Oxyfuel technology: Oil shale desulphurisation behaviour during unstaged combustion

Fuel 158 (2015) 460–470

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Oxyfuel technology: Oil shale desulphurisation behaviour during unstaged combustion L. Al-Makhadmeh a,⇑, J. Maier b, M. Al-Harahsheh c, G. Scheffknecht b a

Environmental Engineering Department, Al-Hussein Bin Talal University, Ma’an 71111, Jordan Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germany c Chemical Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan b

h i g h l i g h t s  First investigation on oil shale desulphurization under unstaged oxyfuel & air-firing conditions.  Oil shale-S conversion rates to SO2 is lower during oxyfuel combustion.  Sulphur self-retention is more pronounced during oxyfuel combustion.  Significant reduction in SO2 emissions is obtained by limestone injection.  There is a clear potential for zero SO2 emissions at high Ca/S molar ratios.

a r t i c l e

i n f o

Article history: Received 10 January 2015 Received in revised form 25 May 2015 Accepted 26 May 2015 Available online 4 June 2015 Keywords: Desulphurisation Sulphur self-retention Oxyfuel Combustion Oil shale

a b s t r a c t Oxyfuel combustion is a promising technology in terms of CO2 and NOX emissions control. Furthermore, CO2-rich atmospheric condition in a furnace leads to in-furnace desulphurisation. The high sulphur content in Jordanian oil shale is considered one of the biggest challenges for its utilisation. Direct sorbent injection studies for desulphurisation in O2/CO2 pulverised coal combustion were very limited and there is none for oil shale combustion. In this study direct limestone injection has been investigated during Jordanian oil shale combustion under unstaged oxyfuel conditions as well as air-firing in a 20 kW vertical reactor. Different molar ratios of Ca/S were investigated in both firing modes. The oil shale-S conversion rates to SO2 are lower during unstaged oxyfuel combustion compared to air-firing; they were 61%, 49%, and 58% for air-firing, OF27, and OF35 combustion, respectively. Sulphur self-retention is more pronounced during oxyfuel combustion compared to air-firing due to the higher concentrations of SO2 and CO2. Significant reduction in SO2 emissions is obtained by limestone addition in both combustion modes. The desulphurisation efficiency increases with Ca/S molar ratio for both air-firing and oxyfuel combustion. At Ca/S molar ratio of 3, the desulphurisation efficiencies were 95%, 100% and 93% for air-firing, OF27, and OF35 combustion, respectively; there is a clear potential for zero SO2 emissions at high Ca/S molar ratios during unstaged combustion. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction One of the most effective ways for oil shale utilisation is direct combustion. However, oil shale combustion causes serious air pollution problems, which restricts its use. The high sulphur content in oil shale is considered one of the biggest challenges for its utilisation. Sulphur in oil shale exists in both organic and inorganic forms [1]. The inorganic sulphur is mostly in the form of pyrite and/or marcasite, together with small amounts of sulphates ⇑ Corresponding author. Tel.: +962 795 834 5597. E-mail addresses: (L. Al-Makhadmeh).

[email protected],

http://dx.doi.org/10.1016/j.fuel.2015.05.059 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

[email protected]

depending on the depth of oil shale bed [2]. The organic sulphur exists either in aromatic rings or in aliphatic functional groups, usually categorised as mercaptans, aliphatic and aryl sulphides, disulphides and pyritic sulphur depending on the rank and total sulphur content of oil shale [3]. In recent years, tremendous effort has been put into the development of Carbon Capture and Storage (CCS) technologies within the process of power generation from fossil fuels in order to reduce the release of CO2 which is seen as one of the major greenhouse gases. The oxyfuel process has emerged as a promising option for applying the CCS technology. During oxyfuel process, a combination of oxygen with a purity of more than 95 vol.% diluted with recycled flue gas consisting of CO2 with/without H2O is used for

L. Al-Makhadmeh et al. / Fuel 158 (2015) 460–470

fossil fuel combustion. Part of the flue gas, consisting mainly CO2, is recycled back to the furnace to maintain the temperature and heat flux profiles, and to entrain the pulverised fuel. The concentration of CO2 in the exhaust gas is significantly increased and the recovery of CO2 becomes feasible with a lower-efficiency penalty [4]. Fundamentally, because of the variation in the oxidant and consequently the in-furnace gas environment, as compared to the conventional air-firing, oxyfuel combustion has an impact on the combustion process as well as on the other processes such as heat transfer. Accordingly, the gas compositions and the different flame characteristics during fossil fuel combustion at oxyfuel conditions are expected to influence the emission behaviour and the associated formation mechanisms of SOx species [5]. Several studies have shown that the oxyfuel process using recycled flue gas results in a higher concentration of SO2 (ppm) than in conventional pulverised coal combustion but a lower emission rate of SO2 (mg/MJ) [6–10]. This is due to the higher conversion of SO2 to other sulphur species throughout the process. Higher concentrations of SOx can lead to increased potential corrosion problems from molten and solid ash in boiler deposits or from acid gas corrosion when operating heat exchangers below the acid dew point. It is to be emphasised that even though the SO2 concentration in the furnace is increased, the absolute amount of exhausted SO2 is much smaller than in conventional coal combustion. Therefore, the conversion ratio of fuel-S into exhausted-S can be much lower than that in conventional coal combustion. Al-Makhadmeh et al. [11] found that oil shale combustion with lower SO2 and NO emissions can be achieved using oxyfuel conditions. They reported that SO2 emissions during OF27 combustion at different excess oxygen concentrations are significantly lower (around 30%) than air-firing. Sulphur self-retention was also observed during OF27 which was proved by the analyses of S in the collected ashes at 2.5 m from the burner using a 20 kW vertical reactor. 2. Desulphurisation background SOx emissions are harmful and cause acid rain. Accordingly, legal restrictions are set in order to control emissions of sulphur pollutants from power plants. In general, these restrictions are met by means of corresponding management of the combustion process as well as by flue gas desulphurisation (FGD) [5]. Many processes have been developed for sulphur oxides removal including wet scrubbing, dry scrubbing, direct dry sorbent injection and re-generable processes. Among these technologies, sulphur removal in furnace is very competitive for controlling the SOx level derived from coal combustion, due to the low capital and operating costs. Sulphur oxides adsorbents may be coal calcium, which may be present as minerals such as calcite, calcium ion-exchanged into the coal matrix, or as calcium stones such as limestone and dolomite, which may be injected in the post flame region [12,13]. Desulphurisation process depends on a variety of individual factors, including the fuel properties (Ca/S molar ratio, particle size, sulphur content, etc.) and the combustion conditions (combustion temperature, residence time of flue gases in the temperature range relevant for desulphurisation, air stoichiometric ratio, mixing of the injected powder with the SO2 produced in the flame, etc.). The Ca/S molar ratio is one of the most important controlling factors in the dense tower desulphurisation process and it directly affects the chemical reaction between the desulphurisation reagent and SO2 [14]. In conventional air-firing, when the limestone is used as sorbent; the SO2 removal process takes place through two reaction steps; calcination of the sorbent to produce high surface area and high porosity CaO, and sulphation in the presence of O2 to form the higher molar volume solid product, CaSO4, according to the following reactions:

461

CaCO3 ! CaO þ CO2

ð1Þ

CaO þ SO2 þ 1=2O2 ! CaSO4

ð2Þ

It has been accepted that the whole sulphation reactions may be divided into two steps which cannot be separated. At first, before the continuous topo-chemical layer of CaSO4 encapsulates the whole particle surface, chemical reaction or pore diffusion is the reaction rate-limiting step. However, when the continuous CaSO4 product layer formed, i.e. the pores at the external layer of particle were plugged with solid product; the rate-limiting step will change to be the product layer diffusion [15]. Indirect sulphation reactions are significantly dependent on reaction temperature. Higher reaction temperature would produce larger pores, i.e. less specific surface area of lime and eventually lead to sintering. Generally, the optimum temperature for sulphation of lime is about 1123 K, above which the decomposition of gypsum would be preferred [16]. The main reason to account for the very low desulphurisation efficiency of in-furnace desulphurisation, in conventional pulverised coal combustion, is the decomposition of CaSO4 formed from a desulphurisation reaction owing to the high temperatures in pulverised coal combustion boilers. In contrast, in O2/CO2 combustion, the calcination is inhibited when the CO2 partial pressure is higher than the equilibrium pressure and the limestone is subjected to a direct sulphation reaction [17,18]:

CaCO3 þ SO2 þ 1=2O2 ! CaSO4 þ CO2

ð3Þ

Furnace temperature is an important parameter for direct sulphation as well. Liu et al. [19] found that initially the sulphation reactions run more efficiently with rising temperatures. However, CaSO4 starts to decompose at high temperatures, whereby SO2 is released. Increasing the temperature, the decomposition reactions reduce the efficiency of the flue gas desulphurisation, until the CaSO4 decomposition outbalances its formation. They reported that in O2/CO2 pulverised coal combustion, a high desulphurisation efficiency can be kept in a wider range of temperatures. At temperatures above 1450 K, the desulphurisation efficiency is zero for conventional pulverised coal combustion, but it maintains appreciably high for O2/CO2 pulverised coal combustion. Meanwhile, the peak temperature corresponding to highest system desulphurisation efficiency is also higher than that of conventional pulverised coal combustion, which is pronounced at higher sulphur content of coal. Moreover, the substantial increase in SO2 concentrations under oxyfuel conditions suppresses the decomposition of CaSO4 and shifts it to higher temperatures [19]. On the other hand, full conversion to calcium sulphate is not reached when limestone is used as sorbent in conventional combustion atmosphere, because CaO is sintered and pores are plugged. In pulverised coal systems, utilisation rate is usually below 20–25% [20]. For pulverised coal fired boilers, the furnace sorbent injection process (FSI) can yield an SO2 reduction of 30– 40% by injecting powdered sorbents into the upper furnace at Ca/S = 2. In a limestone injection multi-stage burner process (LIMB), limestone added to the periphery of a‘‘low NOx’’ burner typically reduces SO2 emissions by 30–50% when Ca/S = 2 [21]. Direct SOx removal by limestone addition or other calcium derivatives has shown a greater desulphurisation capacity in oxyfuel combustion [19,22–24]. Studies aimed at desulphurisation in O2/CO2 pulverised coal combustion are very limited and there is none for oil shale combustion. Most of these studies were carried out using a fluidised-bed system and thermogravimetric analysis (TGA). So, there exist many unknowns concerning in-furnace desulphurisation in O2/CO2 combustion such as mixing, namely the problem of injecting a sorbent in a huge volume (cross section) of an utility

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pulverised fuel boiler to make it effectively meet the sulphur compounds, which is solved by injection with the fuel. In a normal flame the temperatures may be higher and this method is questionable. Additionally, the reactions between sorbent and SO2 lie between two temperature windows; they are of low rates at low temperature, while at high temperature sintering occurs. Therefore, if the limestone is injected with some other method, the temperature field becomes critical during load changes. Therefore, in this study direct limestone injection has been investigated during Jordanian oil shale combustion under oxyfuel conditions using a 20 kW vertical reactor. The effect of firing media on desulphurisation efficiency was analysed. 3. Experimental

Table 2 Ash main and trace elements contents of El-Lajjun oil shale. Ash main elements (oxide form)

(wt.%)

Ash trace elements

(mg/kg)

Al2O3, BaO CaO, Fe2O3 K2O MgO Na2O Mn2O P2O5 SO3 SiO2 SrO TiO2

5.70 0.007 44.50 1.65 0.561 1.076 0.009 0.193 3.67 16.44 25.85 0.151 0.189

As Ba Cd Cr Cu Mn Mo Ni Hg Pb Sb Se Sr V Zn

11.5 ± 0.3 52.7 ± 1.5 49.5 ± 0.5 364 ± 1 73.5 ± 3 23.3 ± 2.8 235 ± 9 198 ± 3 n.b.(0.085±.016) 0.555 ± 0.181 10.2 ± 5.4 34.6 ± 1 824 ± 6 167 ± 4 599 ± 18

3.1. Materials Oil shale sample used in this study was obtained from El-Lajjun area in Jordan. The proximate and ultimate analyses, heating value, ash main and minor elements and size distribution were analysed and are shown in Tables 1 and 2. Limestone sample with 96 wt.% CaCO3 was ordered from Kalk-Laden & Kalk-Schule. The chemical composition of the sample was determined using RTG fluorescence and is listed in Table 3. The particle size distribution of limestone sample was measured by laser diffraction and shown in Table 4. EDX analysis and Scanning Electron Microscope (SEM) analyses were also carried out as shown in Figs. 1 and 2, respectively. The used limestone is very fine; 90 vol.% of the particles have a diameter equal to or less than 15.4 lm which is proved from Fig. 2. It composed mainly of Ca, C, O and Si (Table 3 and Fig. 1). 3.2. Experimental parameters and approach El-Lajjun oil shale combustion experiments were performed in a 20 kW vertical furnace, the configuration of the furnace had been described previously [11]. Unstaged combustion experiments were performed under air and oxyfuel conditions. All the combustion experiments were performed at a wall temperature of 1200 °C. The input parameters for El-Lajjun oil shale combustion under unstaged air and oxyfuel firings conditions are listed in Table 5.

Main elements (Oxide form)

Trace elements

(mg/kg)

TOC (wt%) TIC (wt%) CO2 (wt%) Al (mg/kg) Ba (mg/kg) Ca (g/kg) Fe (mg/kg) K (mg/kg) Mg (mg/kg) Na (mg/kg) Mn (mg/kg) P (mg/kg) S (mg/kg) Si (g/kg) Sr (mg/kg) Ti (mg/kg)

As Cd Co Cr Cu Hg Mo Ni Pb Sb Se Sr Tl V Zn

18.3 <0.5 4.08 42.5 39.7 <0.4 3.9 18.1 33.9 24.7 20.6 402 8.43 29.5 22.7

0.062 10.98 40.3 3920 26.1 395.1 235 276 1490 60.4 191 115 94.1 16.6 34.5 16.5

Table 4 Limestone particle size distribution. Size distribution D10 (lm) D50 (lm) D90 (lm)

Table 1 Proximate, ultimate analyses and size distribution of El-Lajjun oil shale.

a

Table 3 Main and trace elements contents of limestone.

Proximate analysis Water (ar, %)a Ash (wf, %)b Volatile (waf, %)c Fixed carbon (waf, %)d

1.08 54.20 99.06 0.85

Ultimate analysis C (waf, %) H (waf, %) N (waf, %) S (waf, %) O (diff, %)d LHV (waf, kJ/kg) e

55.68 4.27 0.87 8.30 30.88 19585.15

Size distribution D10 (lm)f D50 (lm) D90 (lm)

1.25 9.86 35.3

As received. b Water free. c Water ash free. d By difference. e Low Heating Value. f Di: represent that i% by volume of the particles that have a diameter equal to or less than Di.

0.988 5.37 15.40

One should keep in mind that this investigation was carried out in a once- through furnace; i.e. single pass (no recycle was used). It was not possible to simulate the actual oxyfuel condition by SO2 injection due to the analysers limitation at the high measured SO2 concentration range. So, the SO2 concentration in the furnace during OF27 and OF35 combustion does not represent the SO2 concentration of an oxyfuel plant with flue gas recirculation, as there was no accumulation of SO2 in the furnace. For emission calculations refer to [11]. SO2 conversion was calculated using the following formula:

SO2 conversion;% ¼

ySO2;meas;d  100 ySO2;th;d

ð4Þ

where ySO2, meas,d is measured volumetric concentration of SO2 in dry flue gas, ppmv and ySO2,th,d is the theoretical maximum possible volumetric concentration of SO2 in dry flue gas if all of fuel-S is converted to SO2, ppmv.

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463

Fig. 1. EDX pattern of limestone.

Fig. 2. SEM of limestone.

and;

Table 5 Input parameters during El-Lajjun oil shale unstaged combustion.

Gas feeding rate

Carrier gas (m3/h at stp. c) Primary gas (m3/h at stp.) Secondary gas (m3/h at stp.)

Oil shale feeding rate (kg/h) Overall oxygen ratio, k O2 excess (vol%) Wall temperature (°C) a b c

Air

OF27a

OF35b

1.5

1.5

1.5

2.04

2.04

2.04

3.06

3.06

3.06

2.21 1.19 3 1200

2.93 1.15 3 1200

3.89 1.12 3 1200

ðCa=SÞ ¼

The theoretical maximum SO2 concentration in the furnace can be approximated by considering combustion in pure oxygen [25] and using the following expression:

V SO2;max;d V G;d

MSO2 cS M S qn;SO2

ð6Þ

where MSO2 is molar mass of SO2, kg/kmol, MS is the molar mass of S, kg/kmol, cS is the fuel mass fraction of S and qn,SO2 is density of SO2 at STP conditions, kg/m3. It is of particular interest to compare the sulphur dioxide emissions and the desulphurisation efficiency based on the system Ca/S molar ratio. The molar ratio of Ca/S is calculated according to the following equation:

OF27: 27% O2/73% CO2 environment. OF35: 35% O2/65% CO2 environment. STP: Standard Temperature and Pressure.

ySO2;th;d ¼

V SO2;max;d ¼

ð5Þ

where VSO2,max,d is maximum possible volume of SO2 related to dry flue gas mass if all of fuel-S is converted to SO2, m3/kg; VG,d is dry flue gas volume related to fuel mass at STP during combustion, m3/kg.

ðCa in sorbentÞ ðfuel  SÞ

ð7Þ

Limestone was mixed with El-Lajjun oil shale at three different Ca/S molar ratios (1, 2 and 3), the mixtures were used during unstaged combustion at 1200 °C. The molar Ca/S ratio of El-Lajjun oil shale is 5.21. To quantitatively compare the desulphurisation processes under different conditions, the definition of desulphurisation efficiency is introduced here as:



gSO2 ¼ 1 

 SO2ðCa=S¼nÞ  100% SO2ðuns;Ca=S¼0Þ

ð8Þ

where SO2(Ca/S=n) is SO2 emission from unstaged combustion of El-Lajjun oil shale with limestone at different molar ratio of calcium

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to sulphur Ca/S, mg/MJ and SO2(uns,Ca/S=0) is SO2 emission from unstaged combustion of El-Lajjun oil shale without limestone addition, mg/MJ. 4. Results and discussion 4.1. SO2 emissions and conversions without limestone addition (Ca/ S=0) The average flue gas concentrations at the end of the reactor for the three investigated cases, air-firing, OF27 and OF35 combustion are calculated and listed in Table 6. The SO2 emissions are very high (17,010–25,550 mg/m3stp), because El-Lajjun oil shale contains high S content (8.3% waf). The SO2 concentration is higher during oxyfuel combustion cases than air-firing, since the oil shale feeding rate is higher (see Table 5). The oil shale-S conversion rates to SO2 are found to be 61%, 49%, and 58% for air-firing, OF27, and OF35 combustion, respectively; the conversion rates to SO2 can be described as moderately low. This can be explained by the sulphur self-retention, an interesting fact that oil shale contains high concentration of calcite CaCO3. Calcite decomposes producing free lime which then reacts with SO2 forming CaSO4. The fly ash which is formed during combustion constitutes of certain elements such as Si, Al, Fe, Na and Ca depending primarily on the mineral matter composition of the respective fuel. For self-retention of sulphur oxides present in the flue gas, the alkaline earth oxides are of major importance with CaO being the governing component [26–28]. El-Lajjun oil shale combustion under oxyfuel conditions (OF27 and OF35) results in a lower SO2 conversion compared to air-firing and this is proved by the S content in the collected ashes at the three investigated combustion cases, (see Table 6). Sulphur content in the collected ashes are 2.76%, 3.3% and 3.01% for air-firing, OF27, and OF35 combustion, respectively. The SO3 content in the collected ashes also proves that as will be discussed later in Section 4.3. Higher sulphur self-retention during OF27 and OF35 can be explained by that direct sulphation of CaCO3 is favoured under such conditions and calcite decomposition is inhibited due to the high CO2 concentration. In addition, higher concentration of SO2 in oxyfuel combustion stabilises the formed CaSO4. Al-Makhadmeh et al. [11,29] have proved higher sulphur self-retention during oxyfuel combustion than air-firing using the same El-Lajjun oil shale. Croiset and Thambimuthu [7] reported that the type of environment (air, O2/CO2 and recycle) has some impact on the conversion of fuel-S to SO2. They obtained a fuel-sulphur conversion ratio of 91% during air-firing of hard coal. For combustion in single pass O2/CO2 atmosphere, the conversion rate drops to 73–79% and for oxyfuel combustion with flue gas recirculation the conversion ratio is 56–66%. Fleig et al. [30,31] found that the oxyfuel combustion produced less sulphur in the flue gas (on a mass emitted basis) compared to the air-fired case. They reported a fuel-sulphur to SO2 conversion of 67% for air-firing and between 41 and 46% for oxyfuel combustion at a 100 kWth pilot plant with lignite as a fuel without Ca addition. Table 6 Measured and averaged flue gas concentrations at the end of the reactor for air-firing, OF27 and OF35 combustion without limestone addition.

O2 (vol%, dry) CO2 (vol%, dry) CO (mg/m3stp dry) NO (mg/m3stp dry) SO2 (mg/m3stp dry) SO2 conversion (%) S in ash (wt%)

Air

OF27

OF35

3.04 15.72 10.6 13687 17010 61 2.76

2.85 96.1 36.6 782.68 17214 49 3.30

2.91 94.8 29.6 1068 25550 58 3.01

SO2 emissions and conversions for different fuels were reviewed and summarised in Table 7 to provide a better insight on the SO2 emission levels during El-Lajjun oil shale combustion. From the current study and also the previous investigation [11], it is clear that SO2 emissions from oil shale combustion in both modes are significantly higher compared to the other cases even oxyfuel combustion tests with recycle flue gases (dry/wet); the S-content of oil shale is significantly higher. On the other hand, oil shale-S conversions to SO2 in both combustion modes are lower compared to the other investigations during air-firing and oxyfuel combustion without flue gas recirculation. 4.2. Desulphurisation tests with limestone addition at various Ca/S ratios To solve the problem of the very high SO2 emissions during oil shale combustion, direct sorbent injection was investigated in this work, limestone was used as sorbent. In this section, the efficiency of desulphurisation during unstaged El-Lajjun oil shale air-firing and oxyfuel (OF27 and OF35) combustion is studied. The desulphurisation efficiency of O2/CO2 combustion and that of conventional combustion is compared based on the same Ca/S ratio. Three molar ratios of Ca/S were used, namely 1, 2 and 3. The SO2 emissions over the three ratios used for the three investigated conditions (air-firing, OF27 and OF35) are shown in Fig. 3. It arises from the presented results that as the Ca/S molar ratio increases the SO2 emission decreases significantly (Ca/S = 0 means no limestone addition with oil shale). During OF27 combustion, SO2 emission reduced from 5136 mg/MJ to 2645 mg/MJ when limestone is added with molar ratio of 1. As the Ca/S molar ratio is increased to 3, SO2 emission is reduced to 16 mg/MJ. Similarly for air-firing, when the Ca/S molar ratio is increased to 3, the SO2 emission is reduced to 297 mg/MJ. For OF35 combustion, at Ca/S molar ratio of 3, the SO2 emission reduced to 419 mg/MJ. The significant reduction in SO2 emissions by limestone addition in both combustion modes (air-firing and OF combustion) can be explained by the high SO2 concentration for this high-S oil shale. CaSO4 decomposition is inhibited owing to the high SO2 concentration inside the furnace [19]. The particle size of the limestone has an important role as well for the significant sulphation found, since smaller particle size means higher surface area, better mixing with the fuel and then with the produced SO2 gas. Under conditions of fine sorbent particles and short residence times, such as in furnace-sorbent injection, the pore size distribution model can explain most of the factors affecting the reaction. The model indicates the dominant factors to be pore size distribution, particle size and the partial pressure of SO2 [34]. Therefore, efficient in-furnace desulphurisation for oil shale air-firing and under oxyfuel conditions is obtained. Chen et al. [20] reported that direct sulphation of limestone was only pronounced when an extra SO2 was introduced into flue gas and excess Ca was present in flue gas; the doping of extra SO2 through the recirculation of flue gas mainly increased the sulphation of limestone and concurrently reduced the unreacted limestone fraction through direct sulphation. Fig. 4 shows the desulphurisation efficiency at different Ca/S molar ratios. An increase in the Ca/S molar ratio results in an increase in the desulphurisation efficiency for all investigated combustion cases. At Ca/S molar ratio of 2, the desulphurisation efficiencies are 80%, 88% and 82% for air-firing, OF27, and OF35 combustion, respectively. The highest desulphurisation efficiencies are obtained for OF27 combustion over the investigated ratios, the maximum efficiency is almost 100% with Ca/S molar ratio of 3. On the other hand, SO2 emissions and desulphurisation efficiencies obtained under air-firing and OF35 combustion are quite similar in spite of major differences in the O2 and CO2 levels. This is due to the nature of the CO2 enriched environment of combustion; it

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L. Al-Makhadmeh et al. / Fuel 158 (2015) 460–470 Table 7 SO2 emission and conversion for different fuels. Reference

Fuel

Conditions

Current work

El-Lajjun oil shale Sdaf = 8.3% Cdaf = 55.68% Vdaf = 99.06%

20 kW vertical reactor 1200 °C

[11]

El-Lajjun oil shale Sdaf = 9.03% Cdaf = 54.33% Vdaf = 99.95%

[25] Klein kopje coal Sdaf = 0.72% Cdaf = 83.93% Vdaf = 27.76% Lausitz brown coal Sdaf = 0.85% Cdaf = 66.78% Vdaf = 57.36% Rhenish coal Sdaf = 0.3% Cdaf = 67.58% Vdaf = 54.18% [7]

[5]

[16]

US eastern bituminous coal S = 0.96% C = 77.65% V=%

SO2 emission (ppmv)

S conversion (%)

5948 6019 8934

61 49 58

6999 5186

72 42

Air-firing OF27

416 646

89 86

Air-firing OF27

653 936

94 94

Air-firing OF27

295 315

98 92

598

91

pure pure

749 916

79 73

pure pure

1766 1649

65 61

pure pure

1509 1736

56 64

Air-firing OF27 OF35 20 kW vertical reactor 1200 °C Air-firing OF27 20 kW vertical reactor 1300 °C

0.21 MW Air-firing O2/CO2 O2 = 28%, 100% O2 = 35%, 100% Dry recycle O2 = 28%, 100% O2 = 35%, 100% Wet Recycle O2 = 28%, 100% O2 = 35%, 100%

Lignite S = 1.69% C = 49.7% V = 41.65%

500 kW furnace Air-firing

1685

OF35 with recycle

5048

DT coal Sdaf = 1.7% Cdaf = 78.6% V = 24.7%

DTF T = 1473 K Air-firing

280

Air-firing

864

YZHS coal Sdaf = 5.7% Cdaf = 76.4% V = 38.8% [32,33] USA coal S = 1.3% C = 69.8% V = 31.4% UK coal (Daw mill) S = 1.9% C = 72.7% V = 29.5%

80 kW down-fired PF furnace Burner oxygen ratio = 1.15, t = 2 s, T = 1100–1200 °C Air-firing 915 Air-firing

is evident that combustion in an enriched CO2 environment is delayed [4,7,25]. CO2 has different properties from N2 which influence both heat transfer and combustion reaction kinetics: the CO2 gas is denser than N2 and the CO2–H2O mixture has a higher specific heat capacity and different radiation and absorption characteristics. The resulting adiabatic combustion temperatures in O2/RFG (recycle flue gas) mixtures are much lower than those in air-firing at comparable oxygen concentrations. Therefore, it is necessary to select a flue gas recycle ratio that yields a mixture in the oxidant that provides combustion and heat transfer characteristics similar

1509

to air-firing [4]. Many literatures reported that the optimum oxygen concentration in O2/CO2 mixtures that gives a similar flame temperature, heat transfer, and emission formation rates is in the range of 25–35 vol.% [7,25]. Higher desulphurisation efficiency during OF27 compared to air-firing is explained by the direct sulphation; this is a direct reflection of the high CO2 partial pressure which inhibited the right shift of reaction (1). On the other hand, our experimental conditions were checked using the equilibrium CaCO3–CaO data [35]; calcination of limestone still occurs during OF27 combustion due

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6279 5136

5000

Ca/S=0

3535

3526

Ca/S=1

2645

3000 2000

Ca/S=2

1238

1000

Ca/S=3

1111 598 16

297

0

air

OF27

OF35

Desulphurisation efficiency, %

OF27 OF35

1

1.5

2

2.5

3

3.5

Ca/S molar ratio Fig. 4. The influence of the molar Ca/S ratio on SO2 desulphurisation efficiency during unstaged air-firing, OF27 and OF35 combustion.

to the high temperature applied in this investigation. Calcination, sintering and sulphation behaviour under high CO2 concentration is different from that in conventional combustion atmosphere, which is possibly an important reason for the high desulphurization efficiency in oxyfuel combustion [36]. Lee et al. [37] found that sorbent particles (CaCO3) rapidly calcined and sintered in the air atmosphere, whereas, calcination was delayed in the CO2 atmosphere due to the higher CO2 partial pressure. Instead, the sintering effect was dominant in the CO2 atmosphere early in the reaction. Their experiments were conducted at varying temperatures,

Table 8 Proximate analysis and S content of ash during unstaged combustion.

Air, Air, Air, Air,

a b c d e

Ca/S = 0 Ca/S = 1 Ca/S = 2 Ca/S = 3

M (wt%)a

V (wt%)b

A (wt%)c

FC (wt%)d

S (wt%)e

<0.1 <0.1 <0.1 <0.1

0.3 <0.1 0.37 0.23

99.6 99.8 99.5 99.7

<0.01 0.18 0.10 0.10

2.76 4.61 5.16 4.51

OF27, OF27, OF27, OF27,

Ca/S = 0 Ca/S = 1 Ca/S = 2 Ca/S = 3

<0.1 <0.1 <0.1 <0.1

0.18 0.37 0.62 0.60

99.7 99.4 99.3 99.4

0.12 0.19 0.1 <0.1

3.30 4.81 5.39 4.80

OF35, OF35, OF35, OF35,

Ca/S = 0 Ca/S = 1 Ca/S = 2 Ca/S = 3

<0.1 <0.1 <0.1 <0.1

0.87 0.17 0.47 0.68

99.1 99.6 99.4 99.2

<0.01 0.20 0.15 <0.1

3.01 4.95 4.82 5.02

M: Moisture. V: Volatiles. A: Ash. FC: Fixed Carbon. S: Sulphur.

471 419

398

400 300 200 100 0

air

0.5

500

419

Fig. 3. The influence of the Ca/S molar ratio on SO2 emissions during unstaged airfiring, OF27 and OF35 combustion.

100 90 80 70 60 50 40 30 20 10 0

NO emission, mg/MJ

SO2 emission mg/MJ

6000

4000

600

6269

air

OF27

OF35

Fig. 5. NO emission during unstaged air-firing, OF27 and OF35 combustion with limestone addition.

residence times, and atmospheric conditions in a drop tube furnace. Direct sulphation enables higher degrees of desulphurisation than those observed from CaO–SO2 sulphation, because the counter-diffusion of the CO2 generated during the direct sulphation reaction forms a porous product layer that offers less diffusional resistance than the essentially nonporous layer formed during the CaO–SO2 sulphation reaction [20,24,38]. Liu et al. [19] identified that the system desulphurisation efficiency in O2/CO2 pulverised coal combustion is increased to about four to six times as high as that of conventional pulverised coal combustion. García-Labiano et al. [12] tested the SO2 retention capacity of two different limestone samples by thermogravimetric analysis at typical oxyfuel conditions in fluidised bed combustors. They observed a clear difference in the sulphation conversion (the ratio of CaSO4 moles in the sample to the total number of moles of the calcium species CaSO4, CaCO3 and CaO) reached by the sorbent, whether the sulphation takes place under indirect or direct sulphation; it is higher under indirect sulphation. However, in spite of this difference, for a given condition and temperature, the CO2 concentration did not affect the sulphation conversion, being its major effect to delay the CaCO3 decomposition to a higher temperature. The maximum sulphation conversion increased with decreasing the particle size and increasing the SO2 concentration. From the chemical compositions of the collected samples at 2.5 m from the burner, shown in Table 8, the collected samples with and without limestone additions contain 99.1–99.8% ash. The burnout in all cases was in the range 99–99.8% (for calculation see [11]) which means that the combustion efficiency was not affected by limestone addition. Furthermore, limestone addition did not affect flame stability. A small increase in ash fixed carbon content is found with limestone addition, a small fraction of volatiles is found as well.

S retention, %

7000

100 90 80 70 60 50 40 30 20 10 0

air

OF27

OF35

4

6

y = 6.775x R² = 1

0

2

8

γs, ash (dry) Fig. 6. Sulphur retention in the ashes during air-firing and oxyfuel conditions.

L. Al-Makhadmeh et al. / Fuel 158 (2015) 460–470

It can be concluded that with direct sorbent injection using limestone, high desulphurisation efficiency during unstaged oil shale combustion is obtained. Meanwhile, high NO emissions during unstaged combustion are found as shown in Fig. 5. The presented values are the average during the injection of limestone (for the three molar ratios of Ca/S). The NO emission is lower during oxyfuel combustion compared to air-firing. In a trial to optimise oil shale combustion with low SO2 and NO emissions direct sorbent injection during staged oil shale air-firing and under oxyfuel conditions is handled, detailed analysis will be given in a future publication. 4.3. Characterisation of the collected ash samples at the end of the reactor Ash samples were collected at the end of the reactor (2.5 m) via an oil-cooled sampling probe. The ash was separated from the flue gas at the end of the oil-cooled probe by means of a filter and then immediately collected in glass container for lab analysis. The proximate analysis and S content of some collected ash samples are reported in Table 8. It was found that S content increases in the ashes when limestone addition was performed. Generally, the S content in the collected ashes during combustion with limestone addition increases as the molar Ca/S ratio increases in both combustion modes proving the sulphation results from the flue gas concentration analysis. Higher S content in the ash is found after oxyfuel combustion compared to air-firing. The sulphur retention during combustion of oil shale under air-firing and oxyfuel conditions is calculated and shown in Fig. 6. The sulphur retention is the fraction of oil shale-S retained in the ashes and is calculated as:

Sretention ¼

ðcS;ash  cash;oilshale Þ

cs;oilshale

ð9Þ

where cS, ash is the mass fraction of sulphur in the ash, cash, oilshale is the mass fraction of ash related to the oil shale and cS, oilshale is the mass fraction of sulphur in the oil shale. The S content in the ash and S retention holds a linear relationship for the three investigated cases, and the highest retention is obtained for the case OF27 combustion. If the amount of SO3, H2S and the absorption of sulphur compounds in condensed water is

467

negligible, the retention of sulphur in ash can be calculated by the conversion of oil shale-S to SO2: Retention = 100% – Conversion. By comparing the S retention in ashes and the values calculated from S conversion using Table 6 (without limestone addition), there is a difference of almost 20% for air-firing and 20–22% for oxyfuel cases; this suggests that there are significant portions of SO3, H2S and other sulphur compounds in the flue gases since higher SO2 concentrations support higher levels of other sulphur species. Kiga et al. [6] showed that O2/CO2 conditions with recycled flue gas enhanced sulphur deposition throughout the combustion process leading to an unaccounted 14–30% of the sulphur balance. Stanger and Wall [39] reported that the oxyfuel combustion produced less sulphur in the flue gas (on a mass emitted basis) compared to the air-firing resulting in higher levels of sulphur in the ash and producing a higher amount of unaccounted sulphur (mass by difference), suggesting that a higher SO3 deposition rate occurs along the cooler transport lines. The conversion of SO2 to SO3 occurs in homogenous and heterogeneous phases. SO3 formation is affected by many factors including SO2 and O2 partial pressures, temperature, residence time, and the content of catalytically active compounds in the ash (e.g. iron and vanadium) [14,39–40]. Further investigations are required to have a realistic information about SO3 concentration in flue gas during the combustion of such high S-oil shale. Fig. 7 below shows the XRD diffraction pattern of raw oil shale, air-firing fly ash and OF27 fly ash without limestone addition. The minerals present in oil shale are calcite and quartz as major phase; fluoroapatite, kaolinite are also present. The ash samples were found to contain anhydrite (CaSO4), CaO, Quartz, several calcium silicate phases (such as larnite – Ca2SiO4 and wollastonite and/or Pseudo-wollastonite – CaSiO3), calcium aluminum silicates (like Gehlinte –Ca2Al2SiO7), calcium phosphate, and hematite. The qualitative XRD analysis showed that, OF27 combustion did not significantly change the main crystalline phases formed in residue ashes compared to air-firing, implying no significant impacts on ash formation behaviours of main oil shale minerals. Lime and the other calcium products were the products of calcite transformations and reactions. Anhydrite was formed from the reaction between lime and SO2 in the presence of oxygen. Lime existence in the analysed ashes is not significant compared to anhydrite indicating the self-sulphur retention in both air-firing and OF27 combustion.

Fig. 7. XRD analysis of El-Lajjun oil shale, air-firing fly ash and OF27 fly ash collected at 2.5 m from the burner without limestone addition.

468

L. Al-Makhadmeh et al. / Fuel 158 (2015) 460–470

Table 9 Major ash contents during unstaged combustion (wt.%).

Air, Ca/S = 0 Air, Ca/S = 1 Air, Ca/S = 2 Air, Ca/S = 3 OF27, Ca/S = 0 OF27, Ca/S = 1 OF27, Ca/S = 2 OF27, Ca/S = 3 OF35, Ca/S = 0 OF35, Ca/S = 1 OF35, Ca/S = 2 OF35, Ca/S = 3 a

Al2O3a

BaO

CaO

Fe2O3

K2O

MgO

MnO2

Na2O

P2O5

SO3

SiO2

SrO2

TiO2

5.32 4.22 3.62 4.16 5.01 4.16 3.69 4.12 5.06 4.19 3.72 3.77

0.015 0.007 0.012 0.005 0.014 0.008 0.011 0.005 0.013 0.007 0.011 0.005

46.4 50.3 50.8 61.1 45.9 49.7 52.3 60.9 46.5 49.5 51.6 59.1

2.21 1.99 1.47 1.12 2.09 1.92 1.49 1.22 2.15 1.91 1.53 1.11

0.89 0.773 0.69 0.673 0.851 0.746 0.719 0.66 0.827 0.834 0.703 0.677

1.04 0.861 0.802 1.07 1.0 0.847 0.804 1.06 1.0 0.875 0.802 1.07

0.007 0.008 0.007 0.014 0.007 0.007 0.007 0.014 0.007 0.008 0.007 0.014

0.208 0.182 0.158 0.155 0.19 0.176 0.159 0.144 0.196 0.191 0.173 0.160

4.02 3 2.86 3.11 4.08 3.02 2.87 2.97 4.08 2.98 2.95 2.86

6.46 10.0 13.0 13.7 7.8 10.6 13.8 13.5 6.96 10.5 13.5 14.2

33.5 28.1 27.4 15.3 33.3 27.3 24.3 15.1 32.9 28.1 24.7 17.0

0.174 0.146 0.126 0.094 0.173 0.143 0.124 0.09 0.173 0.144 0.126 0.089

0.272 0.306 0.206 0.190 0.262 0.276 0.204 0.185 0.261 0.304 0.203 0.178

Unit: wt.%.

Table 10 Minor ash contents during unstaged combustion (mg/kg).

Air, Ca/S = 0 air, Ca/S = 1 air, Ca/S = 2 air, Ca/S = 3 OF27, Ca/S = 0 OF27, Ca/S = 1 OF27, Ca/S = 2 OF27, Ca/S = 3 OF35, Ca/S = 0 OF35, Ca/S = 1 OF35, Ca/S = 2 OF35, Ca/S = 3 a

Asa

Be

Cd

Co

Cr

Cu

Hg

Mo

Ni

Pb

Sb

Se

Sr

Ti

V

Zn

14.0 10.2 8.3 9.33 1.8 13.2 16.9 14.9 20.8 33.1 16.7 20.0

<1.0 <1.0 <1.0 4.32 <1.0 <1.0 <1.0 4.37 <1.0 <1.0 <1.0 4.59

13.5 2.65 <1 12.4 10.2 <1 3.2 27.3 11.4 <1 3.54 22.2

5.64 5.19 3.92 6.81 4.88 3.11 3.36 6.09 4.61 4.51 5.87 14.3

850 765 603 391 793 696 572 325 776 759 581 354

225 174 138 121 219 172 135 119 181 177 179 121

0.22 <0.05 0.11 <0.05 0.11 0.063 0.14 <0.05 0.16 <0.05 0.10 <0.05

245 219 177 180 218 200 167 155 221 225 173 171

302 256 219 206 275 248 216 239 284 261 219 211

23.2 16.5 21.4 1.80 30.9 19.4 26.1 12.5 25.8 21.9 20.8 21

13.0 27.4 25.8 14.7 31.4 25.9 24.1 11.0 7.6 29.7 25.0 6.41

27.2 36.4 32.5 31.4 16 24 23.1 42.6 16.3 40 28.1 39.7

1440 1270 1240 940 1430 1260 1260 900 1730 1260 1260 890

16.7 13.8 16.6 4.61 22.3 1.28 12.5 8.81 26.6 16.7 22.3 26.0

271 195 151 128 238 205 197 130 239 169 171 133

1030 763 710 632 943 751 737 634 946 810 722 680

Unit: mg/kg.

Nevertheless, it was observed there are some differences in the relative intensities of the XRD peaks for some phases formed in both combustion modes. Anhydrite peaks are more intense in OF27 ash, while CaO peaks are more intense in air-firing ash, which is consistent with the above findings regarding sulphur retention. Our findings are in agreement with Sheng and Li [41], who studied the mineral transformations and ash formation during O2/CO2 combustion of pulverised coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions of O2/N2 and O2/CO2, the temperatures were 1200 and 1400 °C. They reported that the main crystalline phases for coal A ashes are mainly mullite and quartz. For Coal B ashes, it was found that the main mineral phases are mullite, quartz, lime, and calcium hydroxide. Besides mullite, quartz, lime, and calcium

hydroxide, the crystalline phases identified included anhydrite, anorthite, hematite, and magnetite were found in the ashes produced from coal C and Coal D. It was found that, O2/CO2 combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in the two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviours of included minerals. The high combustion temperature (1200 °C) enabled the formation of several cement phases including larnite, wollastonites and gehlinite. Off course such mineral combustion of ash enables the use of such material in cement industry, however, the main

SO3

1.4

1.4

1.2

1.2

1.2

1

1

1

0.8 0.6

Efactor

1.4

Efactor

Efactor

CaO

0.8 0.6

0.8 0.6

0.4

0.4

0.4

0.2

0.2

0.2

0

0

Ca/S=0

Ca/S=1

Ca/S=2

Air-firing

Ca/S=3

0 Ca/S=0

Ca/S=1

Ca/S=2

OF27 combuson

Ca/S=3

Ca/S=0

Ca/S=1

Ca/S=2

Ca/S=3

OF35 combuson

Fig. 8. Efactor for Ca and S in the collected ashes at 2.5 m during air-firing, OF27 combustion and OF35 combustion at different Ca/S molar ratios.

L. Al-Makhadmeh et al. / Fuel 158 (2015) 460–470

limiting factor of its utilisation is the presence of such phases as calcium phosphate. Nevertheless, proper dilution of this ash would make its utilisation in cement industry feasible. The major and minor ash contents during unstaged combustion are shown in Tables 9 and 10, respectively. There are no major differences between the ashes produced under air-firing and oxyfuel combustion except the S content. The main constituents of oil shale ashes are CaO and SiO2 as major oxides, and Al2O3, P2O5, Fe2O3, MgO, Na2O, K2O, and TiO2 as minor once. The sulphur trioxide content in the ash represents the fixed sulphur in the form of Ca–S compounds. Again SO2 capturing is proved and it is greater during OF27 and OF35 combustion compared to air-firing. The enrichment factor Efactor is used to compare the relative enrichment of Ca and S in the collected ash samples relative to its concentration in the oil shale ash. The enrichment factor Efactor is defined as:

Efactor ¼

X F;i X ash;i

ð10Þ

where XF,i is the mass fraction of the element Ca and S (expressed as oxide) in the filter ash (collected at 2.5 m) and Xash,i is the mass fraction of the element Ca and S (expressed as oxide) in the initial ash of the oil shale prior to combustion. Fig. 8 shows the Efactor at the three investigated combustion modes with and without limestone addition. All the collected ash samples in both combustion modes are enriched with CaO and depleted in SO3. The enrichment of CaO is due to the addition of limestone, S is oxidised to SO2 and due to the addition of limestone SO2 is captured. The Efactor for SO3 increases with increasing the molar Ca/S ratio indicating higher desulphurisation efficiency during the three investigated combustion cases, these results are also in agreement with the results obtained from the flue gas concentration analysis. From the minor ash contents of the collected ashes Sr, Zn and Cr represent the main constituents, with low contents of Cu, Mo, Ni and V and trace amounts of As, Cd, Pb, Sb, Ti, Co, Hg and As. Sheng et al. [42] studied ash particle formation during O2/CO2 combustion of pulverised coal. They found that O2/CO2 combustion did not significantly affect the size distribution of the residue ash particles but significantly affected the mass and composition size distributions of both the submicron particles and the fine fragmentation particles. Nevertheless, O2/CO2 combustion did not change the formation mechanisms of the submicron particles and the fine fragmentation particles.

5. Conclusions Sulphur emission and conversion during El-Lajjun oil shale combustion in air and oxyfuel conditions are studied. Desulphurisation by direct limestone injection mechanism during air-firing and oxyfuel combustion (27% O2/73% CO2 and 35% O2/65% CO2) in a 20 kW vertical furnace have been investigated as well. Three molar ratios of limestone were used during unstaged air-firing and oxyfuel combustion (Ca/S = 1, 2 and 3). The major conclusions drawn are as follows: The SO2 emissions without limestone addition are very high (17,010–25,550 mg/m3stp), this is due to the fact that El-Lajjun oil shale contains high S content (8.3% waf). The SO2 concentration is higher during oxyfuel combustion cases than air-firing, since the oil shale feeding rate is higher. The oil shale- S conversion rates to SO2 are found to be 61%, 49%, and 58% for air-firing, OF27, and OF35 combustion, respectively; the conversion rates to SO2 can be described as moderately low. Sulphur self-retention is the reason, which was proved by the analyses of S in the collected ashes at 2.5 m from the burner and from the major ash content analysis.

469

Significant reduction in SO2 emissions is obtained by limestone addition in both combustion modes. The molar Ca/S ratio is a very important parameter; the desulphurisation efficiency increases with Ca/S molar ratio for both air-firing and oxyfuel combustion. The desulphurisation efficiencies with Ca/S molar ratio of 3 are 95%, 100% and 93% for air-firing, OF27, and OF35 combustion, respectively. The high SO2 concentration in the furnace accounts for the higher system desulphurisation efficiency because a high SO2 concentration inside the furnace facilitates the desulphurisation reaction and inhibits CaSO4 decomposition. During oxyfuel combustion the high concentration of CO2 causes different behaviour of calcination, sintering and sulphation compared to air-firing resulting in higher desulphurisation efficiency. There are no major differences between the ashes produced under air-firing and oxyfuel combustion except in the S content. Anhydrite peaks are more intense in OF27 ash, while CaO peaks are more intense in air-firing ash during combustion without limestone addition. Acknowledgments The authors wish to acknowledge the financial support of the Deutsche Forschungsgemeinschaft (DFG). The authors are grateful to Mattias Pagano, Marta Escoto de Tejada, Mario Krautz, and Wolfgang Ross from Institute of Combustion and Power Plant Technology-Stuttgart University for their help in performing the experimental program and lab analysis. Thanks also go to Jordanian Natural Resources Authority for facilitating collection of oil shale samples from El-Lajjun area in Jordan. References [1] Torres-Ordonez R, Calkins W, Klein M. Geochemistry of sulfur in fossil fuels. Washington DC: American Chemical Society; 1990 [chapter 17]. [2] Guffey F, Barbour F, Cummings R. Analysis of oil shale retort production stream gases for sulfur species. Liquid Fuels Technol 1983;1(4):235–57. [3] Lin W. Interactions Between SO2 and NOx Emissions in Fluidiesed Bed Combustion of Coal. 1994, Delft University of Technology: The Netherlands, p. 9–30. [4] Scheffknecht G, Al-Makhadmeh L, Schnell U, Maier J. Oxy-fuel coal combustion—a review of the current state-of-the-art. Int J Greenhouse Gas Control 2011;5:S16–35. [5] Müller M, Schnell U, Scheffknecht G. Modelling the fate of sulphur during pulverized coal combustion under conventional and oxy-fuel conditions. Energy Procedia 2013;37:1377–88. [6] Kiga T, Takano S, Kimura N, Omata K, Okawa M, Mori T, et al. Characteristics of pulverized-coal combustion in the system of oxygen/recycled flue gas combustion. Energy Convers Manage 1997;38:S129–34. [7] Croiset E, Thambimuthu KV. NOx and SO2 emissions from O2/CO2 recycle coal combustion. Fuel 2001;80(14):2117–21. [8] Yamada T. Pilot scale experiments giving direct comparison between air and oxy firing of coals and implication for large scale plan design. In: 2nd Workshop of the IEA international oxy-combustion research network. Hilton Garden Inn, Windsor, USA, 2007. [9] Weller A, Rising B, Boiarski A, Nordstrom R, Barrett R, Luce R. Experimental evaluation of firing pulverized coal in a CO2/O2 atmosphere, in Argonne National Laboratory Report: ANL/CNSV-TM-168, 1985. p. 342. [10] Grathwohl S, Lemp O, Schnell U, Maier J, Scheffknecht G, Kluger F. Highly flexible burner concept for oxyfuel combustion in IEA 1st Oxy-fuel combustion conference. Radisson Hotel, Cottbus, Germany, 2009. [11] Al-Makhadmeh L, Maier J, Al-Harahsheh M, Scheffknecht G. Oxy-fuel technology: an experimental investigations into oil shale combustion under oxy-fuel conditions. Fuel 2013;103:421–9. [12] García-Labiano F, Rufas A, de Diego LF, de las Obras-Loscertales M, Gayán P, Abad A, et al. Calcium-based sorbents behaviour during sulphation at oxy-fuel fluidised bed combustion conditions. Fuel 2011;90(10):3100–8. [13] Sheng C, Xu M, Zhang J, Xu Y. Comparison of sulphur retention by coal ash in different types of combustors. Fuel Process Technol 2000;64(1–3):1–11. [14] Spörl R, Maier J, Scheffknecht G. Sulphur oxide emissions from dust-fired oxyfuel combustion of coal. Energy Procedia 2013;37:1435–47. [15] Han K, Lu C, Cheng S, Zhao G, Wang Y, Zhao J. Effect of characteristics of calcium-based sorbents on the sulfation kinetics. Fuel 2005;84(14– 15):1933–9. [16] Zhang L, Sato A, Ninomiya Y, Sasaoka E. In situ desulfurization during combustion of high-sulfur coals added with sulfur capture sorbents. Fuel 2003;82(3):255–66.

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