Social and Environmental Engineering
New Process for Resource Utilization of Converter Gas and Simulation on the Combustion of Converter Gas
2018 Volume 58 Issue 4 Pages 776-783
Details
2018 Volume 58 Issue 4 Pages 776-783
A new process for resource utilization of converter gas is proposed to produce CO2 at a lower cost in this paper. Converter gas is burned in O2–CO2 atmosphere instead of O2–N2 atmosphere in a closed combustion furnace. Flue gas with high concentration of CO2 can be used as low-grade CO2 directly or be purified as raw gas for preparing high-purity CO2. Both the low-grade CO2 and high-purity CO2 can be recycled for the converter blowing and sealing. In order to analyze the combustion characteristics of converter gas, numerical simulations based on a three dimensional combustion furnace model were carried out. Volume fraction of N2 in combustion flue gas dropped from 63.37% to 3.92%, meanwhile CO2 concentration in flue gas reached 95.08% when the N2 in air was totally replaced by CO2. In addition, to control the CO content in flue gas within the safety criterion, lower stoichiometry is an optimal solution for the converter gas compete combustion.
Converter is widely used in the steelmaking process globally and has been the main steelmaking method. Generally, the oxidation reaction of carbon in hot metal generates large amounts of carbon monoxide (CO) and carbon dioxide (CO2), which are the main compositions of converter gas.1) Beside CO and CO2, the volume fraction of nitrogen (N2) in converter gas is also relatively high, which is generally 10%–30%. The origin of N2 in converter gas includes nitrogen sealing of oxygen lance hole, air engulfment from furnace mouth and N2 bottom blowing. The nitrogen interfuse will reduce the calorie value of converter gas and restrict its application.2,3)
Currently, the converter gas is usually burned in heating-furnace or boiler under air atmosphere. Converter gas combustion in air leads to large amounts of nitrogen oxides formation which pollutes the environment seriously.4,5) Furthermore, the flue gas of combustion is valueless to be reclaimed and recycled utilization, because the nitrogen content in the flue gas is high.
Many researches have reported that top and bottom blowing CO2 in converter can reduce the amounts of smoke dust and iron loss, the contents of nitrogen and phosphorus in molten steel is lower as well.6,7) And CO2 can be converted to CO by the utilization of decarburization reaction during converter steelmaking process.8) However, it is a big problem for steel plants to obtain CO2 gas source. Although the techniques of CO2 purification and preparation is mature,9,10) but it is hard to obtain appropriate raw gas. In China, many plants tried to take the flue gas of lime kiln as the raw gas for CO2 purification, the volume fraction of CO2 in which is only 20%–35%. Lower CO2 content in raw gas means that higher cost is needed for purification process.11,12) Besides, the flue gas of lime kiln contains lots of harmful gases, like nitrogen oxides and sulfur dioxide et.al. Therefore, better raw gas for CO2 purification is in demand.
This article presents a new process for resource utilization of converter gas, producing raw gas with high CO2 content. Raw gas produced by this process can not only be used for CO2 purification, but also be used in converter blowing directly. The new process and its operation mode will be described in detail. The key technique in present process is combustion of converter gas. In order to analyze the combustion characteristics of converter gas under different atmospheres, a three dimensional combustion furnace model based on rolling reheating furnace was established. The Eddy Dissipation Concept (EDC) model with overall and detailed chemical kinetic mechanisms (DRM–19) was adopted as the combustion model in simulation works, because previous studies has found that the EDC model can provide accurate results in the research field of combustion.13,14,15,16)
Figure 1 shows the schematic diagram of the new process, including gas supply system of converter, steelmaking converter, converter gas cabinet, oxidant preparation system, combustion and heat utilization system, flue gas clean-up and storage system, CO2 purification and preparation system. Gas supply system of converter provides all the gases of top blowing, sealing of oxygen lance hole, atmosphere protection of converter mouth, and bottom blowing for steelmaking converter. In order to obtain better metallurgical effect and reduce the nitrogen content in converter gas, operation strategies of top blowing O2–CO2 mixture, CO2 sealing of oxygen lance hole, CO2 atmosphere protection of converter mouth, and bottom blowing CO2 are adopted. Converter gas cabinet is used to store converter gas. Oxidant preparation system is designed to mix O2 and CO2 in accordance with a certain proportion, which provides the oxidant for converter gas combustion. Then the converter gas is transported into combustion and heat utilization system together with oxidant gas, where the converter gas is burned in closed furnace and heat released from combustion is utilized in several ways. Generally, the flue gas generated by converter gas combustion is not clean, containing small amounts of water, oil and dust et.al probably. The flue gas clean-up and storage system is just set up for removing the impurities in the flue gas and storing the clean flue gas. As raw gas, the clean flue gas is purified for preparing high-purity CO2 in CO2 purification and preparation system.
Schematic diagram of the new process. 1-gas supply system of converter, 2-steelmaking converter, 3-converter gas cabinet, 4-oxidant preparation system, 5-combustion and heat utilization system, 6-flue gas clean-up and storage system, 7-CO2 purification and preparation system, 8-top blowing oxygen lance, 9-sealing of oxygen lance hole, 10-atmosphere protection of converter mouth, 11-bottom blowing tuyeres, 12-O2 supply pipe. (Online version in color.)
The detailed operation mode of above mentioned new process is described as follows. This process is a cyclical process, and CO2 plays an important role in the cycle. CO2 is not only the product of this process, but also the injection gas and sealing gas of the steelmaking converter. In initial state, it is necessary to obtain a small amount of high-purity CO2 from the outside and store it in the CO2 purification and preparation system. Then the CO2 is transported into gas supply system of converter through pipelines, mixing with other gas mediums and providing all the gases of top blowing, sealing, atmosphere protection and bottom blowing for steelmaking converter. Based on the gas supply system, the steelmaking converter can inject O2–CO2 mixture by top blowing oxygen lance and inject CO2 by bottom blowing tuyeres, which could reduce the dust generation amount and enhance the stirring effect of molten bath. At the same time, the oxygen lance hole is sealed by CO2, and the converter mouth is protected by blowing CO2, which can inhibit the involvement of nitrogen and reduce the nitrogen content in converter gas effectively. Converter gas with little nitrogen can be collected and stored in the converter gas cabinet by adopting that blowing scheme.
It should be noted that if converter gas is burned in air atmosphere, large amount of N2 will be involved into the combustion flue gas, leading to the dilution of CO2 concentration in flue gas and the increase of nitrogen oxide formation. Furthermore, if the converter gas is burned in pure oxygen atmosphere, temperature of combustion flame will be too high for most combustion furnace and the energy utilization efficiency will decrease. Therefore, the selection of oxidant for converter gas combustion is a crucial problem. In order to solve the N2 involvement and high combustion flame at the same time, taking O2 and CO2 mixture as the oxidant becomes the best option and oxidant preparation system is just set up to achieve that function. Those CO2 in oxidant is also provided by CO2 purification and preparation system. Converter gas with little nitrogen and O2–CO2 mixture oxidant are transported into combustion and heat utilization system and burned. Because the flue gas needs to be recycled and mixed with other gas for injection, it is necessary to ensure its complete combustion due to the security risk. Consequently, the combustion and heat utilization system is equipped with temperature sensor and flue gas analyzing device, data collected by that is used for feedback to adjust the flow rate and proportion of oxidant. The qualified flue gas with recycling value should be rich in CO2, and contains small amount of N2 inevitably as well as small amount of excess O2. Both the qualified and unqualified flue gas will be transported into flue gas clean-up and storage system. For the unqualified flue gas, it is cleaned to meet the national waste gas emission standard, then it is released into the atmosphere legally. For the qualified flue gas, it is cleaned and stored as clean flue gas. Since the clean flue gas is rich in CO2, it can be used as low-grade CO2 directly in many fields. In addition, the clean flue gas can also be purified as raw gas for preparing high-purity CO2 in CO2 purification and preparation system with low cost and high efficiency. Thereafter, this new process achieve circulating mode. Both the low-grade CO2 and high-purity CO2 can serve as the source of CO2, and be transported into gas supply system of converter. The source of CO2 for converter blowing can be solved by the new process, and there is no need to obtain CO2 from the outside any more. Actually, the preparation yield of high-purity CO2 from converter gas combustion can meet the demand of converter blowing easily. Extra high-purity CO2 can be used in other field or be sold.
N2 in the combustion air replaced by CO2 will induce changes in the combustion rate of converter gas as well as in radiation properties and heat capacity of the flame and flue gas. Numerical simulation based on the software ANSYS-FLUENT was performed to analyze the combustion characteristics of converter gas under O2–N2 and O2–CO2 atmosphere. The temperature distribution field and species concentration distribution in the combustion furnace have been studied in the numerical research. The numerical models used in this simulation and results obtained from this work is described and discussed in detail.
3.1. Geometry Model and Simulation SchemeA square combustion furnace based on rolling reheating furnace was considered to study the combustion characteristics of converter gas. To ensure the reliability of the results, a three dimensional model was built with the ratio of 1:1 in the numerical simulation. Figure 2 shows the geometric representation and dimension parameters of the furnace.
Geometric representation and dimension parameters of the combustion furnace. (Online version in color.)
To reduce the computational time of the numerical simulation, only one quarter of the total computational domain was simulated by splitting the entire domain. The computational domain along with the detailed grid used in the simulation is shown in Fig. 3.
The computational domain with the detailed grid.
Based on the actual conditions and theoretical calculation, the assumed converter gas composition and its thermophysical properties are shown in Table 1. Because the designed combustion power in this research is 10 KW constantly, flow rate and turbulence parameters of the converter gas jet can be calculated and also shown in Table 1. Where turbulent intensity is the key parameter describing the characteristics of jet turbulence, which is equal to the ratio of turbulent pulse velocity to average velocity.
| Converter gas composition | Molar mass | Dynamic viscosity (Pa s–1) | Calorific value (MJ kg–1) | Flow rate (kg s–1) | Turbulent intensity (%) | Jet momentum (kg m s–2) | ||
|---|---|---|---|---|---|---|---|---|
| CO | CO2 | N2 | ||||||
| 75% | 15% | 10% | 30.4 | 1.74e–5 | 6.98 | 1.43e–3 | 5.335 | 8.21e–3 |
Both the effect of oxidant composition and stoichiometry on the combustion were studied in this work. It should be noted that stoichiometry is defined as the ratio of theoretical oxidant mass required for fuel complete combustion to actual supplied oxidant mass. When the oxidant composition varies, the stoichiometry was kept 0.9, which means a little oxygen-rich state in the furnace. Accordingly, when the stoichiometry was varied from oxygen-rich to oxygen-lean, the oxidant composition was maintained. The specific simulation conditions are shown in Table 2.
| NO. | Oxidant compositions | Stoichiometry | Flux of oxidant (kg s–1) | Turbulent intensity (%) | Jet momentum (kg m s–2) |
|---|---|---|---|---|---|
| 1 | 21%O2+79%N2 | 0.9 | 2.70e–3 | 5.413 | 7.67e–3 |
| 2 | 21%O2+79%CO2 | 0.9 | 3.88e–3 | 5.080 | 1.10e–2 |
| 3 | 40%O2+60%CO2 | 0.9 | 1.93e–3 | 5.587 | 2.87e–3 |
| 4 | 60%O2+40%CO2 | 0.9 | 1.20e–3 | 5.972 | 1.20e–3 |
| 5 | 80%O2+20%CO2 | 0.9 | 8.45e–4 | 6.294 | 6.30e–4 |
| 6 | 100%O2 | 0.9 | 6.29e–4 | 6.583 | 3.75e–4 |
| 7 | 40%O2+60%CO2 | 0.7 | 2.48e–3 | 5.414 | 4.75e–3 |
| 8 | 40%O2+60%CO2 | 1.0 | 1.73e–3 | 5.661 | 2.33e–3 |
| 9 | 40%O2+60%CO2 | 1.1 | 1.58e–3 | 5.729 | 1.92e–3 |
| 10 | 40%O2+60%CO2 | 1.3 | 1.33e–3 | 5.850 | 1.38e–3 |
As stated above, the detailed chemical kinetic modeling was performed using codes from the CHEMKIN library. Because there was no hydrocarbons in the converter gas, so the relative simplified mechanism (DRM-19) was applied for calculation. The reaction mechanism, involving 21 species and 84 elementary reactions, consists of oxidation subsets for H2, CO and hydrocarbons et al. In this simulation, the most important reaction is the oxidation of CO, whose oxidation mechanism can be expressed as follows:
| (R1) |
| (R2) |
| (R3) |
| (R4) |
Reaction of CO+O2 (R1) was previously thought to be important in CO oxidation under conditions without hydrogen containing species, but recent works have proven that its reaction rate is much slower than indicated by the early estimates. A small amount of H2O or H2 has great influence on the oxidation rate of CO. Because the oxidation steps with OH group (R3) is much faster than that with O2 or O. During the simulation procedure, it was found that converter gas cannot be ignited even if the ignition temperature was more than 4000 K before H2O was added, which validates the correctness of above-mentioned theory. Then 1 pct. H2O was added before ignition, the converter gas was ignited easily by ignition temperature of 2000 K. So the CO oxidation steps can be described as follows:
Although CO+O2 (R1) is very slow and have little contribution to CO2 formation, but it plays a role in inspiring chain reactions. The actual oxidation of CO is achieved by CO+OH (R3), which is a chain transfer reaction, a hydrogen atom is produced simultaneously. This hydrogen atom further reacts with O2 to generate OH and O (R4). Then these radicals return to the oxidation step (R3) and the first chain branching reaction (R2). For the whole reaction mechanism, the reaction of CO+OH=CO2+H (R3) is the most critical step of CO oxidation.
According Mardani et al.13) and Christo et al.,17) there is a marginal difference between the results calculated with and without considering the radiation influence. Considering the great difference of radiation properties between N2 and CO2, so the Discrete Ordinate (DO) radiation model with Weighted Sum of Gray Gas Model (WSGGM)18) was applied to calculate the combustion flame in this work. Meanwhile, combustion flames under O2–N2 and O2–CO2 atmospheres with and without considering the radiation influence were compared and discussed in present work.
3.3. Turbulence Models and Computation ProcedureStandard k–ε model with the standard wall function was implemented for modeling the turbulent flows, which was a semi-empirical model based on model transport equations for the turbulence kinetic energy (k) and its dissipation rate (ε). It is known that the k–ε turbulence model predicts the flow features of the multiple jets with little deviations due to the anisotropy of the turbulence.19) However, this model was still used in most numerical works because it is easy to get reasonable solutions quickly with the standard k–ε model.
The steady, pressure-based solver and the implicit method was used to discretize and solve model equations. The SIMPLE algorithm method was used to solve the pressure-velocity coupling. In order to improve the accuracy of the simulation results, the second-order upwind scheme was utilized for discretizing the density, momentum, turbulent kinetic energy, et al. And the pressure was discretized by standard scheme. Convergence was accepted when the residuals were less than 10–6 for the energy and 10–5 for all the other variables.20)
3.4. Boundary Conditions and Assumptions(1) Converter gas entrance and oxidant entrances were set as mass flow inlet.
(2) Temperature of inlet converter gas and inlet oxidant were set to 298.15 K.
(3) The flue gas exit was set as pressure outlet, and the gauge pressure of outlet was set to 0 Pa, the backflow temperature of outlet was set to 1500 K.
(4) All the walls were set as constant temperature wall. According to the experimental and simulation results obtained by other researches15,16), combined with the specific model used in this work, temperature of wall inside the combustion furnace was set to 1473 K, and temperature of converter gas entrance wall, oxidant entrance wall and flue gas exit wall were set to 298.15 K because of the water cooling effect.
As mentioned above, lots of studies has proved that EDC model coupled with detailed chemical mechanism can provide accurate simulation results of combustion.13,14,15,16) P. F. Li21,22,23) from Peking University carried a large number of simulation works and experiments on gas fuel combustion, whose results verified the accuracy of the model used in this research.
Further, numerical simulation on the Jet in Hot Coflow (JHC) Burner using above mentioned model was carried out and the simulation results were compared with the experimental data measured by B. B. Dally.24) Figure 4 shows the cross-section of jet in hot coflow burner and the detail grid arrangement of the two-dimensional computational domain. Figure 5 shows the radial temperature distribution at the axial location of x=30 mm obtained by numerical simulation and experiment, respectively. It can be seen from Fig. 5 that the simulated results agree well with the experimental results. Therefore, the validation of the model used in this work is acceptable.
(a) cross-section of jet in hot coflow burner, (b) detail grid arrangement of the two-dimensional computational domain. (Online version in color.)
Radial temperature distribution at the axial location of x=30 mm.
It is well known that monatomic and diatomic molecules with symmetrical molecular structure have no ability of radiation absorption and emission, such as O2 and N2. But CO2 has considerable power of radiation, which will change the combustion characteristics significantly after N2 replaced by CO2 in oxidant.25,26) Figure 6 shows the temperature field distributions with and without loading the radiation model during numerical simulation under three different oxidant atmospheres. The effect of radiation will be discussed in this section.
Temperature field distributions with and without loading the radiation model. (Online version in color.)
As can be seen from Fig. 6, the common feature under three oxidant atmospheres is that the start position of high-temperature zone is closer to the entrance with radiation than without radiation. When the converter gas burns in pure O2 and O2–N2 atmospheres, the effect of radiation is relatively small, because both O2 and N2 have no ability of radiation. As a contrast, when the converter gas burns in atmosphere with high CO2 concentration, big difference of temperature distribution appears. The contour indicates that high CO2 level will increase the area of high-temperature zone and ascend its start location significantly.
In order to analyze the temperature changes quantitatively, the static temperature distribution on the center axis is shown in Fig. 7. The maximum temperature with radiation is higher than that without radiation under all three oxidant atmospheres, although the degree of deviation varies widely. When the oxidant is pure O2, the difference of combustion temperature is very small, which is almost negligible. But for the cases of O2–N2 and O2–CO2, this phenomenon is obvious.
Temperature distribution on the center axis with and without radiation. (Online version in color.)
The reason for position advance and temperature increase after loading radiation model can be expressed as that the CO and CO2 in both combustion reactants and products can absorb radiation energy and convert it into internal energy. Because the extra energy income, temperature of gas inside the furnace increase faster, and the maximum temperature rise accordingly. However, the reason why radiation has little effect on the case of pure O2 is that the radiation absorption coefficient decrease with the increase of temperature, and the temperature of combustion flame in pure O2 atmosphere is much higher.
5.2. Effect of Oxidant CompositionTraditionally, the converter gas is burned in air atmosphere, the composition of which is 21%O2+79%N2. In order to compare the oxidation of converter gas under highly diluted conditions in N2 and CO2 respectively, the case of oxidant composition of 21%O2+79%CO2 is discussed firstly. Figure 8 shows the temperature distributions inside the combustion furnace when the oxidant compositions are 21%O2+79%N2 and 21%O2+79%CO2, respectively. As can be seen from Fig. 8 intuitively, although the shapes of combustion flame are quite similar, but both the maximum temperature and the area of high-temperature zone are higher when the oxidant composition is 21%O2+79%N2. Specific parameters indicating the combustion characteristics of above two cases are shown in Table 3. The exothermic power of both two cases is consistent with the designed combustion power of 10 KW basically, which demonstrates the validity and correctness of the models used in this work. Although the exothermic power of combustion reaction is approximately equal, but the maximum temperature and global mean temperature inside the furnace of 21%O2+79%N2 is 158 K and 93 K higher than that of 21%O2+79%CO2, respectively. It is analyzed that the difference of specific heat capacity between N2 and CO2 caused the difference of combustion temperature field. At 1473 K, the specific heat capacity of N2 is 1.57 KJ Nm–3 K–1 and the specific heat capacity of CO2 is 2.63 KJ Nm–3 K–1. Meanwhile, the difference gets bigger along with the increase of temperature. Under the condition of similar exothermic power, bigger specific heat capacity will necessarily lead to lower temperature, which explains the reason why the flame temperature is lower in CO2 than in N2.
Comparison of temperature distributions in O2–N2 atmosphere and O2–CO2 atmosphere. (Online version in color.)
| Oxidant compositions | Maximum temperature (K) | Global mean temperature (K) | Exothermic power (KW) | Compositions of flue gas | ||
|---|---|---|---|---|---|---|
| N2 (vol. %) | CO2 (vol. %) | O2 (vol. %) | ||||
| 21%O2+79%CO2 | 1647 | 1577 | 10.01 | 3.92 | 95.08 | 0.92 |
| 21%O2+79%N2 | 1805 | 1670 | 9.98 | 63.37 | 35.67 | 0.93 |
The most important purpose to use CO2 instead of N2 in oxidant is to reduce the N2 content and increase the CO2 content in flue gas, which helps to reduce the nitrogen oxides generation and lower the cost of CO2 purification. The area-weighted average composition of flue gas is also shown in Table 3. The volume fraction of N2 in flue gas drops from 63.37% to 3.92% when the N2 in air is totally replaced by CO2. Flue gas consisting high proportion of CO2 and little N2 is obtained easily by this method.
Now that this method works, it is necessary to investigate the effect of CO2 content in O2–CO2 mixture on the converter gas combustion characteristics. In present research, CO2 content varies from 79% to 0, and the temperature fields of five cases are all shown in Fig. 9. Obviously, the flame temperature increase along with the decrease of CO2 content, and the high-temperature zone expands significantly. Figure 10 shows the axial temperature distribution at the centerline of combustion furnace. For all the cases, the temperature on the centerline increases rapidly just after the jet exit from the entrance until it reaches a peak, then the temperature begins to fall slowly. With the decrease of CO2 content in O2–CO2 oxidant, the peak temperature increase gradually and the position of peak temperature gets closer to the entrance.
Effect of CO2 content in O2–CO2 oxidant on the temperature field. (Online version in color.)
Effect of CO2 content in O2–CO2 oxidant on the axial temperature distribution. (Online version in color.)
Table 4 shows the specific parameters indicating the combustion characteristics. As shown in Table 4, when the converter gas is burned in pure O2, the peak temperature reaches 2383 K, and the distance between gas entrance and peak temperature position is the shortest, which will shorten the campaign life of combustion furnace significantly. It should be noted that the exothermic power of all five cases are kept around 10 KW, but the flame temperature varies obviously. The main factor for that phenomenon is the difference of flue gas volume. Because with the decrease of CO2 content, the volume of oxidant gas decrease accordingly, resulting in the decrease of flue gas volume consequently.
| Oxidant compositions | Maximum temperature (K) | Global mean temperature (K) | Exothermic power (KW) | Peak temperature position (m) |
|---|---|---|---|---|
| 21%O2+79%CO2 | 1647 | 1577 | 10.01 | 0.68571 |
| 40%O2+60%CO2 | 1996 | 1746 | 10.09 | 0.64538 |
| 60%O2+40%CO2 | 2191 | 1822 | 10.13 | 0.58487 |
| 80%O2+20%CO2 | 2302 | 1860 | 9.99 | 0.52437 |
| Pure O2 | 2383 | 1886 | 9.97 | 0.48403 |
As the CO2 content in oxidant gas increases, the N2 content in flue gas is diluted and the CO2 concentration in flue gas increases, which helps expand the application of flue gas and reduce the cost of CO2 purification. As shown in Table 5, when the N2 in air is totally replaced by CO2, the CO2 concentration in flue gas reaches 95.08%, which can be used as low-grade CO2 product directly in many fields.
| Oxidant compositions | Compositions of flue gas | ||
|---|---|---|---|
| N2 (vol.%) | CO2 (vol.%) | O2 (vol.%) | |
| 21%O2+79%CO2 | 3.92 | 95.08 | 0.92 |
| 40%O2+60%CO2 | 5.98 | 92.46 | 1.19 |
| 60%O2+40%CO2 | 7.31 | 90.88 | 1.37 |
| 80%O2+20%CO2 | 8.89 | 89.02 | 2.05 |
| Pure O2 | 9.78 | 87.93 | 2.25 |
For the new process, it is very important to control the CO content in flue gas, because of the poisoning and explosion risk of CO. Therefore, it is necessary to study the effect of stoichiometry on the CO content in flue gas. As mentioned above, the composition of oxidant is maintained as 40%O2+60%CO2, and the stoichiometry varies from 0.7 to 1.3. Figure 11 shows the axial mole fraction of CO at the centerline of combustion furnace, meanwhile the detailed compositions of flue gas at furnace outlet are shown in Table 6. The original mole fractions of CO in converter gas are all 75%, then they start to decline and reach respective steady value rapidly. Both the steady values of stoichiometry 0.7 and stoichiometry 0.9 are very small, meeting security requirements. However, once the stoichiometry is bigger than 0.9, mole fraction of CO in flue gas increases obviously. Even when the stoichiometry is 1.0, mole fraction of CO in flue gas achieve 4.07%, and it goes up steadily along with the increase of stoichiometry. The reason for that phenomenon can be explained as that with the increase of stoichiometry, the atmosphere in combustion furnace changes from oxygen-rich to oxygen-lean, leading to the incomplete combustion of CO and lots of unburned CO remained in flue gas. High CO content is fatal for the application of flue gas, because it may induce workers’ gas poisoning and explosion. Hence, lower stoichiometry is an optimal solution for the converter gas compete combustion. Flue gas with very little CO and few excess O2 is safe and suitable for converter blowing.
Effect of the stoichiometry on the axial mole fraction of CO. (Online version in color.)
| Oxidant compositions | Stoichiometry | Compositions of flue gas | |||
|---|---|---|---|---|---|
| N2 (vol.%) | CO2 (vol.%) | O2 (vol.%) | CO (vol.%) | ||
| 40%O2+60%CO2 | 0.7 | 5.25 | 87.54 | 7.18 | 4.07e–3 |
| 0.9 | 5.98 | 92.46 | 1.19 | 5.84e–3 | |
| 1.0 | 6.23 | 87.72 | 1.03e–5 | 4.07 | |
| 1.1 | 6.50 | 83.86 | 1.38e–6 | 8.05 | |
| 1.3 | 6.93 | 77.26 | 1.81e–7 | 14.84 | |
A new process for resource utilization of converter gas is proposed by the authors. Converter gas is burned with O2–CO2 mixture instead of air (O2–N2 mixture) in a closed combustion furnace. CO2 in flue gas can be highly concentrated while N2 in air is totally replaced by CO2, which could be used as low-grade CO2 directly or be purified as raw gas for preparing high-purity CO2. This new process could produce CO2 at a lower cost, and utilize the combustion flue gas as resource, reducing the emissions of greenhouse gases and nitrogen oxides.
In order to analyze the combustion characteristics of converter gas in O2–CO2 atmosphere, a three dimensional combustion furnace model was developed. Conclusions obtained from the simulations are as follows:
(1) When the N2 in air is totally replaced by CO2, both the maximum temperature and the area of high-temperature zone in the combustion furnace decline. Meanwhile, volume fraction of N2 in combustion flue gas drops from 63.37% to 3.92%, and CO2 concentration in flue gas reaches 95.08%, which increases the utilization value of flue gas and reduces the formation of nitrogen oxides significantly.
(2) As the CO2 content in O2–CO2 oxidant increases, peak temperature of combustion flame decreases gradually and the position of peak temperature gets further away from the entrance. In terms of reducing CO2 consumption, combustion in pure O2 atmosphere is preferred, although the flame temperature is high. But the problem can be expected to solve by optimize the burner structure and configuration.
(3) When the stoichiometry is 0.7 and 0.9, volume fraction of CO is very small, meeting security requirements. Once the stoichiometry is bigger than 0.9, volume fraction of CO in flue gas increases obviously. This is because the combustion state changes from oxygen-rich to oxygen-lean with the increase of stoichiometry. Therefore, lower stoichiometry is an optimal solution for the converter gas compete combustion.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51574021 and 51474024). The authors would like to extend their sincere grateful to those who provided help and assistance.
