2013 Volume 53 Issue 9 Pages 1599-1606
Decrease of carbon dioxide emission is a serious subject in the steel works. Utilization of biomass as a carbon-neutral agent is an attractive one for iron ore sintering. Sinter pot tests were carried out with using raw biomass and biomass carbonized char. It is not good on yield and exhaust gas that raw biomass is used directly as carbon material for iron ore sintering. While, it is good on the productivity and the exhaust gas (NOx, SOx, dust, dioxins) that biomass carbonized char is used as carbon material. With using biomass char for the sintering, it is necessary to optimize operation (size control and moisture control of the biomass char), because combustion rate of the biomass char is too high. Biomass carbonized char is evaluated on sinter yield as similar as anthracite or coke. The biomass char is effective to decrease CO2, NOx, SOx, dust etc. emission in sinter exhaust gas.
For the background of the increased steel production on a global scale, the short supply and poor qualities of iron ore and coal are the most serious issues. On the other hand, for the preservation of the atmospheric environment, the reduction of CO2 gas generation in a global scale, the reduction of NOx and SOx generation in a regional scale, have been strongly demanded. For the production of iron ore sinter, the high productivity and the reduction of air pollutants in flue gas have been demanded, in the situation with increasing fine size iron ore and shorting carbon material on the sinter raw material.
These subjects on the sinter production, a lot of research and technological development have been carried out except for the reduction of CO2 through the ages. For reducing CO2, there are many reports of energy-saving technologies1) and the theoretical study2) for the lowest energy of sinter production. However for the terms of reducing CO2 without energy saving, there are not many research reports3,4,5,6,7,8,9,10,11,12,13,14,15,16) on the conversion of carbon material type. The use of biomass is interesting as the terms of CO2 reduction with carbon neutral. As an idea to take advantage, sinter production process with utilization of biomass is scattered to the patent gazette,13) however the technical discussions are poor. By M. Zandi et al.14) the sintering tests with small scale pot at that the coke powder was swapped for the biomass-based raw agricultural were carried out, and by R. Lovel et al.15,16) the sintering pot test at that the anthracite coal and the coke powder are swapped with a woody biomass-char was carried out. For the sintering pot tests, the improving trend on the sintering speed was observed, but the results are not good on the yield because of the poor exothermic energy.
Therefore, in this study, the sintering pot tests of scale 54 kg with using raw biomass or biomass-char are carried out. The effects of the biomass on sintering operations and the sinter quality is investigated and the possibility of reducing CO2 by biomass utilization is discussed.
For the size of the sintering pot, we used with the standard equipment of 300 mm diameter and 500 mm height. The experimental conditions are shown in Table 1. There are two mode conditions. At the first condition, the mass and moisture content of sinter mix charged into the pot, the flow rate of exhaust gas and sintering time are all constant. Because by adopting the first condition of the mass factor constant, the carbon material is evaluated with limit to the only combustion properties. At the 2nd. condition, the mass of sinter mix charged into the pot is constant as same as the 1st. condition. However, at this 2nd condition, to evaluate the actual sintering operation, the suction pressure is kept at 10 kPa, and is varied gas flow rate according to the permeability of the sinter bed. In order to optimize the carbon combustion, the moisture content of sinter mix and particle size of carbon material are changed.
| Condition 1 | Sinter mixture | 59.0 wet-kg (moisture 7%) |
| Charging method | 380 mm drop, uniform | |
| Ignition | 1300°C×60 s | |
| Suction | 1.3 Nm3/min constant | |
| Sintering time | 35.0 min | |
| Hearth layer | Sinter ore: 10–15 mm: 2.0 kg | |
| Sinter products | 2 m drop × 4 times: +5 mm | |
| Condition 2 | Sinter mixture | 59.0 wet-kg (moisture 7%) |
| Charging method | 380 mm drop, uniform | |
| Ignition | 1300°C×60 s | |
| Suction | –10 kPa constant | |
| Sintering time | Peak temp. of exhaust gas | |
| Herath layer | Sinter ore: 10–15 mm: 2.0 kg | |
| Sinter products | 2 m drop × 4 times: +5 mm |
The blending ratio conditions of the sinter mix are shown in Table 2. After mixing the blended materials in the drum mixer for 2 min, the water is added to the mixed material. They are granulated in the drum mixer for 4 min. The sinter mix is weighed amount (59.0 wet-kg) 53.9 dry-kg, and then is set in the charging device located 380 mm upper position from top surface of the sinter pot. The sinter mix is fallen into the pot and is charged to flatten the surface. In addition, because 2.0 kg (20 mm) hearth layer is laid in advance, the sinter bed height becomes about 420–450 mm depending on the properties of the carbon material.
| Brand | mass% |
|---|---|
| SF. Pisolite | 54 |
| SF. Austlaria | 16 |
| SF. Brazil | 4 |
| Limestone | 12 |
| Return fine | 14 |
| sum | 100 |
| Carbon material | 4.0–5.0 |
At the 1st condition, the sintering time of the test is set for 35 min according to the results of the past study.17) At the 2nd condition, the sintering time is set to 3 min after the time indicated maximum temperature of the exhaust gas. Then the hot sinter cake is cooled by air, and is crushed by 4 times fall with using the shatter test equipment (2 m drop × 4 times: JIS-M8711). The crushed sinter cake is classified with a sieve 5 mm, and the deposited sinter ores on the sieve are evaluated as sinter product.
2.2. Properties of Carbon MaterialsThe properties of the carbon materials with used in the pot tests are shown in Table 3. Since it is not clear indicator of the carbon material properties to determine the optimal carbon blending ratio of the sinter mix, the other pot tests with many kinds of carbon material other than biomass-char shown in Tables 3 and 5 are also carried out. In addition, the pot test with using raw biomass is challenged, but it could not be sintered (less than 30% yield of product). Then the sinter pot tests were carried out with using biomass-char carbonized biomass. The blending ratio for the standard condition is set at 4.0 mass%, and is set at 5.0 mass% in the case of the low yield. In addition, it is well known that the particle size distribution of carbon material will affect very largely on sintering, as carbon material ratio. In order to eliminate the effect of the particle size distribution, the particle size distribution of the carbon material becomes constant. The size of the coke18) for standard condition is shown in Table 4, which is generally used for the sintering. In addition, in order to study the effect of the particle size of biomass-char on sinter yield, the size is changed at some pot tests.
| Brand | Fix.C. | V.M. | Ash | *E (Low Temp.) | Consumption | |
|---|---|---|---|---|---|---|
| % | % | % | kJ/mol | Fix.C.-kg/s-t | ||
| A | Coke | 86.0 | 1.1 | 12.9 | 128 | 50.5 |
| B | Coke | 88.0 | 0.7 | 11.3 | 116 | 53.9 |
| C | Coke | 86.0 | 0.9 | 13.1 | 129 | 52.4 |
| D | Coke | 87.5 | 0.6 | 12.0 | 114 | 52.5 |
| E | Anthracite | 88.5 | 6.4 | 5.0 | 100 | 54.2 |
| F | Anthracite | 88.8 | 6.4 | 4.8 | 102 | 55.0 |
| G | Anthracite | 81.2 | 9.2 | 9.6 | 119 | 50.0 |
| H | Bituminous Coal | 68.7 | 22.4 | 8.9 | 90 | 47.7 |
| I | Bituminous Coal | 59.7 | 32.5 | 7.9 | 67 | 50.1 |
| J | Subbituminous Coal | 50.0 | 49.2 | 0.8 | 62 | 74.0 |
| K | Biomass char | 65.3 | 32.0 | 2.8 | 104 | 96.7 |
| L | Biomass char | 67.5 | 30.3 | 2.1 | 107 | 100.1 |
| M | Biomass char | 93.1 | 2.8 | 4.2 | 118 | 98.8 |
| N | Biomass char | 91.7 | 3.2 | 5.1 | 120 | 109.2 |
| O | Biomass char | 76.1 | 19.2 | 4.7 | 144 | 101.8 |
| P | Biomass char | 89.8 | 1.9 | 8.3 | 114 | 85.2 |
| Q | Raw biomass | 18.9 | 77.5 | 3.6 | – | – |
| R | Pitch | 97.8 | 0.7 | 1.5 | 175 | 54.3 |
| S | Pitch | 60.6 | 38.8 | 0.6 | 105 | 60.9 |
| T | Graphite | 76.8 | 4.3 | 18.9 | 162 | 60.4 |
| U | Graphite | 99.8 | 0.2 | 0.0 | 221 | 57.2 |
| V | *Blend (Plant) | 86.8 | 1.0 | 12.3 | – | 57.5 |
| (mm) | (mass%) |
|---|---|
| 4.76–2.0 | 24 |
| 2.0–1.0 | 21 |
| 1.0–0.25 | 35 |
| –0.25 | 20 |
The sinter yield is investigated by the sinter pot tests with the 1st. condition (constant gas flow rate). The carbon material used at the pot tests is shown in Table 3. The particle size distribution of the carbon material is constant as shown in Table 4. In general, sinter yield is determined by the temperature pattern in the bed and iron ore particle size. The temperature pattern is also affected by the gas flow rate and the bed height, and is basically determined by the heat generated by the combustion of carbon material. Therefore, it is important that the amount of heat generated by combustion of carbon material charged into the pot. The volatiles in the carbon material thermally decomposed at low temperature and carbonate mineral thermally decomposed at about 800°C are existed in the sinter mix. It is considered that the calorific value of carbon material and Total.C in the sinter mix are not important, but the Fix.C content and Free.C content in sinter mix are important. These value shall be deeply related to the temperature in the bed. The Fix.C value in carbon material is measured by industrial analysis of JIS-M9212, and is calculated by the following equation.
Effect of Fix.C amount on yield.
Next, using biomass-char with rich Fix.C (90%>) which was carbonized at high temperature (900°C), the effects of sinter yield were investigated. The result of the comparison between the coke and biomass-char are shown in Fig. 2. The sinter yield with biomass-char is significantly lower than that of coke at the condition of the same level Fix.C. The sinter yield is ensured by increasing the blending ratio of 4% to 5%. However, the increase in the blending ratio has still high Fix.C consumption as shown in Table 3, and the swapping coke to biomass-char is lead to a significant increase of energy consumption.
Effect of biomass char (carbonized at 900°C) on yield.
Figure 319) shows the results of sinter pot tests which were separately investigated for the effect of the input caloric energy on sinter yield. In this figure, the effect of the caloric energy inputted to sinter pot on sinter yield is indicated at the condition of 3 cases (① ignition time (LPG volume), ② blending ratio of the –5 mm particle of reduced iron ore, ③ blending ratio of biomass-char). The caloric energy is calculated from assuming of the complete combustion on ignition LPG gas, the complete re-oxidation to Fe2O3 on reduced particle (m-Fe, Fe (II)), and the complete combustion of Fix.C on biomass-char. The sinter yield of the biomass-char is lower than that of the change cases with ignition gas and the reduced iron powder. Then, it is considered that something of biomass-char except the caloric energy exists as the cause of the decrease in sinter yield.
Effect of input heating energy on sinter yield (Circle mark: Changing of the ignition time, Triangle mark: Use of the reduced iron ore, Square make: Swapping to the biomass char).
The change in gas composition (CO, CO2) in the exhaust gas, at the condition of gas flow rate constant, is shown in Fig. 4. The combustion rate of biomass-char is faster than that of coke and anthracite, and the biomass-char have a higher ratio of CO / (CO + CO2) than that of coke and anthracite on the exhaust gas composition.
Effect of biomass char on CO and CO2 concentration in exhaust gas at sinter pot test (1.3 Nm3/min constant).
Figures 5 and 6 shows the concept of effect on the temperature pattern in the sinter bed, combustion behavior and combustion rate of the carbon material in the sintering process. It is considered that the oxygen, which is supplied to the particle surface of carbon material, raises the CO and CO2 gas in the reaction boundary film surrounding the carbon material particles by the combustion reaction with carbon. The bulk gas through the sinter bed contains enough oxygen, however the gas flow rate at surrounding the carbon material is low and does not lead to diffuse enough oxygen into the boundary film. For this reason, when the combustion rate of carbon material is too high, the oxygen concentration is low within the boundary film. The combustion of the lack of oxygen generates rich CO gas and the CO is released to the outside film. When incomplete combustion with CO generation occurs, the amount of heat generation decreases and the temperature in the sinter bed decreases at a whole.
Effect of combustion rate of carbon material on react zone and gas component in sinter bed.
When the combustion rate is increased, the rate of heat generation is increased and the maximum temperature is also higher on the temperature pattern in the sinter bed as shown in Fig. 6. However, when the combustion heat of carbon material in sinter mix is assumed to be constant, high combustion rate means short on combustion time, it leads to the result that the holding time at high temperature in the sinter bed becomes short. For sintering reaction which is done by melt bonding between the solid particls, it is important to ensure the melting temperature and the time required for the melt to move. When the combustion rate is too high, it is considered that the sinter yield is decreased because of the shortage of the holding time. Of course, at the constant condition of gas flow rate, higher combustion rate and lower ignition temperature of the carbon material have a trend to improve sinter productivity, because of the shortage of sintering time by the increase of flame front speed.
Concept of heat pattern in sinter bed for combustion rate.
The physical properties of the used carbon material are shown in Table 5. For the ignition temperature of carbon material, the temperature of biomass-char is lower than the temperature of coke and is similar to the temperature of anthracite. For the measurements of activation energy20) in low-temperature combustion shown in Table 3, the value of biomass-char is lower than that of coke and is similar to that of anthracite. As the bulk density of biomass-char is the lowest and saturation value of absorbed water is the highest, biomass-char is higher porosity and shape factor. For specific surface area which is related to the micro reactivity, some biomass-char are a very high level, or any others are as same as coke and anthracite.
| Carbon material | *Ignition temp. | Surface area | Absorbed water | Bulk density | N | S | |
|---|---|---|---|---|---|---|---|
| Brand | (°C) | BET (m2/g) | (mass%) | (g/cm3) | (mass%) | (mass%) | |
| Coke | A | 454 | 6.5 | 4.2 | 0.77 | 1.22 | 0.58 |
| Coke | C | 528 | 13.1 | 4.7 | 0.78 | 1.24 | 0.64 |
| Anthracite | E | 314 | 10.6 | 4.6 | 0.86 | 1.31 | 0.56 |
| Anthracite | F | 297 | 3.7 | 2.5 | 0.83 | 1.26 | 0.52 |
| Anthracite | G | 310 | 1.4 | 4.2 | 0.88 | 1.24 | 0.48 |
| Biomass char | M | 327 | 8.9 | 15.1 | 0.42 | 0.83 | 0.06 |
| Biomass char | P | 288 | 224.7 | 12.8 | 0.58 | 0.95 | 0.03 |
The mechanisms that the effect of biomass-char on sinter yield shows in Fig. 7. Because biomass-char is porous and large shape factor, the combustion rate of the biomass-char particles are higher than that of coke and anthracite. Then, biomass-char is heated in the sintering bed to a high temperature in a shorter time. So it seems that the sinter yield is reduced by reducing the holding time for high temperature with sinter reaction (more than 1200°C). Furthermore, it is discussed that the yield reduction may due to reduce the oxygen concentration in the boundary film of combustion and to increase the ratio of incomplete combustion with more CO generation, and to reduce the heat generation because of high combustion rate.
Mechanism flow of carbon material on sinter yield and productivity.
In order to suppress the high combustion rate of biomass-char particles, course sizing (1–5 mm) and granulating with high moisture were carried out. The results of the pot test is shown in Fig. 8. The blending ratio is 4 mass% for the coke case, and is 5 mass% for the biomass-char case. At the condition with the usual size coke and 4 mass% blend, the sinter yield is 70 mass%. At the condition with the usual size coke and 5 mass% blend, the sinter yield was 80 mass% (not illustrate in the figure). In general, the optimum size of coke particle on sinter yield is reported to be 2–0.5 mm.21) In the range of these test results, the small size without –0.5 mm fine for coke particle is the best. On the other hand, the coarse size (1–5 mm) for biomass-char becomes good results and the sinter yield at biomass-char M or P are 80 mass%. So, 1–5 mm biomass-char can be swapped to the usual size coke at the condition of equivalent 5 mass%. In addition, by the high moisture in sinter mix (7→8 mass%), the sinter yield with coke decreases, while the sinter yield with biomass-char increases. The improvement mechanism by increasing the moisture with biomass-char is not clear. It is seemed that the improvement due to directly suppress combustion by water evaporation, or due to suppress combustion by ore adhesion to the surface of biomass-char particle with granulation, or due to ensure good granulation and permeability by filling water into the pore of porous biomass-char.
Effect of coarser size of biomass char and higher moisture of the mixture on sinter yield. (Coke ratio: 4.0 mass%, Biomass char ratio: 5.0 mass%).
Therefore, It is suggested that the optimum operating conditions (carbon material size, moisture of sinter mix) for coke, anthracite and biomass-char are individually different. As the biomass-char is concerned about catching fire naturally, biomass-char is needed to have rich moisture considering the ship transportation and much storage. Therefore high moisture condition of biomass-char cannot be said to be necessarily bad. While coarsening carbon material affects the distribution of Fix.C content at the height direction in the bed, the evaluation for carbon material on sinter yield is difficult by sintering pot test with a uniform charge.22) Further, it is necessary to test and study with the condition of an any air flow rate and an any mix carbon material blended some carbon material for the swapping evaluation to biomass-char.
3.2. Sintering ProductivitySintering productivity is basically consists of product of sintering vertical speed, sinter yield and bulk density in sinter mix bed. Sintering speed is due to permeability at the condition with heat equivalent. The sintering speed is due to combustion rate of carbon material, too. The sintering speed is faster, when the combustion rate of carbon material is faster. Previously, sintering technology added raw biomass to the sinter mix are reported.23,24) The technologies improve to permeability and sinter productivity by utilizing granulation binders23) and by forming gas flow space24) after the raw material combustion. Then, the pot tests added with raw agriculture biomass (average 8 mm particle) to sinter mix carried out at the 2nd.condition. The relative influence of each factor for the test results is shown in Fig. 9. The relative effects on bulk density, permeability in the bed of sinter mix and sinter yield are small. But the relative effects on permeability during sintering and sintering speed are significantly large and they increase. It is considered that the coarse biomass extinguished after combustion and formed voids in sinter bed, and that has improved permeability.
Effect of rawbiomass (Q) addition on relative factors for sinter productivity.
On the other hand, when the pot tests with using biomass-char (particle size: Table 4) carried out at the condition of gas flow rate constant (1st condition), the test results on flame front arrival time (FFP: exhaust gas rising time) and pressure drop of sinter bed are shown in Fig. 10. The pressure drop in sinter bed is reduced, FFP is shortened. From these results, biomass-char may increase sintering speed while maintaining sinter yield and improve productivity. An example of the results on productivity tested with biomass-char (1–5 mm) at the condition of constant suction pressure (condition 2nd.) are shown In Fig. 11. The sinter productivity in the case of biomass-char is not clearly higher than that in the case of coke or anthracite, and the detailed analysis and evaluation are items for future research.
Pressure drop, flame front point (FFP) in exhaust gas at sinter pot test of condition 1. (1.3 Nm3/min, Particle size const.; C·F→M·N·P: Blending ratio 4 mass%→5 mass%, Moisture 7 mass%→8 mass%).
Effect of biomass char on sinter productivityby sinter pot test of condition 2 (10 kPa, Blending ratio 4.5 mass% const.; C·F→M·N·P: Particle size usual→1–5 mm, Moisture 7 mass%→8 mass%).
The sinter ore that tested at the condition of gas flow rate constant (1st. condition) are almost same level on sinter yield is selected. The qualities of the sinter ore is evaluated and the results is shown in Fig. 12. Whereas there is no significant difference in coke and anthracite, there is a difference on TI (ISO3271), RDI (ISO4696-2), FeO (Fe[II]) in biomass-char M, and TI in biomass-char P. In addition, side-by-side, the qualities of the sinter ore obtained by sintering at condition of a constant pressure drop (2nd. conditions) are shown in Fig. 12. In the 1st condition, the sinter yield has secured by increasing the blending ratio with same particle size of the carbon material, so the biomass-char has a slight excess thermal and melt. It is considered that secondary hematite in the case of biomass-char is formed in large amounts because of rich melt under the limitation of the gas flow rate in last period. For this reason, It is seemed that the RDI in biomass-char M has deteriorated significantly. On the other hand, in the 2nd.condition, the combustion of biomass-char was optimized by high moisture in sinter mix and size coarsening for the particle. From the fact that the permeability has been improved, the gas flow rate and the cooling rate increases in last period. Then, it is seemed that FeO in sinter ore are increase, while RDI and RI (ISO7215) decrease.
Effect of biomass char on sinter qualities at sinter pot tests (Solid mark: Condition 1:1.3 Nm3/min, Size, Moisture conct.; Blending ratio C·F=4 mass%,→M·P=5 mass%) (Open mark: Condition 2:10 kPa, Blending ratio 4.5 mass% conct.; CF→M·P, Size usual→1–5 mm, Moisture 7 mass%→8 mass%).
In the sintering machine of Japan, the emission concentrations (SOx, NOx, dust, dioxins) are highly regulated. On these concentration, the effect of carbon material for sintering is large, then these items are very important,18,25) when the carbon material is swapped. The waste-based biomass with rich Cl, N, S component has a problem for sintering operation because of SOx, NOx, dust and dioxins. On the other hand, forestry and agriculture based biomass is low-S and low-N component compared to coal type. Then, SOx and NOx in the exhaust gas at sinter plant is expected to decrease by using the biomass-char.
Some of forestry and agriculture biomass has rich P content and rich K content, and has minute oil-bearing that is different from the kind of coal and coke. The behavior of these chemical component on the carbonization process and on the sintering process have to be kept in mind on the exhaust gas. The pot tests with adding the raw agriculture biomass Q (oil content: 0.55 mass%) to the sinter mix (blending anthracite and coke) was carried out, and was investigated17) the effects of the raw biomass Q on dust concentration in sintering exhaust gas. In addition, the pot tests with adding the raw biomass Q to not only the standard mix (excluded dust) shown in Table 2, but also the pot tests with sinter mix involved the contaminated dust (2 mass%) at the actual plant, was carried out, too. The results of these tests are shown in Fig. 13. Even in the case with adding the raw biomass to any sinter mix, it is recognized that the dust concentration in exhaust gas is increased with the adding ratio of raw biomass. As it seems to evaporate the volatilized components of raw biomass during sintering, the carbonized biomass-char have to be added to sinter mix.
Effect of raw biomass addition on dust and oil concentration in exhaust gas at sinter pot test (plant: mix with dust at actual plant, standard: mix without dust at Table 2).
Then, the effects of biomass-char by the pot test at the 1st. condition, was investigated17) on concentration of NOx, SOx, dust and dioxins in the exhaust gas. The results are shown in Fig. 14. The NOx and SOx concentration in case of biomass-char is greatly reduced compared with that of coke or anthracite. As S content and N content in the biomass-char are poor as shown in Table 5, the SOx and NOx concentration in the exhaust gas seemed to be reduced. The dust concentration in case of biomass-char is also reduced compared with coke or anthracite. As the compound of Cl, S, and N contents have been the main factors determined dust concentration in exhaust gas,25) the reduction of dust concentrate due to the poor N content and S content in biomass-char. Furthermore, on dioxin concentration, biomass-char is lower than coke, but is higher than anthracite. Discussing of the small absolute value and the accuracy of analysis, the result that anthracite is lower than coke is correspond to the prior knowledge.18) Also, the dioxin concentration in case of biomass-char are not observed to increase significantly at least.
Effect of carbon material on NOx , SOx, dust and dioxins concentration in exhaust gas at sinter pot test.
(1) With direct using a raw agriculture biomass as the carbon material for the sinter production, the biomass cannot be heat source and rise the temperature. In addition, the exhaust gas treatment must to be enhanced.
(2) With using a biomass-char as a carbon material for the sinter production, when the biomass is carbonated up to rich Fix.C (low volatile) and the size of the carbonated biomass-char is course, the biomass-char is a level equal to anthracite or coke on sinter yield and sinter productivity.
In addition, the using with biomass-char is reduced emissions CO2 (certified), NOx, SOx, and dust in exhaust gas.
