Hydrogen Chloride Bonding with Calcium Hydroxide in Combustion

Article pubs.acs.org/EF

Hydrogen Chloride Bonding with Calcium Hydroxide in Combustion and Two-Stage Combustion of Fuels from Waste Sławomir Poskrobko,*,† Danuta Król,‡ and Jan Łach§ †

Białystok University of Technology, Wiejska 45C, 15-351 Białystok, Poland Silesian University of Technology, Konarskiego 18, 44-101 Gliwice, Poland § Technical University of Radom, Krasickiego 54, 26-600 Radom, Poland ‡

ABSTRACT: The present study deals with the problem of limiting HCl mobility in processes of combustion of two solid recovered fuels (SRF1 and SRF2) having significant but different contents of chlorine. Large-scale experiments were carried out at a temperature of 1050 °C with the use of a stoker-fired flat grate boiler with the heat output of 1 MW, and with a two-stage combustion system for medical waste incineration (MWI) with the same heat output in which first the process of gasification in the temperature of 650 °C occurs, and subsequently the previously obtained syngas is fired in the temperature of 1000 °C in the reburning chamber of the gas generator. In such a way, two different technologies of combustion have been compared, with regard to HCl capture efficiency using widely available calcium hydroxide sorbent Ca(OH)2. In order to simulate the increased concentration of chlorine in SRF1, the fuel was being supplemented with three various additions of poly(vinyl chloride) (PVC) recyclate. Before combustion, SRF1 and SR2 were also blended with Ca(OH)2 whose weight amount was being changed: from 0 to 3 wt % with a step equal to 0.5 wt % (combustion) and from 0 to 1.5 wt % with a step equal to 0.5 wt % (two-stage combustion). For both of the methods of SRFs combustion, series of experimental findings have been presented, showing trends in the changes of HCl concentration in flue gas, depending on (i) the amount of PVC recyclate added to SRF1 and amount of Ca(OH)2 added to both fuels; and (ii) the molar ratio Ca/Cl2. Finally, the ratio of HCl concentrations in flue gas after combustion and two-stage combustion, depending on the amount of added PVC and Ca(OH)2 to the fuels was analyzed. The achieved results indicate that in a two-stage process (gasification and recombustion), HCl can be much more effectively bonded than during combustion in stoker-fired boiler.

1. INTRODUCTION The main purpose of this paper is to compare two different technologies of combustion of solid recovered fuels (SRFs) containing significant concentrations of chlorine, with regard to HCl capture efficiency, using an inexpensive and widely available calcium hydroxide sorbent (Ca(OH)2). The first approach embraces combustion in a stoker-fired boiler, the second one involves a two-stage combustion in a medical waste incinerator (MWI). In principle, all solid fuels contain chlorine. In hard coals, this element is encountered as such in three major forms: chloride ions (Cl−), inorganic chlorides, and organochlorine compounds.1 This is similar to the case of various biomass species which, in the nutrient cycle, assimilate chlorine from the atmosphere and soil in such predominant inorganic forms as KCl and NaCl. In practice, due to a relatively high content of sulfur in coal, the presence of chlorine in coal combustion is usually not observable.1,2 The situation is different for biomass fuels, especially biomass of agricultural origin. The practical experience gained in recent years shows that biomass is a difficult source of renewable energy, in view of the selection of proper technology of its combustion. This is caused by (i) the significant chlorine content facilitating the mobility of potassium and other inorganic compounds;3 and (ii) low content of sulfur. The low value of S/Cl molar ratio unfavorably influences boiler operating conditions, as it facilitates high-temperature corrosion and formation of deposits from potassium compounds.4 Analogous problems arise during operation of waste heat recovery boilers. Chlorine contained in fuels from waste exists in both organic and inorganic forms, which has a considerable impact on its mobility, availability, and reactivity. © 2011 American Chemical Society

Almost all of the organic chlorine and part of the inorganic chlorine released during combustion is easily converted to HCl.5 It should be emphasized that polyvinyl chloride (PVC) and NaCl are predominant natural sources of chlorine in waste-based fuels such as refuse-derived fuels (RDF) and municipal solid waste (MSW).6−10 It has also been shown in experiments conducted in a bench scale11 that food and plastic are the main inorganic and organic sources of HCl from MSW, respectively. Laboratory tests were carried out for materials such as plastic, wood, textiles, food, rubber, and paper. In addition, measurements of HCl concentration in the flue gas proved (i) its rapid increase when the temperature exceeds 500 °C, especially during combustion of plastic in which PVC is the predominant source of chlorine, and relatively low increase in the case of paper and food waste with predominant inorganic form of chlorine; and (ii) considerable percentage span of the total chlorine (from 30% to 70%) passes in the gas phase into the flue gas at 700 °C. The latter phenomenon may result from different distribution of chlorine content in the fuel between organic and inorganic forms, and also from the fact that individual components of MSW may contain various quantities of alkali-metal and alkali-earth-element compounds (mainly potassium and sodium). In high temperatures (i.e., between 1000 and 1200 °C, as in combustion processes), alkali chlorides pass into flue gas in the form of a vapor phase (KCl(g), NaCl(g))12 which condenses on small fly ash particles, forming a Received: June 4, 2011 Revised: December 21, 2011 Published: December 28, 2011 842

dx.doi.org/10.1021/ef2016599 | Energy Fuels 2012, 26, 842−853

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individual request of the authors. In Poland, this variety of SRF is known by its mnemonic name, PAS’r, and is mostly used in the cement industry as a substitute for conventional fuels. It is composed of such waste materials as paper, cardboard, plastic films, rags, textiles, plastic containers, tapes, cables, cleaning cloths, and tires. This type of waste may be contaminated by oils, fats, lubricants, paints, etc. In addition, components such as clothes confiscated by customs officers, plastics, paper and cardboard tainted by organic solvents or oil derivatives, oil filters, food products past their sell-by date, plastic wrappings contaminated by pesticides and detergents, rubber waste (in small quantities, up to 5% only), and bulky waste (pieces of furniture such as couches, armchairs, etc.) are also employed in the forming of such fuels. The properties of SRF1 and SRF2 are given in Table 1. They may be adjusted to the

deposit on boiler heat-transfer surfaces at a temperature of ∼450 °C. Therefore, the risk of high-temperature corrosion is considerably increased.12,13 Detailed characteristics of fly ashes collected from two Chinese MSW incinerators are given in ref 14. The aforementioned compounds may occur mainly in the solid and liquid phase12 at temperatures between 600 and 800 °C, which is typical of fluidized-bed furnaces and gas generators. This suggests that the main part of chlorine released from the fuel in the gasification process is of an organic origin. Thus, the form and amount of HCl released in a combustor are dependent on the technology of thermal decomposition of fuels (pyrolysis, gasification, combustion, cofiring) occurring at different temperatures. The serious consequences of the release of chlorine as HCl in the gas phase during the thermal processing of high chlorine waste-derived fuels (RDF, MSW) mean that one of the important issues is to reduce the risk of deposition and corrosion, 1 as well as to prevent the formation of polychlorinated dibenzo-pdioxin (PCDD) and polychlorinated dibenzofuran (PCDF)7,15,16 by using calcium hydroxide to bond hydrogen chloride already in a boiler furnace and during the first stage of combustion in MWI. The reason why such research has been undertaken is the growing economic and ecological interest of power engineering in production, combustion, and both direct and indirect cofiring of various solid recovered fuels (SRFs), together with a base fuel in power boilers.17,18 The standards developed by the European Committee for Standardization (CEN)19,20 permit the use of such fuels in power production, even though they have a relatively high (but admissible) concentration of chlorine. The relevant literature tends to focus on bonding HCl with widely available inorganic alkali compounds. Accordingly, the ability of solid sorbents with active species such as Na2CO3, CaO, and MgO to reduce the HCl concentration in a bench scale fixed-bed reactor to a very low level which decreases linearly with temperature was shown in ref 21. The successful dechlorination of waste thermal decomposition products using the calcium-based sorbents has been demonstrated in refs 22 and 23. The first work suggests that the calcium carbonate carbon composite sorbent (Ca−C) can be effectively used for the removal of HCl and chlorinated hydrocarbons from the pyrolysis of the mixed plastics (PVC/PP/PE/PS). The second one evaluates the effectiveness of reduction of HCl concentration at 600−800 °C in a fixed-bed reactor by means of two sorbents obtained by calcining CaCO3. Analysis of the formation of HCl from PVC and NaCl contained in several types of RDF and the dechlorination capacity of calcium hydroxide Ca(OH)2 at 650−800 °C during pyrolysis at various air/fuel ratios and syngas combustion in recombustion chamber integrated with the reactor is presented in ref 24. All in all, reducing the mobility of chlorine already in the combustor creates favorable conditions for: (i) limiting the formation of aggressive alkali chlorides deposition on heat transfer surfaces; (ii) controlling high-temperature corrosion; (iii) lowering the concentration of HCl; and (iv) curbing the formation of PCDDs and PCDFs. Furthermore, since a major part of chlorine existing in solid fuels leaves the boiler as gaseous HCl within the flue gas,1 the risk of low-temperature corrosion in flue gascleaning system also can be reduced.

Table 1. Properties of the Solid Recovered Fuels (SRF1 and SRF2) Fuel physical quantity

symbol

SRF1

SRF2

moisture content W 5.74 wt % 6.18 wt % Technical Properties of the Fuels on a Dry Weight Basis flammable fraction 82.80 wt % 81.99 wt % nonflammable fraction 17.20 wt % 18.01 wt % higher heating value HHV 20974 kJ/kg 18420 kJ/kg lower heating valuea LHV 19559 kJ/kg 17147 kJ/kg Elemental Composition of the Fuels on a Dry Weight Basis carbon C 52.91 wt % 51.28 wt % hydrogen H 6.29 wt % 5.66 wt % nitrogen N 2.94 wt % 2.65 wt % sulfur S 1.04 wt % 0.16 wt % chlorine Cl 0.43 wt % 0.87 wt % oxygenb O 19.19 wt % 21.37 wt % ash A 17.20 wt % 18.01 wt % Concentration of Alkali and Alkaline-Earth Elements on a Dry Weight Basis calcium Ca 25 100 ppm 26 030 ppm potassium K 630 ppm 880 ppm sodium Na 1080 ppm 930 ppm Concentration of Heavy Metals on a Dry Weight Basis cadmium Cd 2.7 ppm 2.9 ppm chromium Cr 182 ppm 100 ppm copper Cu 757 ppm 633 ppm manganese Mn 249 ppm 309 ppm nickel Ni 68.2 ppm 20.9 ppm lead Pb 82.6 ppm 61.5 ppm zinc Zn 2265 ppm 1497 ppm mercury Hg 0.8 ppm 0.9 ppm a

Calculated based on the value of HHV and the fuel elemental composition. bClosing balance of elements.

customer’s demands. From the viewpoint of experimental tests, particular attention should be paid not only to the different concentrations of chlorine, but also to different mass fraction of sulfur and the high content of calcium. The source of organic forms of chlorine are PVC and PCB-containing wastes. In either of the tested fuels, chlorine is bonded by means of different types of chemical bonds. The speciation analysis of SRF1 and SRF2 was ineffective, because each batch of this type of SRF is a conglomeration of different wastes and, thus, has diverse elemental composition. Therefore, only its basic physicochemical properties and the content of heavy metals are standardized (including the total chlorine concentration, without specifying its chemical form). If so, for any given volume of the fuel, one deals with irregular thermal

2. THE RESEARCH PROBLEM: MATERIALS AND METHODS The first question involves the selection of a model fuel, especially considering the use of SRF in the power industry. This is a relatively new issue and remains open to debate. In this study, two types of SRF have been tested; they are denoted here as SRF1 and SRF2. The fuels had been produced by SITA Company on the 843


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Table 2. Elemental Composition of SRF1 Fuel Supplemented with PVC Recyclate Granules Expressed on a Dry Weight Basis Composition (wt %) fuel FUEL FUEL FUEL FUEL a

PVC addition (wt %)

C

H

N

S

Cl

Oa

A

0 0.5 1 2

52.91 52.87 52.84 52.77

6.29 6.29 6.28 6.28

2.94 2.93 2.91 2.89

1.04 1.04 1.03 1.02

0.43 0.51 0.59 0.74

19.19 19.22 19.26 19.32

17.20 17.14 17.09 16.98

I II III IV

Closing balance of elements.

Figure 1. Schematic of the boiler furnace. Denoted features are defined in the text.

decomposition of chlorine compounds, making the situation dissimilar to the combustion of such a model fuel as extracted rapeseed meal.25,26 The considerable quantity of calcium naturally facilitates the reduction of HCl released when the fuels are gasified and combusted. The relatively moderate content of sodium and potassium should not have an observable impact on the ash softening temperature and, at the same time, on the structure of slag. Just as in the case of chlorine compounds, compounds of calcium, sodium, and potassium remained unidentified, which translates into their various reactivities. Next, in order to simulate the increased concentration of chlorine in SRF1, the fuel was being supplemented with three various amounts (Table 2) of poly(vinyl chloride) (PVC) recyclate granules, the elemental composition of which has been given previously.25 Here, we recall that PVC is often a component of MSW, RDF, and other waste-derived fuels. Relatively uniform fuel mixtures were obtained after a period of extensive blending. It may be assumed that this procedure produced an entire family of four SRF1 fuels having diverse mass fractions of chlorine given in Table 2. Note that only the amount of chlorine contained in the PVC admixture was specifically determined through known organic chemical bonds. In this way, municipal solid waste (MSW), combustible hazardous wastes, as well as other fuels from waste such as refuse-derived fuels (RDF) can be modeled. As for the comparatively high concentration of chlorine in SFR2 (Table 1), it remained unmodified during the tests.

Before combustion, thus-formed model fuels were also blended with Ca(OH)2, whose weight amount was changed: (a) from 0 to 3 wt %, with the step equal to 0.5 wt % (combustion); and (b) from 0 to 1.5 wt % with the step equal to 0.5 wt % (two-stage combustion). In this way, a possibly long contact time between the sorbent and the reaction environment was ensured. The added calcium sorbent had the form of a fine powder. Its commercially obtainable form did not require any pretreatment. To some extent, the presence of alkali agent originated from the fuel itself. Subsequently, SRF1 without and with the admixtures of PVC and Ca(OH)2 underwent combustion in a stoker-fired boiler, as well as two-stage combustion in MWI. SRF2 with the addition of the aforementioned sorbent underwent the former process only. This means that 35 combustion and 16 two-stage combustion tests were carried out. In addition, each single test was repeated three times. During all these processes, measurements of basic gaseous products concentration were carried out, including denotation of hydrogen chloride levels. A spectrophotocolorimetric method employed for measuring the HCl concentration in exhaust gas25,26 was preceded by the extraction of exhaust gas samples using an aspirator (see Figures 1 and 2). Therefore, the research problem included: (i) determining trends in changes of hydrogen chloride concentration in flue gas, depending on the amount of PVC recyclate added to SRF1 and amount of Ca(OH)2 added to both fuels; and (ii) comparing between the two aforementioned technologies of combustion of waste-derived fuels containing significant concentrations of chlorine with regard to the HCl reduction efficiency 844


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Figure 2. Schematic of two-stage combustion system (MWI − Medical Waste Incinerator). Denoted features are defined in the text.

continuous uniform mechanical feed of the fuel onto the grate of the gas generator. Just as in the tests in a stoker-fired boiler, the installation allowed a quick startup, prompt reaction to adjustment of the parameters, and a possibility of observing, through the eyehole, the feeding, pyrolysis, gasification and the removal of ash to the lower grate into the incineration zone. Because of the continuous removal of ash from the grate of the primary chamber, the entire system was adapted to a 24-h work cycle. The stoker-fired boiler (Figure 1) is comprised of a pressure part (1) and a mechanical grate (2), with the functional length and width of 2150 and 900 mm, respectively, and with the airblast system (3). The three-pass flue gas system consisted of the open-type half-rolled smooth furnace tube (4), the rear reversal chamber (5), two sets of second-pass (6) and thirdpass (7) smoke tubes, and the smoke conduit (8). The combustion chamber, and, at the same time, the first convective flue gas pass, were formed by a furnace tube (4) with a grate (2) and an ignition vault (9). The pressure part (1) was constructed of welded sheet metal and included the following elements: an external jacket, front and rear ends of the boiler, tube sheets, reversible sheets, and a boiler base. The boiler was normally used for the combustion of specified hard coal. The experiments were carried out in the beginning of the heating season, when the boiler was functioning from a single startup, continuously in a 16-h cycle. Before the fuel feed, the furnace was heated up using hard coal. The feeding proceeded at a constant rate from the intake hopper into which the fuel was provided manually. Adjustment of the optimum combustion parameters was possible by controlling the thickness of the coal layer on the grate by means of a coal gate, and also by providing adequate primary air inflow under the grate, together with proper movement of grate. Furnace efficiency, monitored continuously, was used as the reference point for the regulation of the boiler parameters. The concentration of mineral products of combustion, excess air number (λ), and furnace efficiency (η) were measured by means of an IMR-3010P flue gas analyzer. Amounts of PVC and Ca(OH)2 added to combusted fuels had very little impact on the boiler and MWI operating parameters. Thus, in Table 3, their averaged values, as well as mean concentrations of gaseous products of combustion, are given. Samples of the gaseous products of combustion used to evaluate the HCl concentration were taken as shown in Figure 1. The following observations were made: (i) ignition of fuels

of the used sorbent. By and large, the study is focused on answering preliminary and partially the following questions: (i) is eco-friendly combustion of SRFs in flat grate boilers operating properly in municipal and industrial heating plants?; and (ii) can SRFs be used in the process of indirect cofiring with fossil fuels ?

3. EXPERIMENTAL TEST STANDS AND RESEARCH METHODOLOGY Large-scale experiments on the combustion of SRF1 and SRF2 were carried out in a stoker-fired flat grate boiler (Figure 1, Table 3), and in a MWI (Figure 2, Table 3). From the perTable 3. Basic Parameters of Both Stoker-Fired Boiler and Medical Waste Incinceration (MWI) parameter nominal heat output fuel consumption (feed rate) water temperature (outlet/inlet) boiler efficiency (η) excess air number (λ) furnace temperature

boiler

1000 kW 200 kg/h 110 °C/70 °C 87.2% 3.3 1050 °C Flue Gas temperature 210 °C oxygen concentration (O2) 14.6% carbon dioxide content (CO2) 6.1% carbon monoxide content (CO) 1318 ppm nitrogen oxides content (NOx) 137 ppm

MWIe 1000 kWa 60 kg/hb 90 °C/70 °C 88.0%c 0.75d/1.57c 650 °Cb/1000 °Ce 250 °C 7.6% 12.7% 5.0 ppm 140 ppm

a Waste heat recovery boiler. bFirst chamber. cTwo-stage combustion system. dGas generator. eRecombustion chamber.

spective of the tests, it was important that the boiler allowed a quick startup, easy adjustment of operational parameters, and uniform manual feeding of fuel into the intake hopper; it also did not have an automatic control system. The used fuels, although finely shredded, had neither a granular structure nor the form of pellets, which precluded their feeding onto the grate by means of a conveying screw. Moreover, a simple design of the furnace allowed for visual observation of the combustion process (i.e., the ignition phase), the flame length, the afterburning of the fuel, and the fuel and ash layout on the furnace grate. With regard to the MWI, it was used for a two-stage combustion of SRF1 fuel; the process was conducted ensuring 845


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ash from chamber (1), were therefore carried off into the channel, providing syngas into the reburning chamber. This phenomenon suggested that the reburning process proceeded with a considerably greater quantities of fly ash than in the case of combustion on the boiler furnace grate.

proceeded properly; (ii) their combustion was stable; (iii) the process took place with relatively high values of λ, mostly because the furnace was designed for combustion of hard coal rather than fuels from waste; and (iv) despite the addition of Ca(OH)2, ash did not form conglomerates at the furnace temperature of 1050 °C. Note that increasing the feeding rate to more than 200 kg/h resulted in increased CO concentration in the flue gas, along with the appearance of unburnt coal on the grate. Turning to the MWI (Figure 2), it consisted of the gas generator chamber (1) with a stationary flat grate (2) with nozzles providing air from under the grate into the reaction zones by means of the air blast system (3). The SRF1 was fed onto the grate (2) in portions using a mechanical feed unit (4). The feeding frequency of each portion depended on the LHV of the used fuel and followed the instructions given by the producer. The ash from the grate (2) of the gasification chamber was mechanically scooped into the incineration chamber (7) by means of a sweep (5) coupled with the mechanism (6). As for the incineration chamber (7), it served for storage of ash and its homogenization. To this end, it was fitted with a stationary flat grate (8) with air nozzles and with the air inflow system (9). The removal of ash was carried out by a sweep (10), which operated according to the procedure implemented by the producer by means of a system of mechanisms (11). Usually, the sweeping was performed periodically after 7 days of uninterrupted functioning of the gas generator. Ultimately, the ash was removed outside through the cleaning hole door (12). Syngas produced in the generator was reburned in the reburning chamber (13), together with the excess secondary air which was sucked into the chamber through adjustable shutters. Conforming to the regulations relating to the MWI, the syngas temperature in the reburning chamber was controlled with the use of gas burners (denoted as 15 and 16 in the figure). In the course of the tests, it was, on average, equal to 1000 °C. In the flue gas/water waste heat recovery exchanger (17), the flue gas was cooled to a temperature of 250 °C, whereas the stable water parameters equaled 90/70 °C. Samples of the gaseous products of combustion, used for denoting the quality of exhaust gases and evaluating concentration of HCl, were taken through the probing hole (18) in the outlet channel of the heat exchanger. The observed low emission of CO signaled efficient reburning of syngas. The measurement results are reported in Table 3, and the measured HCl concentrations are analyzed below. In the next stage of the process, the exhaust gases were directed into the cleaning installation and ejected into the atmosphere through the chimney, using an exhaust fan. The installation also featured an emergency chimney (19). The MWI startup involved initial heating of the chambers (1, 7, and 13) by means of gas burners (20, 15, and 16). After reaching appropriate temperatures (Table 3), the process of fuel feeding commenced. During the tests, combustion proceed stably, and the heat output of waste heat recovery boiler was retained. In the temperature of 650 °C, ash did not exhibit the tendency to form heterogeneous structures, such as, e.g., conglomerates. Optimum conditions for combustion, fuel feeding, and scooping of the fuel and the ash of the grate were set according to the directives of the manufacturer of the entire system. As a reference point, the lower heating value (LHV) of the used fuel was taken. The observations made during the experiments showed that the incineration of fuel takes a comparatively much longer time than the process of combustion on the boiler furnace grate. Light fractions of the ash collected in chamber (7), together with the

4. RESULTS AND DISCUSSION A graphical representation of the test results of measurements of HCl concentration in flue gas achieved during the combustion of SRF1 in the stoker-fired boiler is shown in Figures 3−5.

Figure 3. Hydrogen chloride concentration ([HCl]) in dry flue gas, relative to the amount of added PVC (combustion of SRF1-based fuels).

Figure 4. Hydrogen chloride concentration ([HCl]) in dry flue gas, relative to the amount of added Ca(OH)2 sorbent (combustion of SRF1-based fuels).

The corresponding results for the two-stage combustion in MWI are presented in Figures 6−8. In either case, each of the absolute values of (HCl) represents an arithmetic mean of hydrogen chloride concentration in flue gas obtained from three independent readings expressed as mg/Nm3. These values have been used to plot the concentrations of HCl in the form of the trendline equations against the amount of added PVC and Ca(OH)2 sorbent, and the molar ratios of Ca/Cl2. In order to prove the reliability of the experimental data, the deviations of 846


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Figure 5. Hydrogen chloride concentration ([HCl]) in dry flue gas and the parameter κb determining the conversion of fuel chlorine to HCl, relative to the molar ratio Ca/Cl2 (combustion of SRF1-based fuels). Figure 7. Hydrogen chloride concentration ([HCl]) in dry flue gas, relative to the amount of added Ca(OH)2 sorbent (two-stage combustion of SRF1-based fuels).

Figure 6. Hydrogen chloride concentration ([HCl]) in dry flue gas, relative to the amount of added PVC (two-stage combustion of SRF1based fuels).

Figure 8. Hydrogen chloride concentration ([HCl]) in dry flue gas and the parameter κMWI determining the conversion of fuel chlorine to HCl, relative to the molar ratio Ca/Cl2 (two-stage combustion of SRF1-based fuels).

three measurements are shown in Figures 3−8. To enhance the clarity of these drawings, only extreme measured (HCl) values (i.e., the maximum and the minimum values) have been taken into account. The aforementioned figures are supplemented by Tables 4 and 5. The effects of Ca(OH)2 addition on the reduction of HCl concentration in the combustion technologies are compared in Figures 9 and 10. Similarly, Figure 11 and Table 6 illustrate the results relating to the combustion of SRF2 in the stoker-fired boiler. The equations for the trendlines showed in Figures 3, 4, 6, 7, and 11 are given in Table 4. Recall that they are based on arithmetic mean values of [HCl]. Table 5 contains calculated values of theoretical maximum HCl concentrations under different experimental conditions (assuming that all of the Cl in the fuel is converted to HCl) as well as parameters κboiler and κMWI, which determine the conversion of fuel chlorine to HCl without the addition of Ca(OH)2 during SRF1 combustion in a stoker-fired boiler and two-stage combustion process in MWI. In Table 6, we present some comparative data on SRF1 (0.1 and 1.47 wt % PVC) and SRF2 combustion.

Figures 3 and 6 show that, for each established amount of Ca(OH)2 addition, a value of (HCl) was increasing linearly with the amount of PVC added to SRF1. This means that, in fact, almost all supplementary organic chlorine released from the fuel is promptly converted to hydrogen chloride. Relying on trendline equations listed in Table 4 (Figures 3 and 6), one can estimate the influence of the added amount of Ca(OH)2 to SRF1 on the rate (R) of increase of HCl concentration in the flue gas originating from the addition of PVC to the fuel. Accordingly,

R [(mg HCl/Nm3)/(wt % PVC)] = a(wt % Ca(OH)2 ) = α − β[wt % Ca(OH)2 ]

(1)

where (i) α = 55.06, β = 6.41, R2 = 0.9964 for the boiler; and (ii) α = 41.85, β = 12.45, R2 = 0.9956 for two-stage combustion. 847


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Table 4. Trendline Equations figure

a

curve

3 1 2 3 4 4 1 2 3 4

b

[HCl] = a × (wt % 54.73 48.85 42.86 35.37

PVC) + b 62.51 52.03 43.15 34.08

[HCl] = a × (wt % Ca(OH)2) + b −22.40 172.58 −15.84 114.90 −11.88 93.01 −10.23 61.43

R2 0.9977 0.9943 0.9939 0.9824

0.9998 0.9949 0.9986 0.9955

1 2 3 4

[HCl] = a × (wt % 41.95 35.18 30.21 23.03

PVC) + b 15.19 12.76 9.21 7.12

0.9989 0.9994 0.9992 0.9982

1 2 3 4

Φ = a × (wt % Ca(OH)2) + b −30.58 99.12 −17.62 57.51 −10.94 34.53 −6.29 16.24

0.9976 0.9992 0.9986 0.9999

A B C

[HCl] = a × (wt % Ca(OH)2) + b −18.90 142.01 −37.43 142.01 −8.31 55.40

1 0.9945 0.9988

6

7

11

Figure 9. Ratio of hydrogen chloride concentrations β = [HCl]boiler/ [HCl]MWI, relative to the amount of added Ca(OH)2 sorbent (SRF1-based fuels).

It can be demonstrated that the value of R for SFR1 combustion is lower than those resulting from the combustion of extracted rapeseed meal.25 This result proves that, in the reduction of chlorine mobility in the combustion systems, apart from the added Ca(OH)2 sorbent, the calcium inherently present in the tested fuels in a high concentration was involved. Assuming that the linear relationship described by eq 1 is valid for the all values of Ca(OH)2, one can calculate (i) the hypothetical values of R corresponding to all the theoretically possible values of the Ca(OH)2 addition to SRF1; and (ii) hypothetical maximum value of Ca(OH)2, i.e., corresponding to R → 0, which could reach ∼8.6 wt % (combustion) and ∼3.4 wt % (two-stage combustion). In addition, the aforementioned trendline equations imply that the concentration of HCl during the combustion and two-stage combustion

Figure 10. Ratio of hydrogen chloride concentrations β = [HCl]boiler/ [HCl]MWI, relative to the amount of added PVC (SRF1-based fuels).

of SRF1 without the addition of PVC can be expressed as follows:

[HCl] [mg/Nm3] = b(wt % Ca(OH)2 ) = γ − μ[(wt % Ca(OH)2 ]

(2)

Table 5. Stoichiometric and Measured Hydrogen Chloride Concentrations ([HCl]) in Dry Flue Gas under Different Experiment Conditions without the Addition of Ca(OH)2 to SRF1 physical quantity

expression

0 wt % added PVC

0.5 wt % added PVC

1 wt % added PVC

2 wt % added PVC

molar ratio, Ca/Cl2 ((HCl)max)max (mg HCl/Nm3) (HCl)max(MWI) (mg HCl/Nm3) (HCl)max(b) (mg HCl/Nm3) (HCl)b (mg HCl/Nm3) (HCl)MWI (mg HCl/Nm3) κb (%) κMWI (%) ab (%) aMWI (%)

((HCl)max)max = (HCl)λ=1 (HCl)max(MWI) = (HCl)λ=1.57 (HCl)max(b) = (HCl)λ=3.3 (HCl)b = (HCl)boiler (measured) (HCl)MWI = (HCl)MWI (measured) κb = κboiler = (HCl)b/(HCl)max(b) κMWI = (HCl)MWI/(HCl)max(MWI) ab = aboiler = 1 − κb aMWI = 1 − κMWI

8.91 398.1 250.4 117.8 61.5 15.8 52.21 6.31 47.79 93.69

4.53 475.5 299.1 140.7 92.8 33.9 65.96 11.33 34.04 88.67

3.02 553.0 347.9 163.6 114.9 57.1 70.23 16.41 29.77 83.59

1.81 708.2 445.6 209.6 172.4 98.7 82.25 22.15 17.75 82.25

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without the addition of Ca(OH)2. It can be directly represented by the parameter

a = 100% ×

([HCl]max − [HCl]measured ) [HCl]max (4)

= 100% − κ

taking values of ab (combustion) and aMWI (two-stage combustion), corresponding to κb and κMWI. Let us add that ab and aMWI include also chlorine existing in different concentrations in fly ashes, mineral deposition, and elements of furnace lining. It is important that, this way, we also have insight into the fundamental differences between the two combustion technologies, resulting in fundamentally different values of κ and a. The detailed results of calculations are given in Table 5. The aforementioned analysis will provide a useful basis for the chlorine mass balances for the combustion processes. When Ca(OH)2 is added to the fuel, then the parameter a also includes HCl bonding with calcium hydroxide. Referring to Tables 4 and 5, one can calculate the values of κ (i.e., κb and κMWI) and a (i.e., ab and aMWI) against two variables: (wt % PVC) and (wt % Ca(OH)2). For finding functional relationships, if necessary, one can use the method of least-squares. We are now in a position to calculate the percentage rate Rκ of κb decrease corresponding to each percent of Ca(OH)2 added to SRF1, i.e.,

Figure 11. Hydrogen chloride concentration ([HCl]) in dry flue gas, relative to the amount of added Ca(OH)2 sorbent (SRF1 (1.47 wt % and 0.1 wt % PVC) and SRF2 combustion).

Table 6. Parameters κb and ab against the Amount of Ca(OH)2 Added to SRF1 and SRF2 1 wt % 2 wt % 3 wt % 0 wt % added added added added parameter Ca(OH)2 Ca(OH)2 Ca(OH)2 Ca(OH)2 κb ab

76.7 23.3

κb ab

55.4 44.6

κb ab

55.4 44.6

SRF1 (1.47 wt % PVC) 66.9 57.3 46.5 33.1 42.7 53.5 SRF1 (0.1 wt % PVC) 46.5 38.7 30.7 53.5 61.3 69.3 SRF2 40.8 26.2 11.6 59.2 73.8 88.4

relationship x = wt % Ca(OH)2 κb = 76.88 − 10.02x ab = 23.12 + 10.02x

R κ (wt % PVC) =

κb = 55.11 − 8.19x ab = 44.89 + 8.19x

Δκ(wt % PVC) Δ(wt % Ca(OH)2 )

(5)

Let us observe that it is also convenient to use the parameter

κb = 55.40 − 14.60x ab = 44.6 + 14.60x

R[HCl] ⎡⎣(mg HCl/Nm 3)/(wt% Ca(OH)2 )⎤⎦ Δ[HCl] Δ(wt% Ca(OH)2 ) R = [HCl]max × k 100

= where (i) γ = 62.06, μ = 9.42, R2 = 0.9985, (HCl) → 0 when Ca(OH)2 → 6.6 wt %; and (ii) γ = 15.24, μ = 5.56, R2 = 0.9906, (HCl) → 0 when Ca(OH)2 → 2.74 wt %. When establishing the percentage addition of PVC, one can talk about a scale of HCl concentration reduction, with respect to the percentage addition of Ca(OH)2 (see Figures 4 and 7, and Table 4). The HCl concentration decreases linearly, and the declining rate of [HCl] increases with greater amounts of PVC added to SRF1. This means that hydrogen chlorine is quickly bonding with almost all supplementary sorbents. Note that the trendline equations listed in Table 4 (Figures 4 and 7) allow us to analyze the influence of the amount of PVC addition to SRF1 on the rate of HCl concentration decrease in the flue gas resulting from the addition of Ca(OH)2 to the fuel. One can also consider a new parameter,

κ (%) =

[HCl]measured × 100 [HCl]max

(6)

which has a straighforward physical interpretation (see Figures 4 and 7). Its values can be established based in Table 4. In addition, such a parameter as the percentage rate Ra of ab increase related to each percent of the added Ca(OH)2 can be also used, i.e.,

Ra (wt % PVC) =

Δa(wt % PVC) Δ(wt % Ca(OH)2 )

(7)

It is apparent that Ra = Rκ occurs here. The above parameters will be used below when comparing the values of [HCl] during the combustion of SRF1 and SRF2. In Figures 5 and 8, the concentrations of HCl are plotted against the molar ratios of Ca/Cl2. Thus, we have an insight into the effectiveness of the sorbent in dechlorination of gaseous products of combustion. In the same figures (as second y-axis), we also show the conversion of fuel chlorine to HCl passing into the flue gas determined by parameter κ. In addition, Figures 5 and 8 may be used to evaluate the hypothetical effectiveness of other calcium-based sorbents. In order to compare the hydrogen chloride concentrations from the two different combustion systems, the absolute measured values of [HCl] are calculated to the same reference conditions (11 vol % O2 content in the flue gas). The ratio of [HCl] in flue gas leaving the boiler and MWI, i.e., β = [HCl]boiler/[HCl]MWI, is shown in Figures 9 and 10. It means that the stoker-fired boiler (Figure 1) is not adapted to burning waste-derived fuels with the characteristics

(3)

that determines the conversion of fuel chlorine to HCl passing into the flue gas. The theoretical maximum HCl concentrations (calculated by assuming that all of the Cl in the fuel is converted to HCl) depend on the experimental conditions. It results from Table 3 that, for the experiments performed, we have [HCl]max(b) = [HCl]λ=3.3 (combustion) and [HCl]max(MWI) = [HCl]λ=1.57 (two-stage combustion). As mentioned previously, the parameter described by eq 3 is denoted as κb in the former case and as κMWI in the latter case. These two values of κ provide valuable information about the initial capture effect of SRF1 ash 849


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dehydration process Ca(OH)2 → CaO + H2O at temperatures above 400 °C. This, in turn, affects its greater reactivity, especially inside first chamber of gas generator (650 °C), in which the aforementioned neutralization proceeds. Thus, the efficiency of HCl bonding with CaO was much higher than in the boiler furnace, where the temperature reached 1050 °C. Note that only a minor decrease in the dechlorination capacity of CaO in temperatures from 600 to 800 °C can be maintained using the sorbent with a properly formed porous structure.23 At the same time, contact between the sorbent and HCl during gasification of SRF1 was much longer than during the combustion on a grate, because gasification is considerably much slower than combustion. The common-sense assumption that a prolonged contact time between reagents improves the HCl capture efficiency is widely confirmed by quantitative data.28 What is more, a fast heatup of the first chamber of gas generator, along with a long period of sorbent and HCl remain at 650 °C, make the Ca → CaCl2 conversion more efficient. This means that the greatest part of calcium chloride remains in the ash. For comparative purposes, preliminary tests of SRF2 combustion in the same stoker-fired boiler (Figure 1) were conducted. In this case, the entire quantity of HCl that was released into the gaseous zone of the boiler originated solely from the inherent properties of this fuel, and it remained unchanged during the tests. Its mobility in the furnace was reduced using a Ca(OH)2 sorbent, just as in the case of PVC-supplemented SRF1. Assuming that the release of chlorine from SFR2 proceeds in the same manner as observed previously, looking at Table 4 and Figure 3 , one may conclude that SRF2 should behave just like the SRF1 containing ∼1.47 wt % PVC (see Figure 11, line A). However, the obtained results prove that the reduction in HCl concentration is greater (Figure 11, line B). It is possible to show that this fact has reasonable justification. First, let us note that, on the one hand, for SRF2, we do have the following: [HCl]b = 142 mg/Nm3 and (HCl)max(b) = 256.5 mg/Nm3, and, consequently, κb(SRF2) = 55.4% and ab(SRF2) = 44.6%. On the other hand, we can easily find [HCl]max(b) = 185.2 mg/Nm3, as well as κb(SRF1) ≅ 76.7% and ab(SRF1) ≅ 23.3% for SRF1 containing 1.47 wt % PVC. It is apparent from the comparison of the above values of κb and ab that the conversion of fuel chlorine to HCl passing into the flue gas, and the initial capture effect of SRF ash without the addition of Ca(OH)2, are different for SRF1 and SRF2. This is mainly due to a significant difference in the sulfur and chlorine content, as well as differences in the S/Cl ratio. In the second step of the analysis, we are concerned with finding relationships: κb = κb(wt % Ca(OH)2) and ab = ab(wt % Ca(OH)2) for both of the fuels. It should be observed that ab includes now not only chlorine existing in SRF ash, fly ashes, mineral deposition and elements of furnace lining, but also chlorine bonded with calcium hydroxide. The calculated values of κb and ab as well as the relationships are given in Table 6. Finally, one can determine the percentage rate Rκ of κb decrease corresponding to each percent of Ca(OH)2 added to both considered fuels. We find

as given in Table 1. However, the combustion of such fuels can efficiently proceed in a system including a prefurnace and recombustion chamber. It should be noted that, under aforementioned combustion conditions, the trendline equations displayed in Figures 5 and 8 can be readily expressed as

[HCl]boiler = 394.18 exp( −0.7148Ca/Cl2) R2 = 0.9631

(8)

[HCl]MWI = 145.97 exp( −1.2428Ca/Cl2), R2 = 0.9602

(9)

It can be presumed that our experimental results complement the findings of similar research14 involving analyzing contents of heavy metals, Cl, KCl, and NaCl in fly ash during the combustion and two-stage combustion of MSW under almost-analogous conditions. A relatively high concentration of HCl during combustion of SRF1 and SFR2 in the boiler (Figure 1) and low concentration of HCl during two-stage combustion in MWI (Figure 2) are indirectly confirmed by data,21 revealing a considerable difference in the chlorine content in fly ash, which is attributed to different technologies of combustion. It seems that an attempt can be made at interpreting the value of β, along with its relation to the percentage additions of PVC and Ca(OH)2 to SRF1 and SRF2. The process of thermal decomposition of PVC with the release of chlorine readily converted to HCl occurs already at temperatures of >150 °C. It is particularly intensive at temperatures in the range of 600− 1000 °C.11,21,23 Studies of the pyrolysis of plastic waste from electronic equipment showed27 that an intensive release of HCl takes place already at temperatures of 200−400 °C. As far as the efficiency of HCl bonding with Ca(OH)2 is concerned, it reaches high values at temperatures of 600−800 °C but diminishes when the temperature exceeds 650 °C.21 It means that the higher the temperature, the less the HCl bonding can keep pace with the release of chlorine in the form of HCl during SFR1 and SRF2 combustion. The ensuing CaCl2 is partly decomposed in high temperatures, causing the re-release of HCl; the entire sequence of these processes occurs faster at higher temperatures. Apart from that, PVC frequently contains chalk, which is used as a filler. Therefore, the released HCl reacts with chalk or CaO resulting from the decomposition of CaCO3, creating a conglomerate of compounds containing calcium chloride, accordingly to the following reactions:

(i): CaCO3 + HCl = Ca(OH)Cl + CO2 (ii): Ca(OH)Cl + HCl = CaCl2 + H2O (iii): 2CaO + 2HCl = CaO·CaCl2 + H2O SRF1 and SRF2 combustion on a grate proceeded at a temperature of 1050 °C, which is higher than in the case of gasification (650 °C), which caused the release rate of HCl to be much higher than the rate of its bonding with the calcium sorbent. The quality of blending the fuels with PVC and Ca(OH)2 may not be without certain impact. Even though our experimental results are not expressed in terms of the porosity of Ca(OH)2, this sorbent forms a more-extended, porous structure during the transformation to calcium oxide in the 850

⎡ ⎤ % ⎥ R κ(SRF1) ≡ 10.02⎢ ⎢⎣ % Ca(OH)2 ⎥⎦

(10a)

⎡ ⎤ % ⎥ R κ(SRF2) ≡ 14.60⎢ ⎢⎣ % Ca(OH)2 ⎥⎦

(10b)


Energy & Fuels

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fuels from waste have an ash content of up to 30% (dry basis). On average, however, it oscillates at ∼20%. In fact, Table 1 shows that SFR1 and SFR2 contain not only elements such as sulfur, chlorine, calcium, potassium, and sodium, but also significant amounts of other ash-forming elements that could eventually influence the concentration of HCl. The dominant component of ash is SiO2, the content of which can amount to several tens of percent. Consequently, chemical reactions occur that produce calcium silicates. A significant role is played by CaCl2, which appears in the process of the chlorine bonding by the calcium-based sorbents. This issue is discussed more extensively in ref 29. from which the following example reactions are quoted:

We also obtain

⎡ mg HCl/Nm 3 ⎤ ⎥ R[HCl]b(SRF1) ≅ 18.56⎢ ⎢⎣ % Ca(OH)2 ⎥⎦

(11a)

⎡ mg HCl/Nm 3 ⎤ ⎥ R[HCl]b(SRF2) = 37.45⎢ ⎢⎣ % Ca(OH)2 ⎥⎦

(11b)

It is evident that the values described by eqs 11 can be immediately obtained from the course of lines A and B in Figure 11. The HCl concentration in flue gas decreases faster against the weight percentage of added Ca(OH)2 during SRF2 than during SFR1 combustion. As mentioned previously, it seems that such a difference can be primarily attributed to a very low sulfur content and a high chlorine content in SRF2, compared to SRF1 (see Table 1). In the case of SRF2, the calcium sorbent reacts mainly with HCl, dechlorinating the gaseous zone of the boiler, and being neutralized to a minor degree during the desulfurization. By comparing the values described by eqs 11, one can roughly estimate the percentage of the total amount of Ca(OH)2 added to SRF1 (see Figure 11, line A) which reacts with HCl and is consumed during the bonding of SO2/SO3. It cannot be excluded that different rates of thermal decomposition of various chemical compounds contained in fuels such as SFR1 (1.5 wt % PVC) and SFR2 can eventually translate into different rates of HCl release. These considerations may also be based on the comparison of the behavior of SRF2 and SRF1, containing ∼0.1 wt % PVC (see Figure 11, line C). We postulate the choice of such SRF1 (see Table 6):

CaCl2(s,c) + SiO2 (s) + H2O(g) = CaSiO3(s) + 2HCl(g),

+ 2HCl(g),

+ 2Cl2(g),

⎡ mg HCl/Nm 3 ⎤ ⎥ R[HCl]b(SRF1) ≅ 10.13⎢ ⎢⎣ %Ca(OH)2 ⎥⎦

(13b)

These values can be immediately obtained from line C (described in Table 4) in Figure 11, the position of which, relative to lines A and B, is clear. As for the relationship between HCl concentration and the molar ratio (Ca/Cl2) for SFR2, it may be expressed in the form of a linear function,

[HCl] = 312.40 − 38.77(Ca/Cl2) Ca/Cl2 ∈ 4.42; 7.28 ; R2 = 0.9946

(16)

c(O2 ) = 5%

(17)

Other components of ash include Al2O3, Fe2O3, MgO, Na2O, K2O, and P2O5. Their content ranges from several percent to several dozen percent. These components can capture HCl in physical processes and in chemical reactions. It is also possible that HCl is subject to occlusion. Moreover, there is some probability that HCl, similarly to CO2 and nitrogen, can form microspheres with aluminosilicates. Planning the experiments, it was assumed that the above processes are of marginal significance. This is because the reaction between HCl and CaO whose content in the combustion chamber is substantially higher than other oxides in forming ashis of primary importance. It comes as a result of the dehydration process of the Ca(OH)2 added to fuel. One of fundamental advantages of combusting waste-derived fuels in two-stage installations (gas generator−recombustion chamber) is the fact that appearance of high-temperature corrosion centers on the steel structural elements, mostly those of the waste heat recovery exchanger, is greatly reduced. The explanation of this phenomenon, as suggested by the relevant literature,12 lies in low temperature (650 °C) in the reaction chamber of the gas generator. In such a temperature the majority of alkali and alkaline-earth metal chlorides, including calcium chlorides, exist in the solid phase, and accordingly are found among solid residues. This fact accounts for a long lifespan of such steel structural elements of the waste heat recovery exchanger as, e.g., tube-sheet bottom and smoke tubes.

The values of κb = κb(wt % Ca(OH)2) and ab = ab(wt % Ca(OH)2) and their linear approximations are given in Table 6. In this case, we have

(13a)

c(H2O) = 1%

2CaCl2(s,c) + 2SiO2 (s) + O2 (g) = 2CaSiO3(s)

(12)

⎡ ⎤ % ⎥ R κ(SRF1) ≡ 8.23⎢ ⎢⎣ %Ca(OH)2 ⎥⎦

(15)

CaCl2(s,c) + SiO2 (s) + H2O(g) = CaSiO3(s)

κb(SRF1, Ca(OH)2 = 0) = κb(SRF2, Ca(OH)2 = 0) = 55.4%

c(H2O) = 20%

(14)

5. CONCLUSIONS The results of large-scale experiments can be used to establish the maximum addition of PVC-containing waste to SRF1 and SFR2, as well as the percentage addition of Ca(OH)2 to these fuels, accordingly to the given amount of added PVC and concentration of HCl in flue gas. Effectiveness of HCl bonding with Ca(OH)2 has a significant impact on formulation of requirements related to flue gas cleaning systems and, finally, on the choice of HCl emission control technology. With regard to the concentration of HCl in flue gas, the experiments have shown that, from the perspective of limiting the consequences of HCl

due to small variations of Ca/Cl2. To a small degree, the difference between the amounts of [HCl] released into the gaseous zone of the boiler during the combustion of the considered fuels is due to their diverse contents of ash. Table 1 specifies that SRF1 has an ash content of 17.2 wt %, whereas SRF2 has an ash content of 18.1 wt % (dry basis). This means that these ash contents differ rather insignificantly. Nonetheless, it must be admitted that the ash contained in the analyzed fuels could influence the quantity of HCl released into the gaseous zone of the boiler. In general, 851


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control of distribution of this part of the combustion air that is fed onto the grate to extend the gasification zone; (ii) regulation of primary air injection onto the grate in the combustion zone; and (iii) control of turbulent injection of secondary air. These actions are aimed at improving combustion of high-chlorine fuels, as they increase the boiler efficiency through the decrease in both the values of λ and the concentrations of CO and O2 in exhaust gases resulting from HCl bonding with Ca(OH)2 in the ignition zone of the grate. Promising outcomes can be expected from the coupling of a gas generator with a pulverized fuel boiler fueled with hard coal. In such a configuration, the gas generator works as a prefurnace. The rationale behind studying the behavior of the SRF lies in the fact that fuels for eco-friendly power generation must meet strictly defined operational requirements, while, at the same time, preserving calorific (economic), technological, and emissive (ecological) CEN standards.19,20

release from SRF, two-stage combustion technology involving gasification (650 °C) and syngas recombustion (1000 °C) is much more effective than combustion of the same fuel from waste in a stoker-fired boiler (1050 °C). In particular, it is important from the viewpoint of potential threat of both high-temperature corrosion of steel structural parts of the flue-gas/water heat exchangers and low-temperature corrosion of flue-gas extractor equipment. It should be also noted that, comparing combustion processes in the two aforementioned systems, one arrives at the conclusion that the values of λ, together with concentrations of CO, CO2, and O2 in exhaust gases are significantly different (see Table 3). This corresponds with the results presented in ref 30, which proves that an increase in HCl concentration in boiler furnace environment inhibits the oxidation of CO to CO2. Such a phenomenon does not occur in the case of two-stage combustion, where the most of HCl is captured in the gas generator. Higher effectiveness of flue gas dechlorination in the twostage process of SRF1 combustion can be explained by the lowtemperature prevailing in the gas generator chamber (650 °C). It is claimed28,29 that temperature is a control parameter in HCl bonding processes by means of calcium sorbents. The mentioned sources suggest that, when it grows above 750 °C, sorption capacity of the sorbents declines, leading to the weakening of HCl bonding reaction. Presumably, this phenomenon comes as a result of a relatively low CaCl2 melting temperature, which is 772 °C. It has been observed that thus forming phase of liquid CaCl2(l) becomes saturated with CaO molecules (CaCl2(l)−CaO) beginning with a temperature of 750 °C, blocking the access of flue gases, including HCl, to the engulfed calcium sorbent. In the temperatures between 700 and 900 °C, there is only little reactivity of the sorbent toward HCl. The peak activity of CaO corresponds to the 500−600 °C temperature bracket, whereas, at 1000 °C, the sorbent is assumed to exhibit only minute activity.29 In the case of the two-stage combustion process, HCl can be secondarily formed in the reburning chamber in the environment of SO2, O2, and H2O, according to the reaction CaCl2(l) + SO2(g) + O2 → CaSO4(s) + Cl2(g) or 2CaCl2(l) + 2SO2(g) + O2 + 2H2O → 2CaSO4(s) + 4HCl(g). As for the furnace of the stoker-fired boiler (T = 1050 °C), the effective capture of HCl by Ca(OH)2 → (400 °C) CaO + H2O occurs most probably on the grate, where the temperature of the fuel bed is lower than the melting temperature of CaCl2. Unfortunately, during the experiments, the distribution of temperature along the grate was not measured. Thus, it can be speculated that the process of HCl capture took place in the ignition zone of the grate, in the drying, pyrolysis, and gasification zones. In addition, in the convective part of the boiler, similarly as in the recombustion chamber, there are conditions for secondary formation of HCl in the environment of SO2, O2, and H2O, which may have an impact on the course of the lines A, B, and C in Figure 11. Based on the obtained experimental results, one can also speak about different technologies of combustion of high-chlorine fuels from waste. Two-stage combustion offers the highest number of advantages resulting from the specific process conditions in the first chamber of a gas generator. In addition, it is possible to control the high-efficiency processes occurring in the recombustion chamber. As for the stoker-fired boiler with a relatively small heat output, the situation is somewhat different. If the concentration of HCl in the furnace should be reduced to the level below the experimental data, then the boiler must be subjected to modernization, which may actually serve as a solution to potential problems related with its further operation. The improvements may include (i) reconstruction of the ignition zone, with the

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48857469205, +48857469200. E-mail address: [email protected].

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Higher Education, Poland (through Grant No. R06 018 02). REFERENCES

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