Combined Removal of NO_x and SO₂ in Circulating Fluidized Beds with Post-Combustion

Abstract

The post-combustion technology of circulating fluidized beds (CFBs) can largely reduce the emission of nitrogen oxides (NO_x) in the process of combustion, significantly reducing the removal cost of NO_x. To explore the potential of the combined removal of NO_x and SO₂ emissions under post-combustion technology, experiments were conducted on a 0.1 MWth circulating fluidized bed test platform. This paper focuses on the effect of temperature in CFB with limestone addition on NO_x and SO₂ emissions under post-combustion technology combined with sorbent injection into the furnace. The low-cost combined removal of NO_x and SO₂ can be realized by denitrification in the furnace and through secondary desulfurization in the furnace and post-combustion chamber. In the optimized experimental condition, with combustion temperatures at 845 °C and sorbent addition in the furnace under post-combustion, the emission of NO_x can be reduced to 47.10 mg/Nm³(@6%O₂), and meanwhile, the emission of SO₂ can be reduced to 92.09 mg/Nm³. Sulfur removal efficiency is higher under lower temperatures in a weakly reducing atmosphere. The reaction of sulfur fixation occurred in the post-combustion chamber and caused the particle size of fly ashes at the tail flue to become bigger and the sulfur content in the fly ash at the tail flue to increase. At 845–905 °C, the combustion temperature had a bigger effect on the SO₂ emission than the NO_x with sorbent addition in the furnace under post-combustion.

1. Introduction

In China, the NO_x and SO₂ emissions totaled 554.3 thousand tons and 338.6 thousand tons in 2022 [1], of which the emissions from coal combustion accounted for 70–80%. Consequently, reducing the NO_x and SO₂ emissions from coal combustion for environmental protection is still a major demand. Circulating fluidized bed boilers are an essential way to deal with coal because of low pollution emissions, fuel flexibility, and so on [2,3,4]. Under the ultra-low NO_x emission standard [5], CFB boilers apply low NO_x combustion technology [6,7] and use selective non-catalytic reduction technology (SNCR) [8,9,10]. Likewise, CFB boilers apply the furnace sorbent injection [11] and flue gas desulfurization technology [12,13,14] under ultra-low SO₂ emission standards. Current methods for the CFB boiler can achieve the combined removal of NO_x and SO₂ emissions [15,16,17], but the methods result in very high costs and secondary pollution due to the application of flue gas removal technology.

To reduce the removal cost of NO_x emissions and secondary pollution, the post-combustion technology of CFBs [18] was developed by the Institute of Engineering Thermophysics of the Chinese Academy of Sciences. For post-combustion technology, coal is mainly combusted in a CFB combustor under a weakly reducing atmosphere and then burnt out in a post-combustion chamber under oxidizing conditions with the air staging method. Zhou et al. [19] found that NO_x emissions can meet the ultra-low NO_x emission standard when the air stoichiometric ratio is 0.96 under post-combustion technology. Song et al. [20,21] found that NO_x emissions for many fuels can reach ultra-low standards under post-combustion. In addition, the NO_x emission of coal slime could be reduced to 38 mg/Nm³ when post-combustion technology was used with a 75 t/h CFB boiler, and ultra-low NO_x emissions could be reached [22]. Post-combustion technology has succeeded in meeting the ultra-low NO_x standard in the process of combustion, reducing the cost of NO_x removal dramatically. However, the question of how to reduce the removal cost of SO₂ emissions and achieve the combined removal of NO_x and SO₂ emissions under post-combustion technology needs further study.

The sorbent injection in the furnace is very economical. Post-combustion technology combined with the sorbent injection in the furnace has the potential to achieve the combined removal of NO_x and SO₂ emissions at low costs for CFB boilers. Combustion temperature and air supply [23,24] are the most important parameters influencing desulfurizing efficiency. Under post-combustion technology combined with the sorbent injection in the furnace, this paper studies the effect of temperature in CFB with limestone addition on NO_x and SO₂ emissions on a 0.1 MWth CFB test platform.

2. Methods

2.1. Fuel Characteristics

The fuel used was Shenmu coal, which was obtained from Shaanxi Province. The proximate and ultimate analyses of the coal are shown in

Table 1. Proximate analysis and ultimate analysis. Reproduced with permission from [Chao Wang], [Journal of Thermal Science]; published by [Springer Nature], [2023] [18].

Item	Proximate Analysis (wt %, ad)			Ultimate Analysis (wt %, ad)						Low Heating Value (MJ/kg)
	Moisture	Ash	Volatile Matter	Fixed Carbon	Carbon	Hydrogen	Nitrogen	Oxygen	Sulfur
Shenmu coal	11.80	9.82	39.01	47.80	62.94	3.88	0.98	10.18	0.40	24.52

The “ad” indicates the abbreviation of “air dry basis”.

. For Shenmu coal, the volatile matter is very high, and the sulfur content is relatively low. Figure 1 shows the particle size distribution of Shenmu coal. It can be seen from Figure 1 that the diameter of particles of Shenmu coal with the biggest content is 500 μm, and the 90%, 50%, and 10% cut mean diameters (d90, d50, and d10) are 603.07 μm, 293.04 μm, and 29.44 μm, respectively.

The bed material used was quartz sand. The sorbent for SO₂ removal was hydrated lime.

Table 2. Hydrated lime composition.

Composition	CaO	MgO	Fe2O3	SiO2	Al2O3	SO3	K2O	MnO	Cl	P2O5	ZnO
Content (wt, %)	74.10	0.96	0.39	0.38	0.19	0.06	0.04	0.03	0.02	0.01	0.01

shows its composition analysis. The CaO content in hydrated lime was 74.10%. Its particle size was 75 μm. Hydrated lime at a Ca/S ratio of 2.5 was mixed with coal in the experiment.

2.2. Test Platform

Figure 2 shows a 0.1 MWth CFB test platform made of stainless steel. It mainly includes a CFB combustor and a post-combustion chamber. The height of the CFB furnace is 5500 mm, and its internal diameter is 150 mm. The total length of the post-combustion chamber is 3500 mm, including a horizontal section with a length of 1000 mm and a vertical section with a length of 2500 mm. The inner diameter of the post-combustion chamber is 80 mm. Coal is fed from the bottom of the furnace. Air is introduced into the experiment system supplied via an air compressor and mainly includes primary air and post-combustion air. The primary air passes through the air preheater and is injected into the furnace. Also, post-combustion air is preheated and divided into three partial flows before entering into the post-combustion chamber. The arrangement of post-combustion air is conducive to the burning of combustible materials and reduces NO_x emissions.

2.3. Measurements

Combustion temperature is the most important parameter influencing NO_x and SO₂ emissions. There are five thermocouples in the furnace, monitoring the temperature at different points. The measurement accuracy of temperature is ±0.5%. The Fodisch MCA14m analyzer can measure the flue gas in Sample 2 in Figure 2, and its measurement error is less than 1%. The analyzer can continuously monitor O₂, CO, SO₂, NO, and NO₂ in cold and dry gases. In addition, nitrogen-containing gaseous products such as NH₃, HCN, and sulfur-containing gaseous products such as H₂S, COS, CS₂, and SO₂ at the cyclone outlet (Sample 1 in Figure 2) are analyzed with the KOMYO vapor detector tube.

Fly ash at Sample 1 and Sample 2 was collected by a vacuum pump, and the sampling time was 20 min in each experiment. The samples of fly ash were taken to the professional testing unit for analysis according to conventional standards. The ash composition of fly ash was analyzed using X-ray fluorescence (XRF) analysis. Sulfur functionalities present in fly ash were identified and quantified by the X-ray photoelectron spectroscopy (XPS) analysis.

2.4. Experimental Conditions

The conditions of the experiments are shown in

Table 3. Experimental condition.

Case	T (°C)	Ca/S	λ	λCFB	Primary Air (m3/h)	Post-Combustion Air (m3/h)	Other Air (m3/h)	Feeding Coal (kg/h)
1-1	900	2.5	1.15	1.15	27	0	6	5.94
1-2	900	2.5	1.15	0.9	27	9.12	6	5.94
2-1	905	2.5	1.15	0.9	27	9.12	6	5.94
2-2	870	2.5	1.15	0.9	27	9.12	6	5.94
2-3	845	2.5	1.15	0.9	27	9.12	6	5.94

Notes: “T” is represented by the highest temperature in the furnace for each case; other types of air include returning air and carrying coal air.

. Temperature is the main reference in the experiments, and the ambient temperature change should not exceed 3 degrees; these are considered stable working conditions. λ_CFB is the air stoichiometric ratio in the CFB furnace; λ_PCC is the air stoichiometric ratio in the post-combustion chamber; and λ is the total excess air coefficient; the formula is as follows:

$${\mathsf{\lambda }}_{\mathrm{C}\mathrm{F}\mathrm{B}}={\displaystyle \frac{{\mathrm{A}}_{\mathrm{P}\mathrm{r}}+{\mathrm{A}}_{\mathrm{R}\mathrm{e}}+{\mathrm{A}}_{\mathrm{C}\mathrm{a}}}{{\mathrm{A}}_{\mathrm{S}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}}}}$$

(1)

$${\mathsf{\lambda }}_{\mathrm{P}\mathrm{C}\mathrm{C}}={\displaystyle \frac{{\mathrm{A}}_{\mathrm{P}\mathrm{o}}}{{\mathrm{A}}_{\mathrm{S}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}}}}$$

(2)

$$\mathsf{\lambda }={\mathsf{\lambda }}_{\mathrm{C}\mathrm{F}\mathrm{B}}+{\mathsf{\lambda }}_{\mathrm{P}\mathrm{C}\mathrm{C}}$$

(3)

$$\mathsf{\lambda }={\displaystyle \frac{21}{21-o_2}}$$

(4)

where A_Stoic is the gas flow rate with stoichiometric complete burnout, m³/h. A_Pr is the airflow rate of the primary air; A_Re, A_Ca, and A_Po are the airflow rates of returning air, carrying coal air, and post-combustion air, m³/h, respectively. O₂ is the actual value measured by the Fodisch MCA14m portable flue gas analyzer.

3. Results and Discussion

3.1. NO_x and SO₂ Emissions with Sorbent Addition: Conventional vs. Post-Combustion

3.1.1. Temperature Profile in the CFB Furnace

Figure 3 shows the temperature distributions in the furnace with sorbent addition for both post-combustion and conventional CFB combustion. The temperature first sharply increased, reached a maximum at T3, and then slowly decreased in the direction of the flue gas flow. The temperature at the bottom of the furnace was lower mostly because the primary air enters the furnace at lower temperatures [25]. The air absorbs a lot of heat, making the temperature lower. The temperature at T3 is highest due to the combustion of volatile matter of coal at this point. After T3, the temperature slowly declined. In addition, before the highest temperature point T3, the corresponding temperature under conventional combustion was higher, and after the highest temperature point, the temperature was higher under post-combustion conditions. The oxygen concentration at the bottom of the furnace under conventional combustion was higher, and the combustion was more complete, thus leading to greater heat release. Therefore, the temperature of conventional combustion below T3 was higher. Under post-combustion conditions, the opposite was true. Due to insufficient combustion, a large number of combustible components moved upward along the airflow direction. Therefore, after T3, the temperature was higher under post-combustion conditions.

3.1.2. NO_x and SO₂ Emissions with Sorbent Addition Under Both Combustion Methods

Figure 4 presents the NO_x and SO₂ emissions with sorbent addition for both conventional combustion and post-combustion. From Figure 4, the NO_x emission under sorbent addition for conventional combustion is up to 241 mg/Nm³. For the traditional circulating fluidized bed boiler, injecting the sorbent into the furnace can lead to an increase in NO_x emissions in most investigations because of the heterogeneous catalysis at the active particle surface of the sorbent, which can enhance NH₃ and HCN conversion to NO [26].

NH₃ + O₂NO + N₂

(5)

HCN + O₂ NO + N₂

(6)

NO_x emissions under sorbent addition at conventional combustion are higher. Compared with conventional combustion, NO_x emissions under sorbent addition at post-combustion is significantly lower. This is because the coal is combusted in a circulating fluidized bed combustor under a weakly reducing atmosphere. First, it can both inhibit and minimize the generation of NO_x emissions. Second, injecting the sorbent into the furnace under a weakly reducing atmosphere has almost no adverse effect on the NO_x emission.

It can be seen from Figure 4 that the SO₂ emission under sorbent addition at conventional combustion is 193 mg/Nm³, while the SO₂ emission at post-combustion is 386 mg/Nm³. Sulfur removal under conventional combustion can be carried out by the in situ injection of the sorbent through the following reaction:

CaO + SO₂ + 1/2O₂ → CaSO₄

(7)

A circulating fluidized bed combustor at post-combustion contains a weakly reducing atmosphere, while the condition of a post-combustion chamber at post-combustion is oxidizing. When the sorbent is injected into the furnace, desulfurization reactions occur; when it enters into the post-combustion chamber, desulfurization reactions also take place. Sulfur removal under post-combustion can be carried out by the in situ injection of the sorbent during the following reactions:

CFB main combustor: CaO + H₂S → CaS + H₂O

(8)

Post-combustion Chamber: CaO + SO₂ + 1/2O₂ → CaSO₄

(9)

3.2. NO_x and SO₂ Emissions with Sorbent Addition: Conventional vs. Post-Combustion

3.2.1. Temperature Profile in the CFB Furnace

Figure 5 presents the temperature distributions in the CFB furnace for different cases. Regardless of the experimental condition, the trend of temperature distributions in the CFB furnace in different cases is similar. In the direction of the flue gas flow, the temperature of the five locations in the CFB furnace first increased sharply, reached a maximum at the point of T3, and then decreased slowly.

3.2.2. Effects of Temperature on NO_x and SO₂ Emissions with Sorbent Addition Under Post-Combustion

Figure 6 presents the NO_x and SO₂ emissions at different combustion temperatures with sorbent addition under post-combustion. As is shown in Figure 6, both NO_x and SO₂ emissions increased nearly linearly with rising combustion temperatures under post-combustion when the sorbent was added into the furnace. Specifically, when the temperature increased from 845 °C to 870 °C, NO_x emission rose from 47.10 mg/Nm³ to 58.50 mg/Nm³, and the extent of the increase was 24.20%. When the temperature increased further from 870 °C to 905 °C, NO_x emissions increased from 58.50 mg/Nm³ to 74.10 mg/Nm³, and the extent of the increase was 26.67%. As far as SO₂ emissions are concerned, when the temperature rises from 845 °C to 870 °C, SO₂ emissions increase dramatically from 92.87 mg/Nm³ to 252.75 mg/Nm³, and the extent of the increase is 172.15%. When the temperature increases from 870 °C to 905 °C, SO₂ emissions rise from 252.75 mg/Nm³ to 452.88 mg/Nm³, and the extent of the increase is 79.18%.

This trend shows that under post-combustion and the addition of the sorbent for sulfur removal, both NO_x and SO₂ emissions increase as the combustion temperature rises. It is essential to lower the combustion temperature in order to reduce NO_x and SO₂ emissions under post-combustion. Notably, NO_x emissions at 845 °C were 47.10 mg/Nm³, which is below 50 mg/Nm³, thus achieving ultra-low NO_x emissions under in-furnace desulfurization conditions, while SO₂ emissions are only 92.8 mg/Nm³. Therefore, the simultaneous removal of both NO_x and SO₂ can be realized under post-combustion. Ultra-low SO₂ emissions (21.93 mg/Nm³) and low NO_x emissions are achieved by sorbent injection into the furnace at 75t/h for pure coal slime circulating fluidized bed boilers with post-combustion technology [27], showing that the collaborative removal of NO_x and SO₂ emissions can be realized under post-combustion conditions. The removal efficiency of NO_x and SO₂ is especially effective at lower temperatures under post-combustion. SO₂ emissions can decrease further and are expected to realize ultra-low SO₂ emissions when calcined lime is injected into the post-combustion chamber. It is also noteworthy that, within the temperature range of 845 °C to 905 °C, the increase in SO₂ emissions is more pronounced than that of NO_x emissions, indicating that SO₂ emissions and desulfurization efficiency are more sensitive to temperature changes. Given the stringent environmental standards, controlling SO₂ emissions requires particular attention being given to the impact of temperature.

3.2.3. Characteristics of Fly Ash at Cyclone Outlet and at Tail Flue

Particle Size Distribution of Fly Ash at Cyclone Outlet and at Tail Flue

The particle size distributions of fly ash at the cyclone outlet and at the tail flue are shown in Figure 7. With respect to fly ash at the cyclone outlet, no large difference was found in the particle size distributions under different temperatures, while the difference in the particle size distributions of fly ash at the tail flue under different temperatures is obvious. The results of the granularity analysis of fly ash are listed in

Table 4. Particle size distributions of fly ash at tail flue and cyclone outlet.

T (°C)	Fly Ash	D10 (μm)	D50 (μm)	D90 (μm)
905	a	4.10	33.12	139.97
	b	7.42	55.15	269.14
870	a	5.30	27.57	149.62
	b	8.23	52.52	452.55
845	a	4.53	28.05	119.98
	b	6.61	34.78	239.18

Notes: “a and b” represent fly ash at the cyclone outlet and fly ash at the tail flue; “D₁₀”, “D₅₀”, and “D₉₀” represent the particle diameters, below which the accumulated percentages are 10%, 50%, and 90%.

. It was obvious that the characteristic particle sizes (i.e., D₁₀, D₅₀, and D₉₀) of the fly ash at the tail flue under different temperatures were relatively larger (

Table 5. Element contents of fly ash at tail flue and cyclone outlet.

T (°C)	Fly Ash	1	2	3	4	5	6	7	8
905	a	Ca (%)	O (%)	Si (%)	Fe (%)	Al (%)	S (%)	Na (%)	Others (%)
		41.25	36.47	8.87	4.38	3.70	2.50	0.63	2.19
	b	Ca	O	Si	Fe	Al	S	Na	others
		36.21	38.76	11.11	4.28	3.51	3.91	0.50	1.72
870	a	Ca	O	Si	Fe	Al	S	Na	others
		39.40	36.93	9.08	5.36	3.93	2.70	0.74	1.87
	b	Ca	O	Si	Fe	Al	S	Na	others
		36.30	38.66	10.41	4.49	3.75	4.17	0.61	1.63
845	a	Ca	O	Si	Fe	Al	S	Na	others
		38.48	37.58	8.93	4.74	4.32	3.46	0.77	1.71
	b	Ca	O	Si	Fe	Al	S	Na	others
		36.33	38.80	10.00	4.49	3.83	4.52	0.56	1.48

Notes: “a and b” represent fly ash at the cyclone outlet and fly ash at the tail flue.

) than those of fly ash at the cyclone outlet under corresponding temperatures. Taking median particle sizes (D₅₀) for example, as shown in Table 5, the D₅₀ of fly ash at the cyclone outlet at 905 °C, 870 °C and 845 °C were 33.12 μm, 27.57 μm and 28.05 μm, respectively, which were relatively smaller than those of the fly ash at the tail flue under corresponding temperatures (55.15 μm, 52.52 μm, and 34.78 μm). This was mainly due to the fact that the reaction of sulfur fixation occurred when fly ash at the cyclone outlet entered the post-combustion chamber.

To illustrate the reaction of sulfur fixation that occurred in the post-combustion chamber, the composition of fly ash at the cyclone outlet and tail flue was analyzed using XRF analysis. The results of the chemical composition of all ashes observed are shown in

Table 6. Binding energies and forms of S (2p) functional groups.

Sulfur Functionality	Binding Energy/eV	Functional Form
Pyrite	162.5 ± 0.1	FeS2
Aliphatic sulfur	163.3 ± 0.3
Aromatic sulfur	164.1 ± 0.2
Sulfoxide	165.7 ± 0.3
Sulfone	168.0 ± 0.5 168.5 ± 0.5

. It can be seen from Table 5 that compared with the fly ash at the cyclone outlet, the sulfur content in the fly ash at the tail flue increased at 905 °C, 870 °C, and 845 °C, which shows that the reaction of sulfur fixation occurred in the post-combustion chamber, resulting in more sulfur being solidified and stored in the fly ash at the tail flue. In addition, the lower the temperature, the higher the sulfur content in the fly ash at the tail flue, and the higher the efficiency of sulfur fixation.

Structural Characteristics of Organic Sulfur in Fly Ash

As shown in Figure 8 and Figure 9, XPS sulfur(2p) spectra are fitted by using a pair of spin–orbit splitting peaks: 2p_3/2 and 2p_1/2 [28]. To obtain an acceptable fit, six pairs of splitting peaks at different energy positions are used to fit the XPS sulfur(2p) spectra of the fly ash. As shown in Table 6, four pairs of splitting peaks are assigned to pyrite, aliphatic sulfur, aromatic sulfur, and sulfoxide, respectively; two pairs of splitting peaks at 168 (±0.5) and 168.5 (±0.5) eV are assigned to sulfone. The integral area in Figure 8 and Figure 9 represents the relative amount of each substance.

As listed in

Table 7. Contents of containing-sulfur functions in fly ash under different temperatures.

T (°C)	Fly Ash	Pyrite (%)	Aliphatic Sulfur (%)	Aromatic Sulfur (%)	Sulfoxide (%)	Sulfone (%)
905	a	2.43	4.18	23.18	15.99	54.22
	b	5.7	6.22	15.12	14.87	58.09
870	a	3.5	0	21.38	13.16	61.96
	b	4.87	1.94	17.19	13.03	62.97
845	a	5.14	0.33	19.32	17.4	57.8
	b	2.22	3.28	18.65	12.66	63.18

Notes: “a and b” represent fly ash at the cyclone outlet and fly ash at the tail flue.

, it is clear that aromatic sulfur and sulfone are the predominant forms of organic sulfur on the surfaces of fly ash, and the proportions of the two forms are over 70%. Moreover, a small amount of sulfoxide sulfur (13–17%) was also present on the surfaces of fly ash, but the amount of aliphatic sulfur was lower (0–6%). Pyrite is quite small in all forms of fly ash. Sulfone is the sum of the relative amounts of two pairs of splitting peaks at 168 (±0.5) and 168.5 (±0.5) eV. This is expected to arise predominantly from sulfate, but contributions from sulfite and sulfone cannot be entirely dismissed [29]. Therefore, it is thought appropriate to consider that sulfone can include sulfate, sulfite, and sulfone. Gorbatyetal [30] found that aliphatic sulfur in coal was easily oxidized in the environment, with the formation of oxidation products such as sulfoxide and sulfone, whereas aromatic sulfur was more stable.

There is no H₂S, COS, or CS₂ at the cyclone outlet, and a very small amount of SO₂ is observed. Therefore, the great majority of S released in coal remains in the fly ash due to a lack of oxygen in the furnace. Sulfone includes sulfate, sulfite, and sulfone, so it is closely related to SO₂ emissions. As Figure 8d shows, the relative amount of sulfone in fly ash at the cyclone outlet experiences the trend of first increasing and then decreasing with a decreasing amount of temperature. Specifically, when temperatures are 845 °C and 870 °C, the relative amount of sulfone in fly ash at the cyclone outlet is higher than that at 905 °C, which shows that sulfur removal efficiency is higher under lower temperatures in a weekly reducing atmosphere. As Figure 9d shows, the relative amount of sulfone in fly ash at the tail flue increases with a decreasing temperature. The lower the temperature, the higher the sulfur content in the fly ash at the tail flue, and the higher the efficiency of sulfur fixation. In addition, it can be seen from Table 7 that compared with the fly ash at the cyclone outlet, sulfone in the fly ash at the tail flue increased at 905 °C, 870 °C and 845 °C, which shows that the reaction of sulfur fixation occurred in the post-combustion chamber, resulting in more sulfur being solidified and stored in the fly ash at the tail flue.

4. Conclusions

In summary, this study proposes a novel method of the combined removal of NO_x and SO₂ emissions under post-combustion technology combined with the injection of a sorbent into the furnace, exploring the potential of combined removal of the NO_x and SO₂ emissions under post-combustion technology. According to the experiment results, the following conclusions can be drawn:

The low-cost combined removal of NO_x and SO₂ can be realized by denitrification in the furnace and through secondary desulfurization in the furnace and post-combustion chamber. In the optimized experimental condition, with the combustion temperature at 845 °C and sorbent addition in the furnace under post-combustion, NO_x emissions can be reduced to 47.10 mg/Nm³(@6%O₂), and meanwhile, SO₂ emissions can be reduced to 92.09 mg/Nm³. Sulfur removal efficiency is higher under lower temperatures in a weakly reducing atmosphere. The reaction of sulfur fixation that occurred in the post-combustion chamber caused the particle size of fly ash at the tail flue to become bigger and the sulfur content in the fly ash at the tail flue to increase. At 845–905 °C, combustion temperatures had a bigger effect on SO₂ emissions than the NO_x with sorbent addition in the furnace under post-combustion.