Effect of CO and CO₂ Content in Suction Gas on Sintering Process for Iron Ores

Abstract

To serve fundamental information for the operation of flue gas circulation sintering (FGCS) process, this study mainly focuses on the influence of the CO and CO₂ content in the suction gas on the iron ore sintering process. The results indicated that the presence of a small amount of CO in suction gas was beneficial to improve the combustion efficiency of coke and the sintering productivity, yield and sinter strength. However, excessive of CO₂ in suction gas worsen the combustion efficiency of coke and sintering indexes. The CO₂ content in the suction gas should be less than 5 vol%.

1. Introduction

The author’s group have been investigating on the flue gas circulating sintering from fundamental point of view: a simulation study was first carried out for systematic understanding the emission rule of O₂, SO₂, NO_x and CO_x in the outlet flue gas by the FGCS process;¹⁾ effect of circulating gas components (SO₂, O₂, CO, water vapor) on the sintering indexes were then examined,²⁾ and the SO₂ behavior in different sintering zones (sinter zone, drying and preheating zone, moisture condensation zone and sinter mixture zone) was also found out.³⁾

Some of the previous works^2,4,5) showed the effect of suction gas components (O₂, CO, humidity) only on the quality and property of the sinter ores, however, the underlying reasons for the changing rule of the sintering indexes were not determined. This note will add them several findings obtained thereafter to clarify the influence of CO and CO₂ content in suction gas. Emphasis was put on the variation of the exhaust flue gas combustion ratio, CO/(CO+CO₂).

2. Experimental

2.1. Materials and Equipments

Materials and equipments were the same as the previous study.²⁾ In particular, the fixed carbon (FC_d), volatile matter (V_d) and ash content (A_d) of coke breeze in dry basis were 81.93 mass%, 2.80 mass% and 15.27 mass%, respectively.

2.2. Conditions for Suction Gas

To examine the CO effect, the experiments were performed by controlling the CO content varying from 1 vol% to 3 vol% under conditions that coke breeze dosage was set at 5.8 mass% and 5.5 mass%, respectively. To examine the CO₂ effect, CO₂ content was set at 0 vol%, 5 vol%, 10 vol%, 14 vol%, respectively. It was notable that N₂ content in the suction gas was correspondingly decreased. The sintering indexes were deteriorated when the O₂ content in the suction gas was less than 18 vol%.^2,3) So the O₂ content in the tests was maintained at 18 vol%. The suction gas compositions of the sintering tests are shown in Table 1.

Table 1. Effect of CO and CO₂ content in suction gas on the sintering process for iron ores.

Run No.	Coke ratio (mass%)	CO (vol%)	CO2 (vol%)	O2 (vol%)	N2 (vol%)	CR0 (%)	CRadj (%)	End-point temp. (K)	VSV (mm·min–1)	Y (mass%)	TI (mass%)	P (t·m–2·h–1)
1	5.8	0	0	18	rest	17.9	17.9	550	21.7	70.3	47.1	1.25
2		1	0			16.3	17.6	557	22.1	76.0	48.6	1.43
3		2	0			15.1	17.4	591	22.4	77.3	51.5	1.47
4		3	0			14.1	17.2	660	22.2	80.3	57.3	1.54
5	5.5	1	0	18	rest	–	–	–	23.6	71.3	47.8	1.38
6		2	0			–	–	–	23.3	73.7	51.2	1.43
7	5.8	0	5	18	rest	–	15.5	555	22.3	69.2	47.0	1.19
8		0	10			–	16.7	603	23.3	69.1	43.7	1.14
9		0	14			–	17.5	578	24.0	68.5	41.7	1.14

2.3. Evaluation Indexes

The sintering indexes including vertical sintering velocity (VSV), yield (Y), productivity (P) and ISO tumbling index (TI) were the same as those shown in the Ref. 2).

Another important parameter, namely the outlet flue gas combustion ratio, defined as CR₀: CO/(CO+CO₂), was used to evaluate the degree of chemical energy utilization of carbon including coke breeze and CO gas in the suction gases during the sintering. Generally, the smaller the combustion ratio is, the better the chemical energy utilization of carbon is. However, the reaction, 2CO + O₂ = 2CO₂, is easy to take place at high temperature if CO exists in the suction gas. Thus, CO₂ generated by the combustion of CO should be taken into consideration when calculating the combustion ratio. Then, the adjusted combustion ratio, defined as CR_adj: CO/[(CO+CO₂–CO_2(gen))], was also used in Section 3.1.3 and 3.2.3, where CO_2(gen) is the CO₂ content generated by the CO in suction gas reacting with O₂.

3. Results and Discussion

3.1. Effect of CO Content on the FGCS Process

3.1.1. Effect on Sintering Indexes

This note confirms the previous findings²⁾ that increasing CO content in the suction gas improved the sintering indexes as given in Table 1. Particularly, under conditions of the circulating CO content 1 vol% and coke breeze dosage 5.5 mass%, TI, Y and P are slightly better than those under reference conditions that no CO gas is circulated and coke breeze dosage is 5.8 mass%. It indicated the presence of CO in the suction gas was favorable to improve the sintering indexes and decrease the coke breeze dosage and inferred that coke breeze dosage was decreased by 0.3 mass% when 1 vol% CO was circulated in the suction gas.

3.1.2. Effect on CO_x Emissions

Figure 1 illustrates the effect of the CO content in suction gas on the CO and CO₂ emissions in the flue gas. With an increase of the CO content from 0 vol% to 3 vol%, after ignition, the CO content in the outlet gas only increases by a small amount, 0.1 vol% or so (in Fig. 1(a)). Simultaneously, the CO₂ content increases from approximately 9.2 vol% to 13.1 vol% or so (in Fig. 1(b)) because CO gas in the suction gas easily reacted with O₂ to generate CO₂ during at high temperature.

Fig. 1.

Emission rules of CO content (a) and CO₂ content (b) of the outlet flue gas in CO circulation sintering.

3.1.3. Effect on Combustion Ratio of Outlet Flue Gas

The combustion ratio shown in Fig. 2(a) is reduced from about 17.9% to 14.1% along with the increase of CO content from 0 vol% to 3 vol%, indicating that the fuels including coke breeze and CO gas were both combusted well even if the CO content in the suction gas was increased.

Fig. 2.

Changing rules of combustion ratio (a) and adjusted combustion ratio (b) of the outlet flue gas in CO circulation sintering.

We further determined the influence of CO content on the combustion efficiency of coke breeze only. It was assumed that the CO in the suction gas was completely combusted into CO₂ during the sintering, then, the newly generated CO₂ could accelerate the gasification of carbon. Figure 2(b) shows that the adjusted combustion ratio decreases from about 17.9% to 17.2% when the CO content increases from 0 vol% to 3 vol%. Moreover, the residual carbon contents in the sinter ores sampled from the upper, middle and bottom sinter bed were detected. It was found that the average residual carbon decreased from 0.20 mass% to 0.13 mass% with increasing the CO content from 0 vol% to 3 vol%. Thus, both the reduced combustion ratio and residual carbon content demonstrated that the increase of CO content in the suction gas improved the combustion efficiency of coke breeze to a certain degree, leading to over 3 vol% CO₂ emission (in Fig. 1(b)).

3.2. Effect of CO₂ Content on the FGCS Process

3.2.1. Effect on Sintering Indexes

The effect of CO₂ content in the suction gas on the sintering indexes was also given in Table 1. As the CO₂ content in the suction gas changes from 0 vol% to 14 vol%, the VSV increases gradually, while Y and P decrease slightly. It is notable that the TI is declined sharply when the CO₂ content exceeded 5 vol%. When the CO₂ content is increased from 5 vol% up to 14 vol%, the TI is decreased from 47.0 mass% down to 41.7 mass%, which indicated that increasing the CO₂ content over 5 vol% was disadvantageous to improve the sintering indexes.

3.2.2. Effects on CO_x Emissions

Figure 3 displays the emission rule of CO and CO₂ content in the fuel gas at different CO₂ contents. With an increase of the CO₂ content from 0 vol% to 14 vol%, the CO content was increased by a small amount of 0.2 vol% (in Fig. 3(a)), meanwhile, the CO₂ content in the flue gas added synchronously to some degree with an increase of CO₂ content in the suction gas (in Fig. 3(b)).

Fig. 3.

Changing rules of CO content (a), CO₂ content (b) and adjusted combustion ratio (c) of the outlet flue gas in CO₂ circulation sintering.

3.2.3. Effect on Combustion Ratio of Outlet Flue Gas

Shown in Fig. 3(c), as the CO₂ content in the suction gas increased from 0 to 5 vol%, the adjusted combustion ratio decreased from about 17.9% down to 15.5%. However, the combustion ratio at 10 vol% or 14 vol% CO₂ content is greater than that at 5 vol% CO₂ content. The results further verified that excessive CO₂ over 5 vol% in the suction gas exerted adverse impact on the combustion efficiency of coke breeze.

Generally, the end-point temperature of flue gas was in the same level under the same sintering conditions especially the same fuels dosage added. The changed end-point temperatures of flue gas, shown in Table 1, indicated that the CO₂ content in suction gas had an obvious effect on the combustion efficiency of coke breeze. The flue gas temperature at the end-point of sintering obtained by the experiments increases gradually from 550 K to 603 K with increasing the CO₂ content from 0 vol% to 10 vol%; subsequently, it falls down to 578 K at 14 vol% CO₂ content. This phenomenon was ascribed to the difference between specific heat capacity of N₂ and CO₂ at different temperatures. The specific heat capacity of CO₂ is bigger than that of N₂ at the same temperature. The difference value (D-value) of specific heat capacity between N₂ and CO₂ were 6.68 J/(mol·K) at 273 K and 16.85 J/(mol·K) at 1673 K,⁶⁾ respectively. The previous work⁷⁾ reports the heat front speed is proportional to the specific heat capacity of the suction gas. Therefore, the heat front speed of the suction gas with higher specific heat capacity was faster. The famous heat flow and sintering experiments conducted by Voice in 1950s had shown that the heat front speed matched the flame front speed (FFS) when ordinary coke breeze was used as fuel and air as the sintering atmosphere.⁸⁾ However, in the FGCS process with CO₂ gas in suction, excessive CO₂ with bigger specific heat capacity would lead to the mismatch between the heat front speed and the FFS.⁹⁾ Further study is being performed to determine how the CO₂ and N₂ affects the heat front speed and FFS.

4. Conclusions

(1) The presence of CO in the suction gas improved the combustion efficiency of coke breeze in the FGCS process. 1 vol% CO addition was able to decrease 0.3 mass% coke breeze.

(2) Excessive CO₂ content in suction gas had negative impact on the combustion efficiency of coke breeze due to the mismatch between the heat front speed and the FFS. The CO₂ content in suction gas must be limited no more than 5 vol%.

Acknowledgments

The authors wish to express their thanks to Teachers’ Research Fund of Central South University (2013JSJJ028) for the financial support of this research.

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