The Fate of Sulfur In Oxy-Fired Fluidized Beds

1.      Introduction

SO₂ emissions from coal combustion have several harmful effects on the environment, including their contribution to acid rain. A variety of approaches, including wet and dry scrubbing and direct sorbent injection, have been developed to control sulfur pollution. Direct dry sorbent injection is a relatively simple and low-cost process. Until now, the fluidized bed has been one of the most popular furnaces for the application of direct sorbent injection. In addition, oxy-fuel combustion is a promising, practical method to reduce greenhouse gas emissions; thus, studies on the mechanisms related to the production of SO₂ emissions under oxy-fuel conditions are important. The major limitation of studying SO₂ mechanisms in existing pilot-scale and industrial-scale fluidized beds is expense. Bench-scale fluidized beds, to some extent, can examine fundamental mechanisms of sulfur under many experimental conditions due to their low cost. However, very few previous studies on the mechanisms of sulfur emissions in single coal particle fluidized beds have been conducted. A few potential mechanisms of SO₂ capture by limestone are presented in the literature^1-5. Little else can be confirmed except that the final product is CaSO₄⁶.

In this work, the capture of fuel sulfur by sorbents in oxy-fired fluidized beds was studied by:

°¤         Performing bench-scale experiments to identify mechanisms and quantify rates;

°¤         Developing a detailed single-particle model to determine rate parameters and test mechanisms;

°¤         Performing pilot-scale experiments to verify bench-scale observations under industry-relevant conditions.

2.      Experimental Method

A bench-scale single-fuel-particle fluidized-bed reactor ^{7, 8} is used to follow the transient evolution of sulfur in mixtures of O₂ with N₂ or CO₂at the same conditions (mass flow rate, O₂ concentration and fluidized bed temperature, etc.). In this way, we can examine the effect of the presence of CO₂ on the formation and capture of SO₂/SO₃ and other participating species. Illinois #6 coal was used as the fuel, due to its high sulfur content.

The single-particle, coal-combustion experimental bench scale fluidized bed is shown in Figure 1. It is a vertical, cylindrical, stainless-steel furnace with a combustion chamber length of 771 mm and an inner diameter of 44 mm. The column has a 2 mm thick perforated plate distributor with 60 holes. The temperatures of the bed material, furnace wall and gas phase are measured by K-type thermocouples. The effluent is measured by a Magna-IR Spectrometer 550 FTIR.

Figure 1.Schematic setup of bench-scale fluidized bed combustor.

3.      Results

The fate of sulfur in the bench-scale fluidized bed experiments were carried out in both N₂ and CO₂ environments at three O₂ concentrations (10%, 20% and 30%) and a range of bed temperatures (765 - 902 C). The sulfur content in each single coal particle was not expected to be uniform; therefore, each experimental condition was repeated at least 5 times and averages of the multiple runs and associated error bars are presented.

Figure 2 (left) shows the comparison of SO₂ emissions without limestone between oxy-fuel combustion and air combustion. It clearly indicates there was no difference in the SO₂ emissions. As a conclusion, the effect of high concentrations of CO₂ on SO₂ emissions appears to be the same as N₂ in the bench-scale fluidized bed.

Figure 2 (right) shows the comparison of SO₂ emissions with limestone between oxy-fuel combustion and air combustion. It is clear that the efficiency of sulfur removal by limestone in oxy-fuel combustion is much lower than during air combustion. Because the CO₂ concentration is very high in the fluidized bed under oxy-fuel conditions, it is anticipated that there would be a great suppression of the decomposition of CaCO₃ (limestone) thereby reducing the efficiency of sulfur removal by limestone.

Figure 2.The fate of sulfur (T= 835 C) in either N₂ or CO₂, without limestone (left) and with limestone (right) present in a quartz fluidized bed.

In order to investigate limestone sulfation behavior in detail, calibration gases of known SO₂ concentration in either N₂ or CO₂were introduced as the sulfur source, instead of generating it from coal particle combustion.

Sulfation behavior of limestone in O₂ /N₂ or O₂/CO₂ is illustrated in Figure 3. As Figure 3 (left) shows, the reactor temperature had little effect on the degree of sulfation over the range considered in O₂ /N₂. It is well established that due to differences in molar volumes between CaCO₃ and CaSO₄, significant pore plugging can occur that would limit the degree of sulfation. If the overall rate is limited by particle diffusion, then the reaction would tend to take place near the particle surface. The pores would become blocked after only a limited outer layer of the particle had reacted to form CaSO₄. The low degree of sulfation is consistent with this hypothesis. In addition, the very limited dependence on temperature would also indicate more of a diffusion-controlled regime.

The degree of sulfation for the oxy-combustion condition is shown in Figure 3 (right), and reveals a clear dependence on temperature. The sulfation mechanism of limestone in O₂ /N₂ or O₂/CO₂ (T=874C) is shown in more detail in Figure 4. This figure provides a composite of the N₂ and CO₂ atmospheres, and plots the degree of sulfation over time as determined from the measured SO₂ concentration. As shown, the transient sulfation kinetic rate in O₂/N₂ is initially much higher compared to O₂/CO₂; however, at longer times the degree of sulfation of limestone in O₂/CO₂ is higher. The crossover occurs at transition point A (t=2100s)for the temperature and conditions of this experiment, as noted in the figure.

It appears that the process of sulfation is controlled by a different mechanism in the presence of CO₂ as compared with N₂, since very different sulfation behavior is observed for the same operating conditions. It is likely that air-combustion and oxy-combustion conditions result in indirect and direct sulfation mechanisms. Also, it appears that the indirect sulfation mechanism is in a diffusion-controlled regime for the conditions evaluated, as there is very little temperature sensitivity and the conversion is low, consistent with the earlier discussion of pore plugging on the outside of the particle. The direct sulfation mechanism may be in a kinetically controlled regime, since the temperature dependence is much more pronounced. In O₂/N₂, if the kinetic rate for the SO₂-CaO (indirect) reaction is relatively fast, then pore diffusion would limit the overall rate and pores would become blocked very quickly, allowing only a limited outer layer of the particle to react to form CaSO4 and limiting overall conversion. In O₂/CO₂, if the kinetic rate for the SO₂-CaCO₃ (direct) reaction is slow, then pores diffusion is not as limiting and the reaction to form CaSO₄ could take place throughout more of the particle. This process would allow the formation of CaSO₄ over a larger surface area and allow greater reaction time prior to pore plugging.

Figure 3. Limestone sulfation behavior at various temperatures in either O₂/N₂ (left) or O₂/CO₂ (right).

Figure 4. Sulfation behavior of limestone in either O₂/N₂ or O₂/CO₂ at T=874C.

 

 

Reference:

 

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