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Abstract

Pakistan has 185 billion tonnes reserves of coal but unfortunately the quality of 80% of this coal is not good. As the country has a shortage of energy so it is necessary to refine the coal before it can be used to produce electricity. In this sense, this research is very important as it enables indigenous coal to meet the increasing energy demand. This study is focused on the control of sulfur emissions during the combustion of high sulfur Pakistani coal from the Trans Indus Range and the Salt Range. Parameters like Ca/S molar ratios (MRs), limestone particle size, bed temperature, and different percentages of biomass in co-firing with coal have been studied. It is showed that desulfurization of coal was maximum with the fine-sized particles of limestone. Co-firing of moderate quantity biomass exhibited a considerable decrease in SO₂ emissions. Results achieved are presented in the form of tables and plots. This study for control of the gaseous emissions from combustion in FBC facility has great potential for new coal based power projects, especially in Pakistan.

Keywords

Fluidized bed combustion coal combustion emissions sulfur control multi variant process

Introduction

Coal is the world’s most abundant and widely distributed fossil fuel, with global proven reserves of nearly 1000 billion tonnes and has been a key component of electricity generation mix worldwide.^1,2 Coal provides more than 40% of the world’s electricity^3,4 and this share is predominant for some countries such as South Africa (93%), Poland (92%), China (79%), India (69%), and the United States (49%). Moreover, the growing energy needs of the developing world are leading to the fact that coal will remain a key component of the power regardless of climate change policy.⁵

In Pakistan, power generation from coal is currently gaining more interest, relative to oil and gas.⁶ Coal in Pakistan is of low grade such as lignite in Sindh (Thar and Lakhra), sub-bituminous in Baluchistan and Punjab (the Salt Range and Trans Indus Range).⁷ The coal reserves of the Salt Range and Trans Indus Range are approximately 500 million tons.⁸ In general Pakistani coal contains high sulphur (ranging from 3% to 11%) except Thar coal. Coal based power generation could represent an economic solution by using low-grade Pakistani coals and their blends with better quality imported coals, which would ensure high performance and compliance with environmental regulations.⁹ FBC technology could prove a better option to utilize such low-grade high sulfur coals in coal fired steam as well as power generation systems with controlled emissions of nitrogen oxides (NO_x) and sulfur oxides (SO_x).^10–12

In the literature, desulfurization of high sulfur Pakistani coals during combustion has not been fully investigated. Zaidi¹³ performed the study of coal reactivity as well as char formation of different coal samples collected from Punjab, Sindh, and Baluchistan. Iqbal et al.¹⁴ studied the effect of particle size on different combustion parameters of the coal from Islamkot Thar. Naveed et al.¹⁵ investigated the gasification perspectives of the Eastern Salt Range coals and found the coal suitable for gasification. Daood et al.¹⁶ performed the study of gaseous emissions of Thar coal such as SO₂, NO_x, CO, and CO₂ in pilot-scale facility. Rehman et al.⁹ have already studied the combustion perspectives along with geology and coalfield stratigraphy of the coals from Salt Range and Trans Indus Range Punjab Pakistan and concluded that the coal has good combustion characteristics, low slagging, and medium fouling Indices. However, the work on desulfurization of high sulfur Pakistani coal has seldom been addressed in the open literature and thus needs to be studied comprehensively using different methods including limestone addition and co-firing with biomass.

In this paper authors have attempted to present the results of SO₂, NO_x, CO, and CO₂ concentration measurements obtained in a pilot-scale FBC, with and without continuous limestone addition. The results of SO₂ were also presented for co-firing of coal with different percentages of biomass, and to offer some qualitative interpretation relevant to the understanding of the SO₂ behavior during coal combustion. The main parameters of the desulfurization process in FBC such as different MRs, adsorbent (limestone) particle size, bed temperature, and co-firing with biomass were analyzed on the basis of a higher initial concentration of SO₂ in combustion furnace.

This research work is very important as the process and methodology given help to utilize the coal available domestically. It also helps to reduce emissions resulting from the burning of coal. A substantial amount of coal reserves can be used as a fuel alternative to meet energy demand for a country like Pakistan.

Materials and methods

The test facility is installed at the Low Carbon Combustion Center, University of Sheffield, UK and it comprises a fluidized bed combustion (FBC) section, having overall dimensions of (1 m × 1 m × 5 m) with an extended freeboard section. Bed of sand, Garside Leighton Buzzard 14/25 range with nominal particle size of 1.18–0.60 mm in this case, was fluidized by air flowing through a standpipe distributor. The internal dimensions of the fluidized bed and the combustion section above the fluidized bed are 0.42 m × 0.38 m and 0.58 m × 0.38 m, respectively. The combustion section is lined with castable refractory backed with ceramic fiber (having low thermal conductivity) based insulation. The bed having an area of 0.16 m², is capable of accepting bio-fuels/wastes feed rate up to 100 kg/h and mineral feed rate up to 500 kg/h depending on plant operating conditions and material type. The rig can be operated with fluidizing velocities of up to 3.5 m/s and fluidized bed heights of up to 700 mm, at combustion temperatures ranging from 750°C to 950°C.

There are three variable speed screws with feed hoppers to form the above–bed solid fuel feed system. All of them discharge into a high speed screw having 125 mm diameter which acts as a part-filled transfer screw and transfers the material onto the surface of the fluidized bed or into the splash zone. The system allows the feeding of three different materials simultaneously and capable to handle a variety of solid granular as well as powder materials.

The fluidized bed is instrumented with temperature and pressure ports for monitoring bed and freeboard conditions. Five thermocouples are arranged in a vertical array at equal space, the bottom and top ones are at 30 and 450 mm from the top of the air distributor. In the access door, an off-take port is provided which allows overflow of bed material when the nominal fluidized height reaches 0.35 m. To withdraw either a fraction of the selected coarse size or the whole bed, an air classifier of 68 mm diameter is given in the base of the fluidized bed.

The positions of all thermocouples and pressure transmitters, to record temperature and pressure respectively, are shown in Figure 1. The transmitters are connected to a National Instrument (NI) TBX-1303 module which is then linked to a NI SCXI-1102 (temperatures) and SCXI-1100 (pressures) and lab-view is used to read them via a NI PCI-6035E card in the computer.

Figure 1.

Schematic of FBC with instrumentation and control equipment.

Manual control valves at primary air and secondary air lines as shown in Figure 1 were used to regulate fluidizing air flow rate through the fluidized bed and above the bed, respectively. The Inverter control of the FBC exhaust fan enables the operator to maintain a negative pressure above the bed, in the mid-freeboard and subsequent sections. Euro-therms PID controllers enable automatic and manual control of the fuel flow of each screw metering feeders into the bed.

Lab view software was used for logging the data of all the temperatures and pressures. The oxygen concentration was measured by a Portable Oxygen Analyzer (Servomex 570). A forced draught fan supplies fluidizing air which enters the bed through a standpipe distributor. A manual damper controls the airflow rate, while start-up is achieved by lighting the premixed gas at the primary burner.

The FBC consists of an air-cooled heat exchanger to reduce the flue gases temperature, a bag filter, and a cyclone to clean the flue gas. The fluidized bed and freeboard sections are made of mild steel with interior refractory lining which results in a low outer casing temperature and consequently low heat losses.

The flue gases leaving the fluidized bed are cooled in the air-cooled heat exchanger using forced draught air. The cyclone is used to separate most of the suspended solids from the flue gases leaving the heat exchangers. The solid particles are discharged into a sealed drum and can be collected for further analysis. The flue gases leaving the cyclone are passed either through the bag filter or discharged directly to the atmosphere via the induced draft fan as shown in Figure 1. The temperature of the flue gases is reduced down to 500°C, depending on operating conditions. Figure 2 and 3 show the main parts of FBC, and process flow diagram of lab scale and pilot scale testing respectively.

Figure 2.

Actual testing facility and main parts of FBC.

Figure 3.

Process flow diagram of lab scale and pilot scale testing.

All reported values of gaseous emissions like SO₂, NO_x, etc. are corrected to 6% O₂ in the flue gas using the following equations (1) and (2).

$S O_{2} at 6 % O_{2} (%) = S O_{2} \frac{(20.9 % - 6 %)}{(20.9 % - O_{2} %)}$ (1)

$N O_{x} at 6 % O_{2} (%) = N O_{x} \frac{(20.9 % - 6 %)}{(20.9 % - O_{2} %)}$ (2)

The bag filter houses ceramic elements to enable the plant to be operated with a range of flue gas exit temperatures, and also to provide information on cleaning and cake properties for full-scale plant proposing to use ceramic filters.

In this experimental work, two different types of coals were used for desulfurization and emission study of high sulfur coal from Trans Indus Range (coal A) and Salt Range (coal B), Punjab, Pakistan. Limestone for desulfurization of the said coals was also taken from the Salt Range area. Coal and limestone were continuously added to the bed, keeping control of the bed height. The feed rate of limestone was in the range of 12–60 g/min, while that of coal was from 150 to 300 g/min. The bed was continuously monitored during the experiments. All the measurements (temperature and gas composition) were taken with the FBC reactor operating at pre-set steady conditions. At specified feeding conditions of air, coal, and limestone, the steady state condition was evaluated by monitoring the temperature and the flue gas composition (in terms of O₂ and CO₂). The steady state condition was considered when the temperature and the flue gas composition were stable, and then a complete set of measurements was obtained.

Coals were sieved from 5 to 15 mm sizes and their properties are shown in Table 1. Limestone was used as an adsorbent and sieved to different size ranges (0.1–5.6 mm) and its properties are given in Table 2.

Table 1.

Results of GCV, proximate, and ultimate analyses of coal samples.

Coal	MoistureARB (wt%)	VM ADB(wt%)	Ash ADB(wt%)	N ARB (wt%)	C ARB(wt%)	H ARB(wt%)	S ARB(wt%)	GCV ADB(kcal/kg)
A	5.46	35.5	34.71	1.17	44.00	3.46	7.23	4826
B	7.27	33.32	34.04	1.25	45.82	3.41	6.75	4949

ARB: as received basis, ADB: air dry basis, VM: volatile matter, N: nitrogen, C: carbon, H: hydrogen, S: sulfur, GCV: gross calorific value.

Table 2.

Chemical analysis of limestone.

Parameters	SiO₂	Al₂O₃	Fe₂O₃	CaO	MgO	K₂O	Na₂O	SO₃	Cl
Analysis results (%)	4.28	0.53	0.39	51.14	1.59	0.09	0.04	0.06	0.004

White wood was used as biomass for co-firing with coal. The results of proximate analysis, ultimate analysis, and semi-quantitative XRF analysis are given in Table 3.

Table 3.

Results of proximate, ultimate and XRF analysis of biomass.

Proximate and ultimateanalysis of biomass		Semi-quantitative XRF Results
Constituents	wt%	Elements	%
N	0.17	SiO₂	40
C	47.05	K₂O	8.81
H	5.39	P₂O₅	1.3
S	<0.1	MgO	4.79
O	47.39	CaO	22.9
Ash	1.14	Al₂O₃	8.46
Moisture	6.6	Na₂O	2.2
volatile matter (VM)	76.32	Br	3.21
Fixed carbon (FC)	11.83	Cl	0.0912
		SO₃	0.292
		Fe₂O₃	4.92
		MnO	1.72
		ZnO	0.174
		NiO	0.0146
		TiO₂	0.502

Gas samples were withdrawn from the bed through water cooled probes and passed through a non-dispersive IR (CO₂, CO, N₂O) and chemiluminescence (NO) analyzers. Due to the existence of radial concentration gradients, the tip of each water-cooled probe was positioned at a distance of 0.07 m from the inner wall. For SO₂ (non-dispersive IR), the samples were taken from the freeboard through a non-cooled movable probe and a heated sampling line. This process restricted the sampling at certain locations in the freeboard, at 0.45, 0.65, and 1.68 m respectively above the distributor plate. Table 4 gives operating condition of the experimental test.

Table 4.

Experimental test operating conditions.

Bed material	Sand, Garside Leighton Buzzard14/25 range (with nominalparticle size of 1.18–0.60)
Fluidized bed height	700 mm
Fluidizing velocities	Up to 3.5 m/s
Typical combustiontemperatures ranging	750°C–950°C
Feed rate of coal	150–300 g/min
Feed rate of limestone	12–60 g/min

Results and Discussion

Effect of limestone quantity on SO₂

Figure 4 shows the influence of different quantities of limestone (added in the bed) in terms of Ca/S molar ratios (MRs) on SO₂ emissions during combustion of coal A from the Trans Indus Range and coal B from the Salt Range.

Figure 4.

Comparison of different MRs and SO₂ emissions for coal A and coal B.

Different values of MR were used depending on limestone particle size and operational parameters. It is clear from Figure 4 that with an increase in MR, SO₂ emissions decrease significantly for both coal A and coal B, reaching upto 40% and 41% at MR = 2 and 49% and 52% at MR = 3, respectively. However, a maximum decrease of 72% for coal A and 78% for coal B (i.e. maximum desulfurization) was observed at MR = 3.5, With a further increase in MR to 4, sulfur capture level decreased to 69% for coal A and 70% for coal B. It shows that an increase in limestone quantity beyond MR = 3.5, is not feasible for SO₂ retention, and thus 3.5 is the optimum value of MR to get the maximum reduction in SO₂ in the case of both coals. It can also be observed from Figure 4 that at MR = 2, a significant reduction in SO₂ emission occurred (40% for coal A and 41% for coal B). With the further addition of 50% in limestone quantity (at MR = 3), only 9% and 11% were added to initial SO₂ reduction for coal A and coal B respectively. Although overall SO₂ emissions are decreased with the increase in MR values but desulfurization efficiency decreased at higher MR values. Therefore, it is evident from the results that desulfurization efficiency strongly depends on the initial concentration of SO₂ in the bed.

The above results are in accordance with the investigation of Bragança and Castellan¹⁷ which reveals that SO₂ retention efficiencies are 48%, 60%, 68%, and 77% at MR values of 1, 2, 3, and 4 respectively during coal combustion in FBC. Their results also show that the amount of limestone utilization lies in the MR range from 1 to 4 to meet the environmental norms and further rise in MR value will not be economically feasible. According to the study of Tarelho et al.,¹⁸ the range of MR is from 1 to 5 for various developed/developing countries to meet the SO₂ emission norms and high SO₂ removal efficiencies can be attained at MR = 3.5.

Limestone reacts with SO₂ in presence of O₂ to form calcium sulfate (CaSO₄).

A number of reactions are possible including the following (equations (4) and (5)):

$CaC O_{3} + S O_{2} + \frac{1}{2} O_{2} \to CaS O_{4} + C O_{2} Δ H = - 303 \frac{kJ}{mole}$ (3)

$CaO + S O_{2} + \frac{1}{2} O_{2} \to CaS O_{4} Δ H = - 481 \frac{kJ}{mole}$ (4)

Stoichiometric calculations show that 1 mol of limestone feed can reduce 1 mol of sulphur.¹⁹ The molar volume of CaSO₄ is greater than that of either CaO or CaCO₃ which leads to the plugging of pores and therefore, complete conversion of the adsorbent particle is impossible. Also, sulfation only proceeds at the outer surface of the CaO particle. The sulfation process continues until external pores are blocked significantly and an impenetrable CaSO₄ shell is formed leaving a considerable amount of unreacted CaO core. The increase in SO₂ emissions for both coals beyond MR of 3.5 value may be due to the equilibrium of sulfation and reduction of CaSO₄.²⁰ According to Cheng et al.²¹ operational parameters including MRs, furnace temperature, residence time, and SO₂ partial pressure affect the sulfation reaction.

Furthermore, the lower SO₂ emissions at all MR values in the case of coal B relative to its counterpart coal A is attributed to its lower inherited sulphur²⁰ as shown in Table 1.

Effect of limestone on NO_x

Figure 5 displays the effect of different MRs on NO_x emissions for coal A and coal B. It is clear from the graph that there is an increase in NO_x emission with the increase in MR from 2 to 3.5 for coal A and coal B and the maximum increase occurs at MR = 3.5. Further increase in the MR beyond the value of 3.5 results in the decrease of NO_x emission. The results are in line with the study of Zhou et al.²² explaining that the NO_x emission increases with the addition of limestone during coal combustion. Some previous studies also show that taking the coal combustion without limestone as a reference, the addition of limestone results in the increase of NO_x emission. An increase in MR to control SO₂ emissions has a prominent impact on NO_x emissions. There are three major mechanisms that explain the reason of this increase in NO_x emission during limestone addition: the catalytic oxidation of NH₃ to NO by CaO surfaces, the catalytic oxidation of CO to CO₂ over CaO, and the catalytic oxidation of HCN to NO over CaO.^23–25 The reaction scheme summarizing the selectivity of CaO toward NH₃, HCN, and CO is given below²⁶ in equations (5)–(7).

$N H_{3} + O_{2} \overset{CaO}{\to} NO + N_{2}$ (5)

$HCN + O_{2} \overset{CaO}{\to} NO + N_{2}$ (6)

$NO + CO \overset{CaO}{\to} \frac{1}{2} N_{2} + C O_{2}$ (7)

Figure 5.

Effect of limestone on NO_x emissions.

The addition of limestone may result in the shift from the homogeneous HCN oxidation mechanism to the catalytic one, which results in higher selectivity for NO formation and a lower one for N₂O.²⁷

A decrease in NO_x emission beyond the MR = 3.5 at which maximum desulfurization occurred, is supported by the fact that catalytic conversion of NH₃ to NO is affected by the desulfurization. Limestone (CaO based sorbent) is an active catalyst for the oxidation of NH₃ to NO but pore plugging of limestone occurs at higher MRs when the maximum conversion of CaO to CaSO₄ is reached, which reduces NO formation from NH₃.²⁸

Effect of limestone on CO

Figure 6 shows the influence of MR on CO emissions. It is clear from the figure that CO emissions decrease with an increase in the value of MR up to 3, for both of the coals. As the MR is further increased to 3.5, CO emissions increase but decrease again as the MR is increased to 4. The maximum decrease in CO emissions is observed at MR = 3.

Figure 6.

Effect of limestone on CO emissions.

Decrease in CO emissions upto MR = 3, is in accordance with the study of Liu and Gibbs²⁶ who investigated the effect of limestone addition on gaseous emissions in a CFB combustor and concluded that with an increase of MR, the concentration of CO decreases during coal combustion in CFB. Tarelho et al.²³ also found that CO and N₂O emissions decrease and NO emissions increase with the addition of limestone during coal combustion in FBC. But for higher MRs, CO emissions increase and maximum concentration of CO is observed at MR = 3.5 at which maximum desulfurization occurred. There is a significant decrease in CO emissions with further increase in MR beyond 3.5 for both coal A and coal B. Armesto et al.²⁹ reported that CO emissions are related to the concentration of O₂, temperature, reaction time/phase, and mixing of fuel with air. Amand et al.³⁰ studied the effect of lime used for desulfurization in 12 MW CFB research plant. Their results reveal that the influence of SO₂ on NO/N₂O chemistry is not dependent on the catalytic effect of the lime surface and CO acts as an important intermediary species in these reactions. They also concluded that increase in SO₂ results in a decrease of NO_x accompanied by an increase in CO emissions. The relation between SO₂ and NO_x has already been discussed and reasoned in the previous section 3.2 of this article. However, the increase of CO with a rise in SO₂ has not been fully understood and needs further investigation. Authors are of the view that the rise in CO emission at and beyond MR = 3.5, might be due to the fact that as NO formation from NH₃ reduces at higher MRs and the rate of catalytic reduction of NO by CO also decreases thereby reducing the consumption of CO. This may result in an increase of CO emissions at higher MRs, however, this phenomenon needs further investigation.

Effect of limestone on CO₂

Figure 7 shows the influence of limestone in terms of MR on CO₂ emissions. It is evident from the Figure 7 that the CO₂ emissions increased at MRs 2, 3 and decreased for higher MRs for coal A. Similarly, CO₂ increased at MRs 2, 3, and 3.5 and decreased for higher MRs for coal B. At high temperatures, CO₂ can be separated by the calcium oxide (sorbent) and high-temperature CO₂ capture systems are based on carbonation reaction when coupled with a calcination step. Grasa and Abanades³¹ experimentally investigated the evolution with the cycling of the capture capacity of CaO. Stanmore and Gilot¹⁹ examined some aspects of using lime from limestone to sequester CO₂ from combustion systems.

Figure 7.

Effect of limestone on CO₂ emissions.

Effect of bed temperature on SO₂

Figure 8 displays the effect of bed temperature on SO₂ emissions for coal B. This coal has been selected here owing to its better reduction of SO₂ emissions as demonstrated in the previous case. In this case, MR of 3.5 is kept which has proved to be optimum value for maximum desulfurization of both coals from the Salt Range and Trans Indus Range. It is clear from the figure that there is a slight increase in SO₂ emission against the bed temperature range from 700°C to 750°C at the beginning of the combustion but there is a decrease in SO₂ emission for the bed temperature window of 750°C–800°C. For bed temperature beyond 800°C, SO₂ emissions increase significantly revealing that further increase in temperature is not prone to control of SO₂ emissions. Therefore, the bed temperate of 800°C is the optimum temperature for maximum desulfurization of the coal under study at MR = 3.5. This behavior is also in accordance with previous research findings that the efficiency of SO₂ removal by limestone appears to decrease with an increase in the bed temperature.¹⁸ According to Bragança and Castell,¹⁷ the optimum bed temperature is 850°C for maximum desulfurization, while Tarelho et al.¹⁸ conducted their tests in FBC pilot plant facility and investigated that SO₂ removal efficiency decreases with the increase in bed temperature ranging from 825°C to 900°C, and thereby a maximum rise in SO₂ removal efficiency occurred around 825°C which has been reported as optimum temperature. Anthony and Granatstein²⁰ are of the view that there is still no consensus over the explanation of maximum temperature in FBC boiler for optimum sulfur retention, and its value depends on the operational parameters, the adsorbent used for desulfurization and types of coal, etc. The seize in SO₂ reduction beyond 800°C in the current study may be attributed either to one or more possible reasons including physical properties of limestone such as solid sintering and choking of porous structure, reductive decomposition of CaSO₄, or a net balance between limestone sulfation and CaSO₄ decomposition, at high bed temperatures.¹⁷

Figure 8.

Bed temperature versus SO₂ emissions for coal B.

Effect of limestone particle size on SO₂

Figure 9 shows the influence of the particle size of limestone on sulfur reduction efficiency. It is evident from the figure that the reduction efficiency is higher for fine particle sizes ranging 0.1–2.0 mm and 2.0–2.8 mm relative to coarse particle size ranging 2.8–4.0 mm and 4.0–5.6 mm, at the same MR of 3.5. Fine particle-size ranges have a higher surface area for reaction than coarse particle-size ranges, as the surface area is the most important factor for better performance of finer particle sizes for SO₂ retention. Maximum sulfur retention efficiency of 72% is achieved at limestone particle-size range of 2.0–2.8 mm instead of finer particle-size range of 0.1–2.0 mm. The lower retention efficiency of finer limestone is accounted for the excessive elutriation occurring at this particle size which is a potential limitation for the investigation of finer sorbents. The particle size of the ground limestone is inversely related to its speciﬁc surface area; decreasing particle size increases the available surface area thereby improving the reactivity of the limestone. However, if the fragments generated from the attrition are too ﬁne, unreacted sorbent would elutriate from the reactor, thus reducing the overall performance of the sorbent.¹⁸ Bragança and Castell¹⁷ investigated two different dolomite particle sizes of 0.47 and 0.32 mm during desulfurization of coal with high oxidizing conditions in FBC pilot-scale facility at bed temperature = 850°C. According to their results, finer particle size achieved 60% while the coarser gained only 30% SO₂ retention efficiency at same MR.

Figure 9.

Limestone particle size versus SO₂ emissions for coal B.

Effect of co-firing on SO₂

Figure 10 shows the influence of different quantities of biomass (added in to the bed) on SO₂ emissions during combustion of coal A and coal B. It is clear from the graph that with the increase in biomass percentage from 30% to 60%, SO₂ emissions decrease from 47.5% to 68.3% for coal A, and 36.3% to 67.8% for coal B. There is no considerable difference in SO₂ emissions at 40% and 50% biomass for coal A but for coal B, SO₂ emissions are 58% and 53% respectively at 40% and 50% biomass. A decrease in SO₂ emissions at 50% biomass in comparison to 40% for coal B could be due to an error in measurement during experimental work. A maximum decrease of 68.3% in SO₂ emissions for coal A and 67.8% for coal B achieved at 60% biomass fractions. This is in accordance with the study of Narayanan and Natarajan,³² they got 50% reduction in SO₂ with the addition of 60% biomass. During their study, it was also found that the decrease in SO₂ emissions remained upto 16% and 36% with cofiring of 20% and 40% biomass, respectively. Study of Demirbas³³ also showed a decrease in SO₂ with an increase in the proportion of the biomass.

Figure 10.

Comparison of biomass percentages and SO₂ emissions for coal A and coal B.

Hein and Bemtgen³⁴ explained the results of 2-years research project launched by the European Commission in which the co-combustion, in laboratory, pilot and full-scale units was studied with the objective to investigate the effect of biomass addition on the gaseous emissions. They confirmed that co-combustion of biomass with coal has a significant effect on SO₂ reductions reaching up to 75%. The reduction in SO₂ with the increase in biomass is ascribed to the dilution effect, as the biomass contains a negligible amount of sulfur. CaO and K₂O present in the biomass, have the ability to capture SO₂ by forming CaSO₄ and K₂SO₄ in the presence of O₂ just like SO₂ capture using limestone.³⁵

Conclusion

In the current study, the potential of limestone and biomass for the reduction in gaseous emissions like SO₂, NO_x, CO, and CO₂ has been demonstrated on two different coal samples sourced from Pakistan using a pilot-scale FBC facility. Using coal as the primary fuel, limestone and biomass were mixed in different proportions to investigate the impact on desulfurization during combustion. The SO₂ reduction was influenced by fuel properties, firing methodology (pure coal/cofiring with biomass), different values of MR, fine/coarse particle sizes of limestone, and the bed temperature. It was observed that maximum desulfurization of coal A and coal B remained 72% and 78% respectively at the optimum MR value of 3.5. There was a predominant impact of bed temperature in the window of 750°C–800°C on SO₂ reduction, revealing the maximum reduction of 70% at 800°C with the same MR of 3.5.

Moreover, the effect of limestone particle size was studied on SO₂ emission which exhibited that maximum desulfurization of 72% occurred at a fine particle size range of 2.0–2.8 mm, relative to finer and coarse mode particles owing to excessive elutriation and small surface area in the former and latter cases, respectively. Also, there was a significant decrease in SO₂ emissions with the increase in the percentage of biomass (30%–60%) during co-firing with coal. Consequently, a remarkable decrease of 68% in SO₂ was noted with 60% biomass relative to other proportions.

This study for control of the gaseous emissions from combustion of high sulfur Pakistani coal in FBC facility has great potential for new coal based power projects, especially under China Pakistan Economic Corridor (CPEC) program. The study would also provide useful knowledge to the international scientific and engineering community for optimization, design, and development of thermal power plants based on high sulfur coal.

Footnotes

We thank all the colleagues and lab supervisors whose help has been a source of light for us.

Handling Editor: James Baldwin

Author contributions

All the authors actively participate in the research work. Muhammad Mahmood Aslam Bhutta conceived and designed the analysis,contributed data,or analysis tools. Shafiq ur Rehman collected the data,contributed data or analysis tools,performed the analysis,and wrote the paper. Asad Naeem Shah conceived and designed the analysis. Muhammad Sajid Kamran contributed data or analysis tools. Ahmad Naveed contributed data or analysis tools. Syed Muhammad Sohail Rehman contributed data or analysis tools,wrote the paper,redraft the paper layout.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This experimental work was supported by Higher Education Commission (HEC) of Pakistan [under the “International Research Support Initiative Program (IRSIP)”];INSPIRE/KEP [HEC and British Council Islamabad funded project titled “Research cooperation for innovation in coal combustion and promotion of modern mining practices in Pakistan”];and the Coal Fired Power Project (CFPP),Government of Pakistan.

Ethical approval/patient consent

Research involves no ethical concerns. However,all applicable international,national,and/or institutional guidelines for conduction of research followed. No harm to nature or its habitants is caused.

ORCID iD

Muhammad Mahmood Aslam Bhutta

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Optimization of multi variant process parameters to reduce emissions of high sulfur coal during combustion in fluidized bed facility

Abstract

Keywords

Introduction

Materials and methods

Results and Discussion

Effect of limestone quantity on SO2

Effect of limestone on NOx

Effect of limestone on CO

Effect of limestone on CO2

Effect of bed temperature on SO2

Effect of limestone particle size on SO2

Effect of co-firing on SO2

Conclusion

Footnotes

Author contributions

Declaration of conflicting interests

Funding

Ethical approval/patient consent

ORCID iD

References

Effect of limestone quantity on SO₂

Effect of limestone on NO_x

Effect of limestone on CO₂

Effect of bed temperature on SO₂

Effect of limestone particle size on SO₂

Effect of co-firing on SO₂