace. The alloy ingot was annealed at 1050°C for 3 h in the Ar-3%H2 atmosphere. Then, it was cut and polished to prepare a plate sample of about 1.0 mm in thickness and 1.5 mm in both length and width.
The plate samples were aluminized applying the pack cementation method. Figure 1 shows the schematic illustration of the aluminizing operation. The sample was put in an alumina tube and filled with powder mixture of “Al: 1 mass%, NH4Cl: 1 mass% and Al2O3: 98 mass%” and then sealed with a heat-resistant cement. The sealed alumina tubes were placed in a horizontal electric furnace and heat-treated at 700–900°C for 3–12 h in the Ar-3%H2 atmosphere. Al powder reacts with Cl released from NH4Cl powder to produce aluminum chloride gas at the holding temperature. The aluminum chloride gas comes in contact with the sample surface, and Al is dissolved and diffused into the sample. Al2O3 powder in the powder mixture acts as spacer.
Schematic illustration of aluminizing by pack cementation method. (Online version in color.)
Oxidation experiments were carried out on the aluminized samples using a thermal gravimetry (TG) with α-Al2O3 as a reference sample. The aluminized sample was heat treated up to 1000°C with 10°C/min and held at 1000°C for 24 h in air atmosphere. The weight change ratio was calculated as follows:
| (1) |
Further, oxidation experiments were conducted after repeated heat treatment to evaluate the stability of the oxide layer. Sample aluminized at 900°C for 3 h was used in the experiments. The sample was heat treated at 1000°C for 24 h in air, and then sample was heat treated ranging from 200 to 1000°C with 10°C/min in N2. The heating and cooling was repeated by 3 times. After that, sample was heat treated again in air at 1000°C for 24 h using TG and the weight change ratio was calculated.
Figure 2 shows the cross-sectional SEM image of the sample aluminized at 800°C for 12 h and locations of the point analysis by EDS. Two layers with different contrasts are observed on the sample. Table 1 shows the normalized molar ratio of Fe and Al at each point. Although Mn and C are dissolved in the sample, the concentrations are much lower than Fe and Al. Further, Mn is substitute for Fe,12) thus influence of Mn concentration on the Al concentration in the alloy is considered to be small. Therefore, point analysis by EDS were normalized by molar ratio of Fe and Al. The molar ratio of Fe and Al in the surface layer at points 1 and 2 is approximately 27:73. From the phase diagram of Fe–Al system,13) Fe2Al5 has composition range of 70–73 mol%Al. Therefore, Fe2Al5 is estimated to form at surface layer. The molar ratio of Fe and Al is 43:57 at point 3 corresponding to the composition of FeAl. The molar ratio of Fe and Al obtained for the point 4 is 88:12. Since Al can be dissolved in α-Fe up to approximately 20 mol%, the phase in point 4 corresponds to the Fe-based alloy.
Cross-sectional SEM image of a sample aluminized at 800°C for 12 h and the location of the point analysis by EDS.
| Point | Fe | Al |
|---|---|---|
| 1 | 26.4 | 73.6 |
| 2 | 26.5 | 73.5 |
| 3 | 42.9 | 57.1 |
| 4 | 88.3 | 11.7 |
Figure 3 shows cross sectional SEM images and corresponding EDS mappings of the sample after aluminizing at 900°C for 3 h. Acicular phase with dark contrast in FeAl layer is observed. EDS mapping shows that Al concentration of the acicular phases is higher than that of the FeAl layer. In the phase diagram of Fe–Al system,13) FeAl coexists with FeAl2 at 900°C and Al concentration of FeAl2 is between 50 and 65%. At 900°C, the Al concentration on the sample surface at the early stage of the aluminizing is considered to be high. Therefore, it is considered that FeAl2 is formed with FeAl as coexisting phase. Further, Zhang et al.14) carried out the aluminizing of cast iron by dipping in molten Al and obtained a similar acicular phase of FeAl2.
Cross sectional SEM images and corresponding EDS mappings of the sample after aluminizing at 900°C for 3 h. (Online version in color.)
Figure 4 shows cross sectional SEM images of samples after aluminizing at (a) 700°C for 12 h, (b) 900°C for 6 h and (c) 900°C for 12 h. FeAl layer is only observed in the sample aluminized at 700°C for 12 h, but not the acicular phase of FeAl2. While, the sample aluminized at 900°C for 6 h shows the acicular phase of FeAl2 in FeAl layer similar to the sample aluminized at 900°C for 3 h. The FeAl layer grows outward and inward across the acicular FeAl2 phase. The former is named as the outer layer and the latter is the inner layer. In the sample aluminized at 900°C for 12 h shown in Fig. 4(c), Fe2Al5 layer is newly observed on FeAl layer.
Cross sectional SEM images of samples after aluminizing at (a) 700°C for 12 h, (b) 900°C for 6 h and (c) 900°C for 12 h.
Figure 5 shows the thickness of aluminized layers of the samples after aluminizing at 900°C for 3 h, 6 h and 12 h. The thickness of the “Total thickness of aluminized layer” is determined as the sum of the thicknesses of the Fe–Al intermetallic compound. The FeAl layer was measured separately for the inner and outer layers. The distinguish between “Inner FeAl” and “Outer FeAl” is based on the center of the needle-like FeAl2 zone. Next, “Inner FeAl” is defined as the thickness until reaching the matrix phase and “Outer FeAl” is the thickness until reaching the surface or Fe2Al5 layer.
The thickness of aluminized layers of sample after aluminizing at 900°C for 3 h, 6 h and 12 h.
The total thickness of the Al-rich layer grows with increasing aluminizing time. The FeAl layer grows at nearly the same rate in both the inner and outer layers. It is considered that FeAl2 is precipitated on the sample surface as coexisting phase of FeAl layer. Since the Al concentration is low inside the sample, the growth of acicular FeAl2 phase is stopped and the inner FeAl layer grows. When Fe2Al5 is formed, the growth of outer FeAl layer is not obtained. It implies that further progress of aluminizing would decrease the Fe concentration on the sample surface and forms Fe2Al5.
Figure 6 shows TG profiles of the samples after aluminizing under different conditions. X axis shows holding time at 1000°C in air. Some profiles were referred from previous study.10) The weight change ratio of sample aluminized at 700°C for 12 h continuously increases during the oxidation experiment. On the other hand, other samples show weight change ratio decreases immediately after the start of oxidation and does not exceed 3% for more than 20 hours. Figure 7 shows cross sectional SEM images of sample after aluminizing at 900°C for 3 h and oxidized at 1000°C for 24 h in air. Oxide layer of approximately 5 μm is observed on the sample surface.10) EDS shows strong mapping of Al in the oxide layer, suggesting that the phase formed on the sample surface is Al2O3. It indicates that the Al2O3 layer acted as a protective layer to inhibit oxygen diffusion into the sample. Also, little mapping of Fe is found in Al2O3. In equilibrium phase, Fe2O3 is considered to dissolve into Al2O3,15) however both Fe2O3 and Al2O3 are stable at high temperature and show high melting points. Therefore, Fe2O3 and Al2O3 are exist separately. Assuming that FexAly is formed in the Al-rich layer, the oxidation reaction is written by following equation.
| (2) |
TG profiles of samples after aluminizing under different conditions.
Cross sectional SEM images of samples after aluminizing at 900°C for 3 h and oxidized at 1000°C for 24 h in air. (Online version in color.)
Figure 8 shows TG profiles of sample after aluminizing, oxidation and repeated heat treatment. The weight change ratio of the sample after aluminizing, oxidation and repeated heat treatment shows approximately 0.3% in the second oxidation. Therefore, it is expected that the improvement of high temperature oxidation resistance is maintained for repeated heat treatment by the aluminizing treatment.
TG profiles of samples after aluminizing at 900°C for 3 h, oxidation and repeated heat treatment.
A new carbonizing and pulverizing process of woody biomass has been proposed, which utilizes sensible heat of industrial flue gas using a heat storage material (HSM). In the present study, Fe–Mn–C alloy was examined as a candidate of HSM, however, it has rather poor high-temperature oxidation resistance. The aluminizing treatment applying the pack cementation method was proposed to improve the high temperature oxidation resistance of Fe–Mn–C alloy. The results are summarized as follows:
• FeAl layer forms in the early stage of aluminizing, followed by Fe2Al5 layer formation. Then, acicular FeAl2 phase precipitates in the FeAl layer.
• Formation of FeAl2 and Fe2Al5 in Al-rich layer contributes to form continuous Al2O3 layer on the sample surface.
• The weight change ratio of the sample after aluminizing, oxidation and repeated heat treatment shows approximately 0.3% in the second oxidation.
