(113d) 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:

1.         Liu,
H., et al., Sulfation behavior of limestone under high CO2 concentration in O₂/CO₂
coal combustion. Fuel, 2000; 79(8): 945-953.

2.         Silcox,
G.D., J.C. Kramlich, and D.W. Pershing, A mathematical model for the flash
calcination of dispersed calcium carbonate and calcium hydroxide particles.
Industrial & Engineering Chemistry Research, 1989; 28(2): 155-160.

3.         Hajaligol,
M.R., J.P. Longwell, and A.F. Sarofim, Analysis and modeling of the direct
sulfation of calcium carbonate. Industrial & Engineering
Chemistry Research, 1988; 27(12): 2203-2210.

4.         Michael
J. H. Snow, J.P.L., Adel F. Sarofim, Direct Sulfation of Calcium Carbonate.
Ind. Eng. Chem. Res., 1988; 27: 268-213.

5.         Tullin,
C., G. Nyman, and S. Ghardashkhani, Direct sulfation of calcium carbonate: the
influence of carbon dioxide partial pressure. Energy & Fuels,
1993; 7(4): 512-519.

6.         Hu,
G., et al., Review of the direct sulfation reaction of limestone. Progress
in Energy and Combustion Science, 2006; 32(4): 386-407.

7.         Sanchez,
A., E. Eric, and F. Mondragon, Fourier Transform Infrared (FTIR) Online
Monitoring of NO, N2O, and CO2 during Oxygen-Enriched Combustion of
Carbonaceous Materials. Energy & Fuels, 2010; 24(9):
4849-4853.

8.         Can,
G., P. Salatino, and F. Scala, A single particle model of the fluidized bed
combustion of a char particle with a coherent ash skeleton: Application to
granulated sewage sludge. Fuel Processing Technology, 2007;
88(6): 577-584.