Article pubs.acs.org/EF
Sulfur Self-retention during Cocombustion of Fossil Fuels with Biomass Janusz A. Lasek* and Krzysztof Kazalski Institute for Chemical Processing of Coal, ul. Zamkowa 1, 41-803 Zabrze, Poland ABSTRACT: SOx emissions from combustion processes can be reduced by the application of in-flame methods, including sulfur self-retention (SSR). Novel indexes for fossil fuels and biomass are proposed for the evaluation of the SSR process. The application of biomass as an additive to avoid SOx emissions from an industrial-scale boiler is presented, and the possible mechanism of the SSR process is discussed. The cocombustion of fossil fuels with biomass containing a high amount of CaO-ash reduces the amount of limestone required for the deSOx process.
1. INTRODUCTION The prevention of SOx emission from combustion processes and the development of efficient SOx removal methods seem to be the most important challenges in combustion science. It is known that SOx emissions can be prevented by using precombustion, in-flame, and postcombustion methods.1 Examples of precombustion methods include fuel switching or fuel (coal) pretreatment to reduce its sulfur content.2 Postcombustion measures are mainly related to scrubbing methods, for example, limestone wet flue gas desulfurization (FGD) and lime wet FGD.3,4 Among these mentioned methods, in-flame measures are very attractive because desulfurization processes (deSOx) can be carried out in situ inside the combustion chamber, and there is no need for an additional reactor. The process can be carried out by the addition of alternative sorbents containing metal oxides or mixtures thereof. The most-used additives for in-flame deSOx processes are calcium-oxide-based sorbents such as limestone, hydrated lime, or lime (CaO);1,5 however, it is known that SOx can be captured by high-content Ca-oxide ash. In other words, the process, defined as sulfur self-retention (SSR), selfdesulfurization, or inherent sulfur retention, involves reactions between the mineral matter in coal ash and the SO2 evolved during coal combustion, and consequently, the emission of SO2 may be significantly reduced.6−8 Grubor et al.6 have discussed the mechanism of formation of gaseous sulfur compounds. It was concluded that SO2 can be formed during devolatilization and char combustion. The main sulfur-containing gaseous compound in this volatile matter is H2S, which is subsequently oxidized to SO2. The conversion of SO2 during char combustion has a different nature. The oxidation of sulfur during this stage of combustion takes place inside the char particle. The formed SO2 diffuses outward through the char pores, and part of the SO2 can react with the base oxides of the ash. It was concluded that the most important base oxide for SSR is CaO, formulating sulfates anchored in the ash matrix;6,9,10 however, other oxides, such as MgO, Na2O, and K2O, play a role in the SSR process.6,11 According to Sheng et al.,9 the highest correlation coefficient exists between sulfur retention and Ca/S, while the correlation coefficients for other compounds (2Na/S, 2K/S, and Mg/S) are relatively small. However, Okasha7 reported that the © 2014 American Chemical Society
effectiveness of SSR using ash constituents was also significantly influenced by the presence of potassium in the biomass. Based on equilibrium calculations, Zheng and Furimsky12 reported that in the presence of SO2, K2O- and Na2O-containing minerals would be preferentially converted to their sulfates. This topic needs to be further investigated. From a mechanistic point of view, it is assumed that SSR is a result of the reaction between SO2 and CaO in the form of uniformly distributed micrograins in char. Most of the SSR with ash occurs during char combustion. Thus, the cofiring of high-sulfur-content fuels with low-sulfur- and high-Ca-ash-content fuels is an attractive method for decreasing SOx emissions because a substantial portion of the sulfur may be retained in the ash, decreasing the amount of limestone that needs to be added.6 Fuels that may specifically be considered for SSR process are biomasses, which are considered fuels with “intrinsic” sorbent capabilities.7 The SSR process depends on the fuel rank; for example, Kazanc et al.13 reported that up to 9% of the sulfur can be retained in the ash during the combustion of bituminous coals, whereas SSR for lignites can reach up to 80%. Even though the SSR process is a well-known phenomenon, it should be mentioned that different biomass fuels have not been classified as potential deSOx additives. Recently, Sommersacher et al.14 proposed fuel indexes for the evaluation of the combustion properties of different fuels. This idea, in an overall sense, was adopted in this paper; however, the main focus was on the SSR process. The main goal of this paper is to introduce potential factors in sulfur self-retention and to evaluate these factors for different biomasses. Additionally, the utilization of SSR during the cocombustion of lignite and willow biomass (Salix viminalis) in an industrial-scale boiler is presented.
2. EXPERIMENTAL SECTION The fuels were analyzed in an accredited laboratory at the Institute for Chemical Processing of Coal (Accreditation Certificate AB 081). Proximate and ultimate analyses of samples were performed on LECO TGA701, LECO TrueSpec CHN, and LECO SC632 devices. The Received: November 25, 2013 Revised: March 14, 2014 Published: March 17, 2014 2780
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process, both in pulverized combustion (PC) and fluidized-bed combustion (FBC).6,8,9 The impact of other elements, such as Mg, Al, Fe, K, and Na, are still under discussion.9,17−19 It is typically assumed, as in Grubor et al.,6,11 that Ca species play a main role in the SSR process. Sulfates of the mentioned species are less thermally stable than CaSO4. Moreover, it was found by X-ray powder diffraction (XRD) analysis that the CaSO4 phase is the main sulfation product retained in fly ash for PC chambers.8 Thus, it can be assumed that the SSR process occurs via the reaction of CaO with SO2 in the presence of O2, according to the overall reaction20
combustion heating values were measured with a LECO AC200 apparatus. Industrial combustion tests were carried out in a circulating fluidized-bed (CFB) boiler under atmospheric pressure inside a combustion chamber. The boiler was fed with lignite (main fuel) cofired with biomass (Salix viminalis). Desulfurization of the flue gas was performed by injecting limestone powder together with the main fuel. The composition of the flue gas was continuously measured using an IR analyzer, and the flue gas probe was placed in the flue pipe (after the electrostatic precipitator). The mass ratio of the cofired and main fuels was determined by certified scales installed in a power plant.
3. RESULTS AND DISCUSSION 3.1. Sulfur Self-Retention Potential Factors. Sulfur selfretention is usually characterized by the percentage of the total sulfur in coal retained by ash after combustion, denoted as ηSR. It can be determined by analyzing laboratory-prepared ash (see eq 1) or according to sulfur balance by measuring the SO2 in flue gas (see eq 2):9 ηSR =
SashA × 100 Scoal
ηSR = (1 − CSO2/C′SO2 ) × 100
CaO + SO2 +
(4)
The main product of the sulfation process is CaSO4. The transformation of the fuel-sulfur into other compounds is possible, for example, CaSO3 or CaS; however, it occurs at temperatures below 750 °C. Usually, CaS is found in FBC residues at levels less than 1%.21 The value of SSRPF indicates the potential possibility of a particular fuel for sulfur selfretention. If SSRPF is close to one, it means that the considered fuel contains an insignificant amount of sulfur that could be transformed to SO2. It should be noted that SSRPF only gives information about the qualitative behavior of a particular fuel. In other words, two considered fuels having the same SSRPF value have different capabilities for SO2 removal. However, SSRPF can be useful for evaluating the fuel upon SO2 conversion. For quantitative calculations, the molar sulfur self-retention factor, MSSRF, is more useful. The MSSRF can be calculated from the following equation:
(1) (2)
where Scoal and Sash are the weight fractions of sulfur in the parent coal and in the resulting ash, respectively; A is the ash content of the coal; and CSO2 and C′SO2 are the measured and theoretically calculated SO2 concentrations in the flue gas, respectively. These factors depend both on the fuel type and combustion conditions and show the real level of sulfur that is captured in the ash. To evaluate the highest potential of SSR, new factors are proposed and defined as follows. An evaluation of the potential availability of fuel as a deSOx additive should be done by standard criteria. New parameters, including the sulfur self-retention potential factor (SSRPF), molar sulfur selfretention factor (MSSRF) and mass sulfur self-retention per unit energy factor (MSSRFE) are presented and estimated for the different fuels. By definition, SSRPF can have values between −1 and 1. It is assumed that all of the combustible sulfur (contained in the fuel) is initially converted to SO2. Thus, all considered SOx species are calculated as SO2. A positive value of SSRPF means that the fuel is capable of capturing more sulfur in its structure. Based on this preliminary description, SSRPF is defined as follows: SSRPF = (SO2capt − SO2conv )/(SO2capt + SO2conv )
1 O2 → CaSO4 + heat 2
MSSRF = SO2capt − SO2conv , kmol SO2 /kg fuel
(5)
The MSSRF represents the molar amount of SO2 that can be avoided due to the sulfur self-retention process. It is clear that negative MSSRF values (below zero) characterize a fuel possessing a higher amount of sulfur than can be potentially captured by the SSR process. Sometimes, a certain quantity of emission (or avoidance thereof) is referred to as an energy unit’. Therefore, if MSSRF is divided by the lower heating value (LHV) of the fuel, then a new factor, the mass sulfur self-retention per unit energy factor, MSSRFE is obtained: MSSRFE = MSSRF × 64/LHV, kgSO2 /GJ
(6)
MSSRFE represents the amount of SO2 that can be avoided per energy unit introduced with the fuel to the combustion chamber. Sometimes, the emission of gaseous pollutants is determined as tons of compound per time unit (e.g., year); thus, MSSRFE can be useful in some environmental calculations. 3.2. Fuel Analysis and SSR Factors. Tables 1, 2, and 3 include the proximate and ultimate analyses and the determined SSR factors of CaO content in fuel-ash, respectively. Almost all of the biomasses have positive SSRPF factors. Oat seeds, residues after maize processing, millet husks, and distillery residues are exceptions that include a relatively high sulfur content and low CaO content. Furthermore, some fossil fuels such as Belchatow and Glogow lignite and Wieczorek and Staszic coals have positive SSRPF factors. SSR during the combustion of hard coal was reported by many researchers; for example, Grubor et al.6 noticed SSR during the combustion of lignite and hard coal. They reported, based on the work of Sheng et al.,9 that the activity of Ca increases as the
(3)
where SO2capt is the molar amount of SO2 per unit of fuel that can be potentially captured by the CaO contained in the fuel, and SO2conv is the molar amount of SO2 per unit of fuel that can be converted from the sulfur (total) contained in the fuel. SO2conv is estimated from stoichiometric combustion calculations. SO2capt is determined by the stoichiometry of SO2 captured by Ca species. It is assumed that the Ca species present in ash of the fuel can be converted to CaO and then to CaSO4 (in the presence of SO2). From equilibrium calculations, it is assumed that CaO is produced at temperatures higher than 770 °C under atmospheric pressure in the air-fired combustion of coal.15,16 However, it should be mentioned that too-high temperature affects CaSO4 dissociation. During the atmospheric-pressure combustion of coal at an excess air ratio λ ≈ 1.3, the temperature of CaSO4 destruction is about 1180 °C.16 It is assumed that calcium plays a dominant role in the SSR 2781
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Table 1. Proximate Analysis of Fuelsa no.
type of fuel
1 2
oat seeds residues after apple processing residues after maize processing buckwheat husks coffee husks conifer sawdust cereal straw briquettes rape straw briquettes sunflower husks Miskhantus giganteus Sida hermaphrodita Salix viminalis chips millet husks distillery residues Posidonia oceanica hard coal LSC (low-sulfur coal) Belchatow lignite Glogow lignite Janina coal Wieczorek coal Sobieski coal Staszic coal Janina coal
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Table 3. CaO Content in Fuel-Ash and SSR Factors
Mr
Ma
Aa
Va
LHVa
%
%
%
%
kJ/kg
11.4 16.5
8.1 6.2
4.2 1.5
71.58 76.41
16286 17542
16.7
5.7
2.5
74.03
16127
41.1 36.7 44.8 9.3 10.7 9.6 8 8.5 9.1 9.7 7.4 8.5 9.4 6.6 50.3 57 20.3 11 15.6 8.2 17
8.4 8.5 7.5 8.8 9.4 8.3 7.2 8.3 8 8.6 7.1 5.5 1.6 3.1 10.7 5.4 13.3 2.8 9.8 1.9 9.4
2.6 10.9 1.1 4.7 5.2 3.9 3.7 2.6 1.5 9.1 5 26.1 27.4 15.1 19 14.7 12.6 17.5 7.5 12.5 5.3
63.75 63.61 75 0 0 67.67 71.3 73.99 72.89 64.55 70.47 54.41 23.39 29.21 36.82 45.12 29.28 26.98 31.57 29.08 30.62
16787 16189 17810 15413 15407 17364 16357 16084 16869 16326 16242 11896 23103 26447 17308 19074 22101 25906 24948 28440 25985
CaO
SSRPF
% oat seeds residues after apple processing residues after maize processing buckwheat husks coffee husks conifer sawdust cereal straw briquettes rape straw briquettes sunflower husks Miskhantus giganteus Sida hermaphrodita Salix viminalis chips millet husks distillery residues Posidonia oceanica hard coal LSC (low-sulfur coal) Belchatow lignite Glogow lignite Janina coal Wieczorek coal Sobieski coal Staszic coal Janina coal
a
M, moisture; A, ash; V, volatile matter; LHV, low heating value; a, “air-dry” state; r, “as received” state.
MSSRF
MSSRFE
(kmol SOx/kg fuel) 106
kg SO2/GJ
2.6 11.93
−0.49 0.06
−37 4
−0.14 0.01
2.5
−0.25
−8
−0.03
8.29 11.82 26.21 24.61 24.93 13.1 5.82 49.07 30.28 0.06 1.62 21.77 2.77 6.46 18.58 17.66 2.66 4.48 4.6 4.3 9.07
0.06 0.48 0.78 0.54 0.57 0.38 0.01 0.90 0.62 −0.93 −0.64 0.78 −0.13 0.12 0.33 0.39 −0.80 0.03 −0.72 −0.20 −0.65
4 149 45 144 169 51 1.0 215 62 −27 −51 890 −39 37 312 260 −465 9 −317 −48 −317
0.02 0.59 0.16 0.60 0.70 0.19 0.00 0.86 0.24 −0.11 −0.20 4.79 −0.11 0.09 1.15 0.87 −1.35 0.02 −0.81 −0.11 −0.78
Table 2. Ultimate Analysis of Fuelsa
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
oat seeds residues after apple processing residues after maize processing buckwheat husks coffee husks conifer sawdust cereal straw briquettes rape straw briquettes sunflower husks Miskhantus giganteus Sida hermaphrodita Salix viminalis chips millet husks distillery residues Posidonia oceanica hard coal LSC (low-sulfur coal) Belchatow lignite Glogow lignite Janina coal Wieczorek coal Sobieski coal Staszic coal Janina coal
Ca
Ha
Na
Oa
Sta
SAa
SCa
wt %
wt %
wt %
wt %
wt %
wt %
wt %
43.3 47.3 44.9 47.1 44.3 49.4 n/a n/a 49.3 44.8 45 46.5 43.9 44.1 35.5 60.1 70.1 48 53 59.2 68.4 65.3 73.6 68.8
5.38 5.18 5.44 5.26 0.12 5.45 5.51 5.39 5.52 5.44 5.42 5.44 5.26 5.92 3.6 3.52 4.15 3.43 4.16 3.56 3.76 4.3 4.35 4.1
1.74 0.04 0.69 1.28 3.48 0.24 n/a n/a 0.86 0.67 0.23 0.74 1 2.92 0.27 0.82 1.14 0.53 0.52 0.88 1.22 1.06 1.26 1.16
37.11 39.72 40.74 35.29 35.68 36.31 n/a n/a 32.04 38.14 38.43 37.78 32.06 34.76 28.85 6.22 6.22 17.96 21.98 8.86 6.16 10.93 6.13 10.15
0.18 0.09 0.06 0.11 0.26 0.02 0.2 0.2 0.13 0.12 0.04 0.06 0.09 0.21 0.4 0.56 0.44 1.02 0.65 1.68 0.42 1.21 0.46 1.29
0.01 0.03 0.03 0.04 0.24 0.01 0.16 0.16 0.05 0.07 0.02 0.02 0.01 0.01 0.22 0.22 0.25 0.64 0.41 0.08 0.26 0.1 0.2 0.2
0.17 0.06 0.03 0.07 0.02 0.01 0.04 0.04 0.08 0.05 0.02 0.04 0.08 0.2 0.18 0.34 0.19 0.38 0.24 1.6 0.16 1.11 0.26 1.09
a
Content (analytic state): Ca, carbon; Ha, hydrogen; Na, nitrogen; Oa, oxygen (calculated); Sta, total sulfur; SAa = SashAa, sulfur in ash; SCa, combustible sulfur. 2782
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rank of coal decreases, and thus, the application of biomass as an SSR additive could improve efficiency. It should be noted that the highest MSSRF (890 × 10−6 kmol SO2/kg fuel) and MSSRFE (4.79 kg SO2/GJ) are attributed to Posidonia oceanica, PO. The thermo-chemical properties of this marine biomass have recently been examined. It was observed that the uncontrolled decomposition of PO caused hygienic and environmental problems, including unpleasant odors and the development of beach flies.22,23 However, this marine biomass has been recognized as an additional fuel for cocombustion with high potential for SSR. The other fuels, biomasses, and lignites listed have adequate SSR factors, especially Sida hermaphrodita, coffee husks, cereal straw briquettes, rape straw briquettes, and the presented lignites. Figure 1 shows the calculated SSRPF as a function of the Ca/ S ratio in the fuel. It was expected that SSRPF should equal
Figure 3. Mass sulfur self-retention per unit energy factor (MSSRFE) for different fuels.
It should be underscored that these parameters are only for theoretical consideration, and capturing all of the sulfur in real combustion chambers is not possible due to certain limitations. One important point is that SSR occurs mainly during the second stage of combustion, that is, char combustion. For example, the presented lignites have high MSSRF values; however, these are caused by their high CaO-ash content, whereas the sulfur content in these fuels remains relatively high. For this reason, biomass is more suitable for SSR, as is explained in further detail in section 3.4. 3.3. Theoretical and Real SSR Processes. It is reasonable that the optimal potential of SSR cannot be attained because of several limitations. It is known that the desulfurization process depends on the properties of the fuel and combustion processes. Cheng et al.8 emphasized that SSR can be affected by the boiler shape, the flame temperature, the fuel residence time in the combustion chamber, the initial molar Ca/S and the reactivity of alkaline components. Moreover, FBC boilers are preferable for SSR compared to PC boilers. This is due to the higher temperature (1300−1600 °C) and shorter residence time (1−2 s) in PC boilers. As a consequence, the SSR observed in these kinds of boilers have not exceed 25%. Moreover, it was experimentally confirmed that increasing the biomass added from 15 to 20 wt % during cocombustion with coal caused increased SO2 emissions. Based on the analysis of flame temperature profiles, this was probably caused by the thermal instability of the CaSO4 that is created during the reaction of CaO-ash and SO2.24 A suitable temperature (800− 900 °C) in FBC boilers avoids the destruction of CaSO4, and a long residence time creates preferential conditions for SSR. The main parameters of the combustion tests are described in Table 4. The fuel particles had a diameter of less than 30 mm, based on the sieve size used after crushing. Table 5 shows the results obtained in a real-scale industrial CFB (bubbling)
Figure 1. Sulfur self-retention potential factor as a function of Ca/S ratio.
zero for Ca/S = 1. Theoretically, fuel such as this is neutral in terms of SO2 emissions, which means that all of the sulfur contained in fuel is captured by Ca-ash species. Of course, the SO2 retention efficiency will be lower in the conditions prevailing in a real combustion chamber. The maximal possible value of SSRPF is 1, which is observed for fuels characterized by a high Ca/S ratio. It was evident that the molar sulfur selfretention factor, MSSRF and mass sulfur self-retention per unit energy factor, MSSRFE were more useful for quantitative calculations. Figures 2 and 3 show the calculated MSSRF and MSSRFE for different fuels. The highest MSSRF and MSSRFE values were observed for the marine biomass Posidonia oceanica.
Table 4. Main Parameters in the Boiler during Combustion Test param.
capacity
lignite combustion
co-firing of lignite and Salix (6.3 wt %)
avg. temp. in the bed, °C
lower higher lower higher lower higher
840 870 35 58 1.3 1.2
840 870 42 57 1.4 1.2
secondary air/primary air ratio, % excess air ratio, λ
Figure 2. Molar sulfur self-retention factor (MSSRF) for different fuels. 2783
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char combustion. Moreover, it is known that most of the SSR by ash occurs during the second stage, or char combustion. Figure 4 shows a simplified scheme of SSR during the
Table 5. Comparison of Sulfur Retention during Combustion of Lignite and Co-combustion with Salix viminalis in Commercial CFB Boiler param.
lignite combustion
theoretical SO2 concn., mg/m3n 1226 at 6% O2 capacity of 157 MWth 395 measured SO2, mg/m3n at 6% sulfur retention, % 68 limestone feed, kg/s Ca/Sa limestone savings, % fuel feed, t/h biomass feed bed temp. capacity of 261 MWth measured SO2, mg/m3n at 6% sulfur retention limestone feed, kg/s Ca/Sa limestone savings, % fuel feed, t/h biomass feed bed temp.
1.1 3.7−3.9 118−122 0 840 395 68 2.1 4.2−4.4 199−201 0 870
co-firing of lignite and Salix (6.3 wt %) 1164
333 71 0.7 2.9−3.1 36 113−117 7 845
Figure 4. SSR during combustion of a pure fuel.
363 69 1.3 2.9−3.1 38 197−201 12 875
combustion of uniform fuel (without cocombustion). During the first step of combustion (devolatilization), mineral matter is not capable of capturing the gaseous compounds of sulfur, mainly H2S. In the second part of combustion, char, the oxidized part of mineral matter, is converted into metal oxides and can react with sulfur compounds, mainly SO2, part of which is converted into CaSO4 (see the right side of Figure 4). SSR occurs differently if cocombusted fuels have different devolatilization rates. A simplified version of this situation is shown in Figure 5. This figure depicts the cocombustion of coal (higher sulfur content) and biomass (lower sulfur content and relatively high ash/CaO content). It is known that the devolatilization of biomass usually occurs at lower temperatures and is faster and less complex than the devolatilization of coal.26,27 Therefore, if the devolatilization of biomass is faster than that of coal, it is possible to capture sulfur (converted during the devolatilization of coal) on CaO-ash compounds in the biomass structure (see part A of Figure 5). Further SSR during char combustion of both coal and biomass is certainly possible; however, SSR during cocombustion is more efficient.
a
Ca/S calculated including the total amount of CaO introduced with lignite, biomass, and lime.
boiler. Analysis was carried out for two capacities: 157 MWth and 261 MWth. The analysis of additional industrial units has been reported by K. Kazalski et al.25 The addition of biomass (Salix viminalis) to the main fuel (lignite) reduced the amount of limestone required, which was used to decrease SO2 emissions. The decrease of emissions (DE), calculated from eq 1, C′SO2(cofiring)/C′SO2(uniform combustion)) × 100, was determined for the theoretical and measured SO2 concentrations. In the theoretical case (SO2 concentration calculated from stoichiometry, e.g. DE = (1−1164/1226) × 100 = 5%; see values in Table 5), only a 5% decrease was obtained, because the amount of fuel-S was decreased by mixing the main fuel (lignite) with biomass. The real decrease of SO2 was higher, reaching 16% and 8% for capacities of 157 MWth and 261 MWth, respectively. Moreover, even though the Ca/S ratio was decreased, the emission of SO2 was also decreased. This phenomenon should be further investigated. However, it can be explained by assuming a high dispersion of ash during the combustion process. Increased surface area improves the overall deSO2 efficiency. Based on the presented results, it is clear that some additional process should occur in the reaction zone. Additionally, if this phenomenon is compared with the reduced limestone consumption during cocombustion, it can be proved that this “additional process” was a result of SSR obtained by biomass addition. Lignite is capable of SSR; however, this process is limited by several issues. Not all of the sulfur can be converted by the original CaO-ash in uniform fuel. The process is more efficient if biomass is used as an additional fuel, such as in a cofiring system. Thus, the relevant mechanism, based on data provided in the literature, is proposed in the following section. 3.4. Possible Mechanism of Sulfur Self-retention during Cocombustion. The formation of gaseous sulfur compounds occurs by two main stages: devolatilization and
4. CONCLUSION The application of biomass as an additive for decreasing SO2 emissions is a possible way to improve deSO2 processes in industrial combustion systems. New indexes for the evaluation of sulfur self-retention have been proposed, including the sulfur self-retention potential factor (SSRPF), molar sulfur selfretention factor (MSSRF), and mass sulfur self-retention per unit energy factor (MSSRFE). SSRPF is useful for the qualitative evaluation of a fuel’s SO2 emission/retention. It was observed that SSRPF = 0 is typical for fuels characterized by a Ca/S ratio of about one. MSSRF and MSSRFE are useful for quantitative characterization of the sulfur self-retention process (SSR); however, the highest possible SSR is lower than the real observed deSO2 process. More efficient SSR is observed for biomass. A 35% reduction in limestone use was observed during the cocombustion of lignite and energy willow in an FBC boiler, which was attributed to the SSR process. 2784
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Figure 5. SSR during cocombustion of coal or lignite with biomass.
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AUTHOR INFORMATION
Corresponding Author
*Tel: + 4832 271-00-41. Fax: +4832 271-08-09. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Research and Development Strategic Program “Advanced Technologies for Energy Generation” project No. 2, “Oxycombustion technology for PC and FBC boilers with CO2 capture,” supported by the National Centre for Research and Development, agreement no. SP/E/2/66420/10. This support is gratefully acknowledged.
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dx.doi.org/10.1021/ef402318z | Energy Fuels 2014, 28, 2780−2785
