From Laboratory Tests to the Full-Scale Combustion Unit

At present, natural limestone is used for the desulphurization of waste gases from the combustion of fossil fuels. However, it is important to save al...
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Energy & Fuels 2008, 22, 3080–3088

Potential of Alternative Sorbents for Desulphurization: From Laboratory Tests to the Full-Scale Combustion Unit Zbyszek Szeliga,† Dagmar Juchelkova,† Bohumir Cech,† Pavel Kolat,† Franz Winter,‡ Adam J. Campen,§ and Tomasz S. Wiltowski*,§ Department of Energy Engineering, Technical UniVersity of OstraVa (VSB), 70833 OstraVa, Czech Republic, Technical UniVersitysVienna, A-1040 Wien, Austria, and Department of Mechanical Engineering and Energy Processes, Southern Illinois UniVersity, Carbondale, IL 62901 ReceiVed March 26, 2008. ReVised Manuscript ReceiVed June 15, 2008

At present, natural limestone is used for the desulphurization of waste gases from the combustion of fossil fuels. However, it is important to save all primary resources, such as limestone, for the future. The researchers focused on finding alternative sorbents for the purpose of desulphurization in a dry additive method, which would become the alternative for natural limestone. This paper is primarily focused on desulphurization tests of selected substances. Tests were initially conducted on the laboratory scale, followed by pilot and full-scale combustion units.

Introduction Alternative sorbents, such as waste substances and byproduct from the paper production industry1 (caustic sludge) and the sugar production industry2 (saturation sludge) (Table 1), can be used to replace the most frequently used sorbent, natural limestone. The reuse of such waste products will reduce the use of naturally occurring sorbents and hence limit the depletion of valuable raw materials. Caustification Sludge from Paper Mill Production. In principle, the caustification process is an important part in paper mill technology, resulting in the transformation of sodium carbonate, generated in the combustion of black precipitated waste liquor, to sodium hydroxide. This occurs with the addition of limestone, which is slaked directly in green liquor. This reaction takes place in two stages CaO + H2O ) Ca(OH)2

(1)

Ca(OH)2+Na2CO3)2NaOH + CaCO3

(2)

Limestone is slaked in green liquor. The suspension is then sent through a series of causticizers into the sedimentation tanks with white liquor, where the sludge is separated. The caustification sludge is washed and vacuum-filtered. Finally, it is calcined in the limestone kiln to produce calcium oxide. The calcination process for the production of CaO from the caustification sludge is similar to the process used at the limestone works. In principle, it is a thermal decomposition of calcium carbonate. CaCO3)CaO + CO2

(3)

* To whom correspondence should be addressed. E-mail: [email protected]. † VSB-Technical University of Ostrava. ‡ Technical UniversitysVienna. § Southern Illinois University. (1) Milichovsky´, M. Basic Chemistry of the Paper Technology, 2nd ed.; VSˇCHT: Pardubice, Czech Republic, 1991; 163, ISBN 80-85113-31-7. (2) Bubnı´ka, Z.; Geblera, J. Introduction to the Sugar Industry Technology, 1st ed.; Sugar Chemistry and Technology Department VSˇCHT Praha a VUC Praha: Prague, Czech Republic, 2006; ISBN 80-23973-15-0.

In comparison to limestone used at the limestone works, the caustification sludge contains 45-55% water, which should be evaporated. For the purpose of drying and calcinations, rotary furnaces or fluidized reactors are used. At large plants, calcium oxide is regenerated from calcium carbonate and recycled again for the preparation of cooking liquor. However, the recycling of calcium carbonate is not always continuous, and in smallscale plants or plants using older technologies, the caustification sludge is simply disposed as a special waste generated in the production of cellulose by the sulfate method. This waste is extensive. For example, the waste disposal site at the paper mill in Sˇteˇtı´ has a lagoon of approximately 5 ha deposited to a depth of 10 m during a decade of paper mill production. Saturation Sludge from Sugar Industry. Saturation sludge is produced as a byproduct in the production of sugar from sugar beets. At sugar factories, both burnt limestone and carbon dioxide are used for the treatment of sugar juice. The basic raw material is lump saturation limestone, which is burnt at a temperature of 1100 °C and is decomposed to carbon dioxide and calcium oxide. The fuel is coke made of the Ostrava hard coal. The consumption of coke is 7-10% of limestone weight. Limestone is burnt in the shaft furnace and is fed from the top. The charge is gravity-fed, and the calcined limestone is removed at the bottom of the furnace. The combustion air needed for coke burning is blown into the lower part of the shaft furnace. The produced waste gases (saturation gas) are extracted in the upper part of the shaft furnace. The saturation gas contains 28-32% of carbon monoxide and 1.5-2.5% of oxygen, with the remainder being nitrogen and trace amounts of sulfur dioxide. Dependent upon the excess combustion air, the saturation gas may contain up to 1% carbon monoxide. However, the production of CO is not desired. Then, the saturation gas is cooled and sprayed, thus removing dust particles and acidic components in the gas, primarily SO2. A suspension of CaO, 215 kg m-3, is prepared from pure or sweet water. The following processes that are involved in the purification of sugar juice: (i) precipitation of high molecular sugars, pectin, proteins, etc., (ii) precipitation of anions in the form of limestone

10.1021/ef800218t CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

AlternatiVe Sorbents for Desulphurization

Energy & Fuels, Vol. 22, No. 5, 2008 3081 Table 1. Chemical Analysis of Alternative Sorbents

sample

SiO2 (%)

Al2O3 (%)

FeO (%)

Fe2O3 (%)

CaO (%)

Na2O (%)

H2O (%)

saturation sludge A, sugar factory caustic sludge B, Frantschach Steti caustic sludge C, Chomutov

3.24 0.78 0.98

1.14 0.91 1.37

0.14 0.14 0.14

0.31 1.09 0.31

51.37 53.93 53.91

0.003 0.530 0.310

45.23 42.83 43.39

salts (phosphates, sulfates, etc.), (iii) precipitation of magnesium ions, and (iv) inversion sugar decolorizing with the production of lactic acid and formic acid. Then, the precipitation is removed with the addition of saturation gas, and Ca(OH)2 is transformed into crystalline CaCO3, which simultaneously serves as the suitable filtration agent on which surface color substances and precipitated nonsugar substances are caught. The abovementioned processes take place in two stages of predefecation and final defecation with subsequent saturation. After saturation, the saturation sludge is separated by sedimentation and filtration. The produced saturation sludge is a significant byproduct from sugar production. From 1 ton of raw sugar beet, the 130 kg of sugar is produced on average. According to the literature, it is 6-8% of saturation sludge for defecation per 1 ton of beet. According to the state-of-the-art technique, the value is approximately 5.5%. The existing use of saturation sludge is as follows: (i) Fertilizer for treatment of acidic reaction of soils. For direct spreading on the field, the dry matter shall be about 70%. The organic mass in sludge is easily transformed into humus, and it is an ideal substrate for microbiological activity. The use of sludge improves soil structure. (ii) Fertilizer for gardeners with the addition of approximately 10% of limestone powder. (iii) Mineral additive into fodder mixtures after mixing with pressed cuttings and molasses. The average balance of saturation sludge production in the Czech Republic is approximately 211 000 tons/year. At present,

Table 2. Chemical Analysis of VFK 55 and VFK 80 SiO2 (%) Fe2O3 (%) Al2O3 (%) CaO (%) MgO (%) SO3 (%) VFK

0.74

0.06

0.22

54.82

0.42

0.05

Table 3. Grain Size of VFK 55 size (mm) 0.045 0.063 0.071 0.09 0.1 0.2 0.5 0.6 0.7 1.0 (%) 69.0 60.9 58.5 55.1 53.5 35.3 7.4 3.3 20.0 0.4

the saturation sludge is mostly used as a fertilizer for agricultural fields. The sludge is continuously transported during the period of sugar beet campaigns, because the sugar factories have no storage areas. The transport vehicles that arrive with sugar beet in the sugar factory take away the raw sludge. To a smaller extent, the saturation sludge is used as an additive for fodder mixtures for animals. However, the saturation sludge is not fully used. This substance could be better used for desulphurization purposes. In addition, the use of this substance by sugar factories could be financially beneficial and bring economic improvements for sugar factories. The high content of CaCO3 (and/or CaO) and high content of water are striking features of these substances. The other important parameter of these substances identified as alternative sorbents is their granularity and their behavior during use. The grain size analysis for saturation sludge is shown in Figure 1. Figure 2 is a microscope photo of the saturation sludge. According to analyses, these substances are ultrafine (see the literature).4,5,8,9,15 Limestone VFK 55 and VFK 80. As a reference substance to alternative sorbents, caustification sludge, and saturation sludge, the limestone from the Sˇtramberk location was used, and it is identified as VFK 55 and VFK 80. Chemical composition (Table 2) and grain size (Table 3) of these types of limestone (VFK 55) are shown. These types of limestone are used for desulphurization in some fluidized-bed boilers in the Czech Republic. The abbreviation VFK means a trademark limestone for fluidized-bed boilers, and the number following is the percent oversize on a 90 µm screen. Results and Discussion

Figure 1. Granular analysis of A (saturation sludge) in micrometers.

Figure 2. Saturation sludge A.

After general research into chemical and physical properties of the investigated waste substances suitable for the dry additive (3) Barret, P. Kinetics of the Gas-Solid Reactions; Academia: Prague, Czech Republic, 1978. (4) Bis, Z. Circulating Fluidization of Poly-dispersion Mixtures; Treatise 63, Wydawnictwo Politechniki Czestochowskiej: Czestochowa, Poland, 1999; ISBN 83-7193-086-0. (5) Bis, Z.; Gajewski, W.; Leszczyn´ski, J.; Nowak, W.; Slomian, C.; Werther, J.; Reppenhagen, J. Utilization of ultra-fine sorbents for immediate desulphurization in circulating boilers. Conference VIII Konferencja Kotłowa, Gliwice, Poland, 2000. (6) Cˇech, B.; Juchelkova´, D. Potential utilization of waste sorbents for ˇ R 101/ desulphurization in fluidized-bed combustion facilities. Project GAC 00/0168, Final Report. (7) Hrazdira, J.; Kusa´, H. Utilization of residual limestone for environmental purposes. Conference of VSˇB-TU. Waste Recycling V, 26.10.2001; pp 131-133, ISBN 80-7078-884-4. (8) Lahovsky´, J. Limestone products for desulphurization of flue gases. Report, Vapenicky´ Seminar, Losˇtice 13.10.1993. (9) Miller, B. G.; Romans, D. E.; Scaroni, A. W. Characterization of limestones for FBC systems, SO2 Emission Control Conference, National Stone Association, Pittsburgh, PA, Sept 9-11, 1990. (10) Miller, B. G.; Weber, G. F.; Gass, D. J. Pilot-scale evaluation of furnace sorbent injection for SO2 control firing Canadian low-rank coals. 14th Biennial Lignite Symposium, Dallas, TX, May 18-21, 1987.

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Α2 ) [CSO2 max(τ2 - τ1)] - Α1

Table 4. Results of Conversion in RRU tested substance

conversion (%)

saturation sludge A caustic sludge (Sˇteˇtı´) B caustic sludge (Chomutov) C VFK 55 VFK 80 Ca(OH)2

70.2 51 50 38 32 53.3

method of desulphurization in fluidized layer and their study,3,11,12 the authors found out the already mentioned set of substances. These substances are appropriate for the desulphurization process not only from the viewpoint of the above-mentioned properties but also because of their availability. Their availability may be one of the criteria for their real implementation in heavyduty facilities. Prior to the application of alternative sorbents in the dry additive method of desulphurization in real equipment, it was necessary to test the behavior of the sorbents in laboratory conditions. For this purpose, the pilot units at TUsVienna and the Czech Academy of Sciences were chosen. The composition of pilot tests was selected primarily for the purpose of a suitable combination of gained knowledge. Fixed-Bed Reactor Tests. Pilot tests started with verification tests of conversion (SO2 sorption capability) of alternative sorbents and conventional limestone used in fluidized technology as reference substances on the equipment with immobile layer, at the laboratory of TUsVienna. The sorbents mentioned in Table 1 and limestones VFK 55 and VFK 80 were used for the tests. The equipment consists of an electrically heated reactor, immobile bed, and system for time measurement and analysis of outlet gases, primarily the content of SO2. The experiments were carried out at 850 °C, with sorbent loading of 0.25 g and gas linear velocity of 0.8 m/s. During the tests, the researchers had to cope with the problem of coagulation of the samples. To overcome this obstacle, siliceous sand in a ratio of 1:9 was added to the samples. Then, this solution guaranteed approximately the same porosity of the layer and, thus, the same velocity of gas flow through all samples. The prepared sample (a mixture of the given sorbent of constant weight and siliceous sand in the given ratio) was positioned on the grate, uniformly spread over and incrementally heated up to the temperature of 850 °C, while N2 was simultaneously flowing as the inert gas. After stabilization of temperature in the reactor, nitrogen was replaced by a mixture of gases (SO2 ) 790 ppm, CO2 ) 15%, O2 ) 6%, and a nitrogen balance). Gas concentrations were constant during the whole period of tests and were monitored on the outlet from the unit. The conversion of SO2 in the given time interval for all substances was chosen as the parameter for evaluation of results from the tests. The SO2 conversion was calculated according to the following equations: conversion )

Α1 Α1 + Α2

(4)

where Α1 )

∫

τ2

τ1

CSO2dτ

(5)

(11) Morrison, J. L.; Romans, D. E.; Liu, Y.; Hu, N.; Pisupati, S. V.; Miller, B. G.; Miller, S. F.; Scaroni, A. W. Evaluation of limestones and dolostones for use as sorbents in atmospheric pressure circulating fluidizedbed combustors. Final Report, PEDAFR-893-4016, June 24, 1994; p 124. (12) Vejvoda, J.; Hrncˇ´ırˇ, J. The use of limestone for reduction of ´ VP exhalations of sulphur oxides from combustion processes. Treatise U cˇ.50.1978.

(6)

These results from the tests are shown in Table 4 and Figure 5. It has to be noted that conversion of Ca(OH)2 was not calculated for the CFBC model, because determining the calcium hydroxide concentration could not be preformed accurately. From the results obtained, it is apparent that the conversion of tested alternative sorbents is greater than that of traditional limestone. This may be attributed to the granularity of the mentioned substances and, thus, to their large, specific reaction surface (Figures 1 and 2). This test confirmed assumptions that these substances may be implemented in dry additive desulphurization. Then, in following tests, it was necessary to investigate these substances in a real process, i.e., in the fluidization process and with the related dwell time of the ultrafine substances in the space of fluidized layer and conditions suitable for the desulphurization reaction. According to theoretical assumptions, the efficiency of the desulphurization process is most likely a question of the ratio between the kinetics of reaction, its velocity, and the dwell time of the particles in the fluidized layer.3,12 Tests on the Model of a Fluidized Furnace with Circulating Layer Circulating Fluidized Bed (CFB). After general information on the behavior of alternative sorbents in the process of dry additive desulphurization with a stationary layer was obtained, the next step for identification of properties of alternative sorbents was made on a model of boiler with a circulating fluidized layer. A chart of pilot equipment located at the laboratory of TUsVienna is shown in Figure 3. The equipment represents a simplified model of the fluidized-bed boiler with a circulating layer. The equipment is thermally insulated, and the temperature inside the equipment is maintained by electrical heating. The simulation of flow of waste gases through the chamber is provided with a mixture of gases. The mixture was supplied to the chamber as the fluidizing medium. The composition of simulated waste gases was SO2 ) 525 ppm, CO2 ) 15%, O2 ) 6%, and the balance was nitrogen. The composition of gas and a temperature 850 °C were maintained at constant values during the tests. As in the previous tests, siliceous sand was used as bed material. After stabilization of all general parameters of unit, the sorbent was one-off-dosed into a fluidized layer. Then, the response was observed as the drop of concentration of SO2 at the outlet of the unit. Results from the tests are illustrated in Table 5. The evaluation of results was subjected to the same criterion as in the RRU tests, i.e., the evaluation of conversion of monitored substances in the specified time interval. The time dependence of the SO2 concentration at the outlet from the unit after implementation of the sorbent batch is shown in Figure 6. When the results of conversion obtained on the fixed-bed reactor are compared to CFBC for the given substances, it is clear that comparable results were obtained on both units. It is very interesting that the conversion of alternative sorbents is better in the process of dry desulphurization on the model with a fluidized layer than in the case of traditional limestone used in the fluidized furnaces. The residence time of alternative sorbents in fluidized layers is shown as being different than what was expected. During these tests, the residence times of sorbent particles by means of the response of concentration of SO2 and other relevant quantities were observed. According to analyses (Figure 2), these substances are ultrafine. The residence times of these materials in the fluidized layer during the test did not correspond to the theoretical residence time.15 During the investigation of this phenomenon, it was concluded that the investigated ultrafine

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Energy & Fuels, Vol. 22, No. 5, 2008 3083

Figure 3. Model of CFB.

sorbents will stick on the particles of bed material that then act as a “carrier” of these grains, and they together circulate in the layer.4,14 Pilot-Scale 100 kW Fluidized Bubbling Bed (FBB) Tests. To approach near full-scale equipment for the purpose of testing, measurements were carried out on the equipment at ´ CHP, a pilot laboratory unit with a rated output of AV CˇR U 100 kW with a stationary fluidized layer (Figure 4). Desulphurization tests took place with the combustion of a mixture of Sokolov brown coal, with individual alternative sorbents and reference substances, natural limestones. The mixtures (coal plus sorbents) were prepared in a molar ratio of Ca/S ) 2.4:1. Then, every prepared mixture was supplied before its testing into the bunker number 1, from which the mixture was dosed by a screw feeder into the combustion chamber of corresponding weight flow. When the reserve of the prepared mixture was exhausted, the dosing with clean coal began from bunker number 2. In this time interval of coal dosing, a new mixture was prepared (13) Harasek, M.; Schausberger, P.; Jordan, C.; Winter, F. Exit geometry effects on the fluidization on fine particles in CFB reactor: Analysis by semi-theoretical and CFD modeling. Chemie Ingenieur Technik (ECCE), 6 2001; 132, S. 628. ˇ KD (14) Husta´k, P. Modeling of processes at fluidized-bed combustion. C Dukla. (15) Szeliga, Z.; Blejcharˇ, T.; Sta´nˇa, M. Residence time of fine grined particles in fluidised bedsNumerical vs. real model. Conference Power Engineering and Enviroment, Modern Energy Technologies and Renewable Energy Resources, Ostrava, Czech Republic, 2007; pp 215-223.

and supplied into bunker number 1. Then, the combustion of coal itself gives the opportunity to obtain data for the determination of the level of SO2 without a supply of sorbent. Also, there is the possibility to eliminate the influence of tests by sorbents from the previous tests. Results from the tests are the average concentration of SO2 during the course of individual tests and the time dependence of the SO2 concentration during the tests (Figure 7). From the obtained results, one may already see the effect of furnace size. Also, the obtained results are in agreement with the results from the laboratory units mentioned above. In comparison to traditional limestone, the alternative sorbents show very good properties in the desulphurization process. Desulphurization Tests in a Circulating Fluidized Bed Combustor (CFBC) Output Power Rated 120 MW. To conclude the investigation of alternative sorbents, the desulphurization test with saturation sludge (A) in a real combustion unit (fluidized-bed boiler with circulating fluidized layer) was carried out.6 The test was carried out on the boiler with a rated output of 120 MW [Compact system, Foster Wheeler by the Department of Power Engineering, Technical University of Ostrava (VSB)]. No modifications were carried out on the equipment or boiler technology for the described test. Natural limestone, a conventional sorbent, is pneumatically dosed into the boiler into four places where the raw fuel is fed into the boiler combustion chamber. Because of its humidity, the saturation sludge cannot be conveyed into the boiler in the

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Figure 4. FBB with an output of 100 kW at the Institution of Chemical Processes of the Czech Academy of Sciences.

Figure 5. Time behavior of SO2 at the outlet from fixed-bed reactor. Table 5. Results of Conversion in CFBC tested substance

conversion (%)

saturation sludge A caustic sludge (Sˇteˇtı´) B caustic sludge (Chomutov) C VFK 55 VFK 80

26.6 22.5 18.4 14.3 14.1

same way as natural limestone. Therefore, it was decided to mix the raw saturation sludge together with coal in an approximate molar ratio of Ca/S ) 2:5, because it is the longterm average for the limestone. The mixture of saturation sludge with coal was prepared in the underground tank. The rear part of the tank contained approximately 170 tons of brown coal,

and the front part of the tank contained 25 tons of saturation sludge. The discharging truck in the rear part discharged coal onto the belt. The truck in the front part dosed the sludge onto the belt with coal in the corresponding weight ratio. It was not the most technical approach, but it was the simplest solution and the only one accepted by the operator (the owner of the facility). This solution requires the minimum costs for performance of the tests and/or for potential long-term operation. The following quantities were measured and observed during the test: (i) measurement of basis gaseous components of emissions CO, NOx, SO2, and relating O2, (ii) measurement of solid emissions (solid contaminants), and (iii) samples of ashes, flue ashes, and fuel were taken for other analyses The following criteria were chosen for test evaluation: (i) the course of SO2 concentration, as the means for evaluating the particular course of the dry additive method, (ii) behavior of the fluidized layer, and (iii) evaluation of the effect of saturation sludge as the substitute sorbent on residues after combustion. From the obtained data from the course of SO2 concentration (Figure 8), it is apparent that, after the limestone dosing and simultaneous supply of saturation sludge/coal mixture into the combustion chamber were stopped, the emissions increased and then further significantly decreased. In the next time cycle, the monitored component of waste gases rapidly increased; the reason for that was the nonhomogeneity of the produced mixture of saturation sludge with coal. According to a visual check, it may be assumed that coal itself was dosed into the boiler in this moment with the minimum quantity of saturation sludge as the sorbent. After this fluctuation, the drop occurs proportionally to the increasing supply of sludge into the combustion chamber. From 13:27, the unit operation stabilized and the test could continue. Dosing of saturation sludge as one sorbent only into the fluidized layer appears satisfactory, although the variability of its flow is quite significant. The test itself under the necessary condition of constant output parameters was running in the time interval from 13:27 to 15:55. In general, this test may be assumed satisfactory as far as the stability of operation and dry additive desulphurization with saturation sludge is concerned. It may be stated that this test was affected by nonhomogeneity of the mixture of coal with sludge. From the results in the time interval from 13:00 to 15:55, it may be concluded that the desulphurization with saturation sludge is possible and applicable with minimum costs for reconstruction of conveyance routes for input materials. In the case of using the above-mentioned system of conveyance of saturation sludge, it is necessary to take into account its nonuniform flow to the combustion chamber. From the observance of SO2 emissions, the dosing of traditional limestone into the desulphurization process in time intervals of insufficient supply of alternative sorbent is necessary. According to data of operating measuring instruments and in direct comparison to operation modes for conventional sorbent and saturation sludge as the total replacement for natural limestone, it is apparent that the supply of saturation sludge as an additive into the combustion chamber did not cause any problems in relation to the behavior of the fluidized layer or boiler systems during the short-term test. It is evident that the granulometric composition of material in the fluidized layer affects an intrinsic behavior of the fluidized layer and also the decomposition of the temperature field in the combustion chamber and other parameters. The granulometry of saturation sludge differs from the granulometry of limestone. However, no important deviations were observed in

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Figure 6. Time behavior of the concentration of SO2 at the outlet from the model of CFB after dosing the sorbents.

ˇ AV, with Figure 7. Time behavior of the concentration of SO2 in waste gases during desulphurization tests on boiler FBB 100 kW, UCHP C alternative sorbents and limestone.

the monitored quantities during the tests, in comparison to the normal operation of the given unit. The saturation sludge coagulated because of its moisture. Some of these formations were identified even in the bed ash. Therefore, it can be stated that during the test such structures did not quite erode in the fluidized layers and the reaction did not occur through the cores of such formations. Of course, this effect has a negligible influence on the total effect of desulphurization. The peaks of emissions of SO2 were monitored during the test; the reason was the already-mentioned nonhomogeneity of the mixture. Further long-term operation may prove the applicability of saturation sludge and verify other factors that were not examined in the short-term test. On the basis of the experience gained during the tests, it is

apparent that the chosen method of dosing of saturation sludge does not guarantee its constant supply to the process of dry additive desulphurization. It is caused by many factors, especially its transportability into the combustion chamber. However, the described process is acceptable by the operator with minimum costs. For future operation, this alternative in connection with the additional supply of traditional limestone seems to be feasible. Then, this method will be used for a long-term desulphurization test. Desulphurization Test in the Powder Boiler Output Power Rated 72.2 MW. The desulphurization test6 was carried out in the powder boiler with an output power rated 72.2 MW. The previously carried out research in the Czech

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Figure 8. Time behavior of SO2 at the outlet of 120 MW CFBC.

Republic in the field of desulphurization using the dry additive method in powdered furnaces with fine-milled limestone did not produce the required results and was abandoned because of low desulphurization efficiency.12 However, the authors of this paper decided to carry out desulphurization tests of a similar character with saturation sludge on the basis of the theoretical knowledge of the chemical/physical properties of this substance. The saturation sludge was used as a sorbent in the dry additive method of desulphurization in the above-mentioned powder boiler firing hard coal. Then, the results can significantly affect the further development of the method, which did not bring the expected results with traditional limestone. Alternative ultrafine sorbents, such as saturation sludge, could bring the revitalization of the method within the framework of adaptation to new and more demanding emission limits. Tests carried out with limestone in this socalled intervention method were performed in boilers with greater output, i.e., with greater dimensions and, thus, with a longer duration of keeping the additive particles in the environment suitable for the dry additive desulphurization reaction. This factor, although apparent, should be emphasized in the analysis and evaluation of the carried out tests. The medium-pressure steam boiler, in which tests were carried out, is designed for the combustion of dirt band from the Ostrava Coalfield. It incorporates two drums with natural circulation, with a two-draft combustion chamber. The lower part of the first draft comprises the radiant furnace, into which the upper part under the roof leads powder burners. The firing and stabilization oil burners are positioned on side walls at the elevation of 19 m. The lower part of the combustion chamber is divided into two granulation discharge hoppers with slag crushers. The milling system consists of a tube mill and interbunkers for coal powder. The condition for the test performance was the requirement of as few as possible interventions in technology. This condition also required determining a suitable approach to dosing the sorbent into the combustion chamber together with fuel. Saturation sludge was mixed with dirt band (coal) by means of a wheel loader. The mixture was poured into the railway carriage and subsequently filled into the underground bunker. The flow of raw fuel and sludge was continuous without any stoppage or outage. This mixture was made in the proportion Ca/S ) 2.5:1. Then, the mixture was conveyed via the typical coal handling routes to the boiler.

Saturation sludge with a humidity of approximately 40% and a chemical composition as described in Table 1 was dosed in the mixture with coal to technological fuel preparation and treatment for the combustion process. Therefore, it was conveyed already in the mixture with coal to the tube mill, where it was dried up together with coal. Any potential coagulated pieces were milled. Afterward, it was carried away with coal powder into the intermediate bunkers of coal powder and then to the combustion chamber burners. It can be assumed that the saturation sludge lost part of its coagulation character because of drying. Because of its granulometry, the saturation sludge was continuously delivered to the combustion chamber. According to the analysis of the obtained data (Figure 9), it is apparent that the saturation sludge shows very good properties in dry additive desulphurization in the powder furnace. Saturation sludge has an ultrafine granulometry and, thus, a large specific surface area. Therefore, it can be theoretically assumed that a substance of this granulometric structure is capable of absorbing a relatively high quantity of SO2 in a very short time. This characteristic is very important for the given technology. The dwell time of the particles in the combustion chamber is stated on the order of seconds (2 s). The course of dry additive desulphurization is conditioned primarily by the range of temperatures appropriate for the above-mentioned reaction. This means that the dwell time of the sorbent particles in the corresponding temperature conditions is very short. Thanks to ultrafine granulometry, the resulting efficiency of desulphurization was very good (approximately 35-40%). This result can be compared to the results of the intervention method, i.e., desulphurization with fine-milled limestone with the technology of a similar type.12 The results obtained in this time period in the boilers of the same technology but with greater output and therefore with better conditions for the course of reaction were comparable to the results achieved using saturation sludge. In the tests with the intervention method and in comparison to the test with saturation sludge, the supply with limestone was regulated with both the quantity and place of supply, i.e., with the choice of temperature field. The goal of the emissions analysis was the comparison of emissions of SO2 in the combustion of pure coal and the mixture of coal plus saturation sludge as the alternative sorbent. This particular test took place from 8:30 to 12:00 (see Figure 9).

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Figure 9. Time behavior of the concentration of SO2 at the outlet from powder boiler with a granular furnace output of 72.2 MW. A, boiler starting up, stabilization; B, stabilized boiler operation, the mixture coal is fired with saturation sludge, average SO2 ) 620 mg mN-3; and C, combustion of pure coal, average SO2 ) 990 mg mN-3.

The start-up and stabilization of the unit took place until 8:30, when a “foreign” powder from the bunker was used. Then, from 8:30, the saturation sludge was present in the mixture. In the time period from 8:30 to 11:30, the sorbent was dosed into the process. The emissions of SO2 varied at an average level of 646 mg mN-3, with short-term growths of emissions up to 780-800 mg mN-3. The minimum achieved values were 540-550 mg mN-3. This fluctuation was apparently caused by homogenization of the mixture and also because of a natural fluctuation of sulfur content in raw fuel. In the period starting from 12:00, the fuel without saturation sludge was fired. The concentration of SO2 grew to 970 mg mN-3. From this, it results in the achievable efficiency of desulphurization of approximately 40%. For this type of boiler, the achieved efficiency is surprisingly high. From the course of emissions between 11:30 and 12:00, it results that saturation sludge came to the combustion chamber primarily through vapor burners. In general, immediately after emptying the bunker of raw coal (where the mixture of coal and sludge was), the emissions of SO2 dramatically increased, despite the fact that the mixture of coal and sludge had to still be in the bunkers. It is very likely that the existing cyclones of milling circuits are capable of collecting primarily coal powder from the carrying air matter and fine particles of saturation sludge pass through the cyclone into the vapor burners. This suggests that the efficiency of desulphurization was reduced during the test because of the fact that two vapor burners were in operation with subsequent non-uniform filling of the combustion chamber with saturation sludge. Even under these unfavorable conditions, the efficiency of desulphurization is very good. From the mentioned data, it is apparent that the results from the verification test have proven the possibility of desulphurization in this type of combustion equipment with saturation sludge. Furthermore, the effects of the content of saturation sludge on parameters affecting the boiler operation were investigated, primarily the softening temperature of ash matter. On the basis of this information and the distribution of the temperature field in the combustion chamber and

primarily on the basis of long-term operation, this method will definitely be examined with respect to its effects on boiler technology. Conclusion Within the framework of presented papers, general research of substances that were appropriate for implementation in the dry additive method of desulphurization in fluidized layer was carried out. The types of substances with appropriate physical and chemical properties and their availability were defined. According to the determination of general chemical and physical properties, saturation and caustification sludge were selected as the most suitable substances. Laboratory desulphurization tests were the first step in the research of these substances, and these tests were carried out on models of combustion units. Research of the behavior of alternative sorbents in laboratory conditions in dry additive desulphurization prepared the base for the performance of tests on real equipment. The duration of stay of ultrafine particles of alternative sorbents in the fluidized layer of the model CFBC was recorded as one of phenomena of behavior of investigated substances for the period exceeding the period calculated on models. Further, the hypothesis was defined and partially examined, which explains this phenomenon. Results from the tests may be evaluated as very good with the prerequisite for use, the testing of investigated substances in real combustion units. On the basis of carried out laboratory and pilot tests, one may expect good results from these real units (equipment with greater output). It may be stated that results of the given sorbents on the given equipment are in mutual accord. Primarily by comparison of results from the model CFBC unit and the FBB 100 kW unit, it may be concluded that the tests on the model CFBC were carried out correctly and the designed unit is satisfactory. The authors in this paper attempted to describe the research of alternative sorbents that have a possibility of replacing a primarily natural raw material, limestone. It should be emphasized that waste substances were identified and inves-

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tigated as alternative sorbents. A desulphurization test was also carried out with the saturation sludge in a powder boiler with a granular furnace 72.2 MW, and the results appear to be very good. The preliminary conclusions from these tests confirm a very good sorption capability of substances identified by authors as alternative sorbents in the dry additive

Szeliga et al.

method of desulphurization. However, this research will need to be followed by tests that are more long term. Acknowledgment. The research was prepared with the support of MSM 6198910019. EF800218T