**2.1. Experimental equipment**

The results of the experimental works presented below have been obtained in laboratory scale installation up to 10 kW, which works under atmospheric pressure. In Figure 1 it is shown schematic representation of installation adapted to two-zone combustion of solid alternative fuel.

Fluidized bed reactor, which is the central part of the installation, consists of a quartz tube with an outside diameter 100 mm, height 500 mm and a wall thickness of 2 mm. It is placed on a perforated plate (distributor) made of chrome nickel steel having a thickness of 1 mm. Distributor has holes with a diameter of 0.6 mm which surface area is 1.8% of the total surface of the distributor. During cold fluidization of bed and autothermal combustion of alternative solid fuel fluidizing factor was air. Ignition and warming up the reactor was carried out by combusting propane pre-mixed with air. Mixing chamber with a distributor, air blower, a set of pipes, valves and rotameters composed supply system of gaseous components and fuels to the reactor.

An open top design of the reactor results in the possibility of placing inside it, at different heights relatively to the distributor, measuring elements, gas sampling probes and the batcher which allows for dosing solid fuel into the reactor. In order to prevent of uncontrolled penetration of gases from the reactor to the environment, in its upper part underpressure is maintained. It is obtained by combination of the reactor hood with exhaust fan. In this part of installation – dedusting part - mixing the gas with the air, a substantial cooling and removing most of the dust in cyclone and ash trap for coarser particles takes place (Figure 1).

The reactor was equipped with a temperature control system consisting of a moveable radiation shield and cold air blower with adjustable airflow. This allows for conduction of autothermal combustion within the temperature 700 - 1000 °C, without changing the compo‐ sition of the air-fuel mixture.

The course of combustion in a fluidized bed reactor depends on the way of providing the reactants and the temperature distribution within it. During one zone combustion the fuel and oxidant are introduced only into the fluidized bed. Through zone above the bed (rare zone) flow then gaseous products of reactions from the fluidized bed, and a considerable amount of air. This creates favorable conditions for the use of this space in reactor as an additional combustion zone.

1 - heated probe for sampling the flue gases, 2 - set of 8 thin thermocouples, 3 - reburning burner, 4 - batcher, 5 - pilot flame, 6 – exhaust fan, 7 – computer storing data from Gasmet DX-4000, 8 - cyclone, 9 - ash trap for coarser particles, 10 – outlet of reburning fuel, 11 - movable radiation shield, 12 – fluidized bed, 13 - rotameters (from left: air and primary and secondary fuel), 14 – rotameter of CO2, 15 - fuel supply valves (from left: fuel supplying the pilot flame, reburning fuel, total fuel, CO2), 16 - blower, for fluidising air, 17 – two thermocouples, 18 - flat, perforated metal plate distributor, 19 - A/D convertor for thermocouple signals; 20 - computer storing chemical analyses quantities and temperature. Analytical block I: A - total organic compounds analyser (JUM Model 3-200), B - ECOM SG Plus, C - Horiba PG250, P – Peltier's cooler. Analytical block II: D – mobile conditioning system of Gasmet DX-4000, E - analyzer FTIR (Gasmet DX-4000), F - MRU Vario Plus.

To carry out the process of reduction of nitrogen oxides in the freeboard of the reactor, applying reburning method, burner comprising eight nozzles was installed. It is designed to distribute uniformly gaseous reburning fuel in the chosen height above the bed (Figure 1, item 3). In the exploded state and in the state of working outside the reactor it is shown in Figure 2. Slightly cross-sectional area of exhaust nozzles (Figure 2, item A) causes that the horizontal velocity of the gas reaches high value which provides high turbulence and rapid and uniform mixing of the reactants. During reactor operation, through the reburning burner small quantity of CO2 was passed continuously in order to prevent the formation of char in the burner nozzles and to ensure better mixing in reburning zone. In this experiment, the reburning fuel nozzles distance from the distributor was 180 mm.

(a) (b)

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**Figure 2.** The reburning fuel nozzle views: exploded state (A – burning nozzle forming channel, B – plate sealing noz‐

Alternative solid fuel was dosed into the reactor from its top through the batcher (Figure 1, item 4). It consists of a reservoir of material dispensed and beneath it a plate with an adjustable

zles) and during working outside the reactor.

**Figure 1.** Schematic representation of the fluidized bed reactor:

Low-Emission Combustion of Alternative Solid Fuel in Fluidized Bed Reactor http://dx.doi.org/10.5772/54158 253

**Figure 1.** Schematic representation of the fluidized bed reactor:

combusting propane pre-mixed with air. Mixing chamber with a distributor, air blower, a set of pipes, valves and rotameters composed supply system of gaseous components and fuels to

An open top design of the reactor results in the possibility of placing inside it, at different heights relatively to the distributor, measuring elements, gas sampling probes and the batcher which allows for dosing solid fuel into the reactor. In order to prevent of uncontrolled penetration of gases from the reactor to the environment, in its upper part underpressure is maintained. It is obtained by combination of the reactor hood with exhaust fan. In this part of installation – dedusting part - mixing the gas with the air, a substantial cooling and removing

The reactor was equipped with a temperature control system consisting of a moveable radiation shield and cold air blower with adjustable airflow. This allows for conduction of autothermal combustion within the temperature 700 - 1000 °C, without changing the compo‐

The course of combustion in a fluidized bed reactor depends on the way of providing the reactants and the temperature distribution within it. During one zone combustion the fuel and oxidant are introduced only into the fluidized bed. Through zone above the bed (rare zone) flow then gaseous products of reactions from the fluidized bed, and a considerable amount of air. This creates favorable conditions for the use of this space in reactor as an additional

1 - heated probe for sampling the flue gases, 2 - set of 8 thin thermocouples, 3 - reburning burner, 4 - batcher, 5 - pilot flame, 6 – exhaust fan, 7 – computer storing data from Gasmet DX-4000, 8 - cyclone, 9 - ash trap for coarser particles, 10 – outlet of reburning fuel, 11 - movable radiation shield, 12 – fluidized bed, 13 - rotameters (from left: air and primary and secondary fuel), 14 – rotameter of CO2, 15 - fuel supply valves (from left: fuel supplying the pilot flame, reburning fuel, total fuel, CO2), 16 - blower, for fluidising air, 17 – two thermocouples, 18 - flat, perforated metal plate distributor, 19 - A/D convertor for thermocouple signals; 20 - computer storing chemical analyses quantities and temperature. Analytical block I: A - total organic compounds analyser (JUM Model 3-200), B - ECOM SG Plus, C - Horiba PG250, P – Peltier's cooler. Analytical block II: D – mobile conditioning system of Gasmet DX-4000, E - analyzer

To carry out the process of reduction of nitrogen oxides in the freeboard of the reactor, applying reburning method, burner comprising eight nozzles was installed. It is designed to distribute uniformly gaseous reburning fuel in the chosen height above the bed (Figure 1, item 3). In the exploded state and in the state of working outside the reactor it is shown in Figure 2. Slightly cross-sectional area of exhaust nozzles (Figure 2, item A) causes that the horizontal velocity of the gas reaches high value which provides high turbulence and rapid and uniform mixing of the reactants. During reactor operation, through the reburning burner small quantity of CO2 was passed continuously in order to prevent the formation of char in the burner nozzles and to ensure better mixing in reburning zone. In this experiment, the reburning fuel nozzles

most of the dust in cyclone and ash trap for coarser particles takes place (Figure 1).

the reactor.

sition of the air-fuel mixture.

252 Advances in Internal Combustion Engines and Fuel Technologies

FTIR (Gasmet DX-4000), F - MRU Vario Plus.

distance from the distributor was 180 mm.

combustion zone.

**Figure 2.** The reburning fuel nozzle views: exploded state (A – burning nozzle forming channel, B – plate sealing noz‐ zles) and during working outside the reactor.

Alternative solid fuel was dosed into the reactor from its top through the batcher (Figure 1, item 4). It consists of a reservoir of material dispensed and beneath it a plate with an adjustable rotating frequency. The amount of material dosed from the plate to the hopper and then to the reactor was regulated by scraper setting. It was verified that such a construction allows a batcher to dose a steady stream of fuel mass to the reactor.

organic compounds based on the method of infrared spectroscopy with Fourier transform

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Application of analyzer Gasmet DX-4000 in measurements, resulted in necessity to adjust the sensivity of the analyzer to the expected range of concentrations of the components analyzed by it. Therefore, analyzers used for the determination of chemical compounds in the flue gases were divided into two separate blocks (Figure 1). Part of the exhaust fumes was taken from area above second combustion zone by heated probe, mounted 475 mm above the distributor (collection point I) and led to the analytical block I (Figure 1). The second part of the flue gases was directed to the analytical block II. It was taken from the reactor after passing through first cross-section measurement, quickly cooled and mixed with secondary air in 1:3 ratio. Then from flue gases stream, partially dedusted in ash trap for coarser particles, sample was collected (collection point II) and passed to analytical block II. The concentration values obtained by the FTIR analyzer should be verified, because in the case of the complex compo‐ sition of the gas sample optimization method can also generate results contain errors. There‐ fore, in the analytical block II was mounted analyzer MRU Vario Plus, which allowed for doubling the measurements of CO2, CO, NOx, SO2 and to measure O2 in this measurement point. The flue gases to be analyzed first had to be diluted. Data obtained from those analyzes were calculated into values before dilution, the aim of this was having values of concentration as they were passing the first measuring point and in turns data from all the analyzers were studied. The dilution degree of flue gases between the first and the second measuring point, necessary for this calculation, was determined on the basis of the mass balance of the two components: CO2 and CO, assuming that the diluting air is practically free from these com‐ ponents (in comparison to their concentrations in the exhaust fumes). This structure of measuring blocks allowed for execution of quantitative determinations of chosen organic substances in the flue gases, when the concentration of these compounds in reburning zone

significantly exceeded the measuring range of the analyzer Gasmet DX-4000.

In the case of flue gases from the thermal utilization of alternative fuels, we have to deal with the presence of organic and inorganic compounds in it, derived from the first and second combustion zone. Their presence is the result of a complex chemical composition of solid fuel (e.g. presence of HCl and HF), the complexity of the combustion of solid alternative fuels and the interaction between the various components of the flue gases. It can be expected that, compared to the combustion of gaseous fuel, in the flue gases from this processes there will be more organic compounds. Analyzer, which makes possible measuring the composition of such a complex mixture is the Gasmet DX-4000 - FTIR. Applied detection method in this analyzer, utilizes the phenomenon of the absorption of infrared radiation by the analyzed components. An important modification of the method is to replace the spectrometer by interferometer. Unit of the equipment, on which method is basing, the does not generate an infrared absorption spectrum (as in the dispersive infra-red spectroscopy - DIR) does not generate directly the absorbance values for the selected wavelength (as in the non-dispersive infra-red spectroscopy - NDIR). In this method, the answer is obtained in the form of a complex relationship between the position of the interferometer mirrors and the size of the measured signal [35]. This relationship is interferogram. Interferogram has the form of implicit informa‐

(FTIR).

As an inert fluidized bed, sand was used with a mass of 250 g and particle size of 0.375-0.430 mm. This material does not wear out during the process, has an adequate mechanical strength, has a high softening temperature (about 1050 °C) and does not react with the compounds present in the reaction environment during the process of combustion.

## **2.2. Analytical and measuring equipment and methodology**

The temperature measurement system was organized in the way to be able to measuring temperature both in bed and in the area above the fluidized bed. The temperature in the bed was measured using two NiCr-Ni thermocouples which connectors were located at a height of 20 and 50 mm above the distributor (Figure 1, item 17). In the zone above the bed temperature measurements were made using a specially designed set of eight thermocouples (Figure 1, item 2). There are made of wires with a diameter of 1 mm. One part of each thermocouple consisted of nickel wire which was joint for every thermocouple, second part consisted of eight wires made of chrome-nickel alloy. Application of one joint wire gave opportunity of setting thermocouples connectors at a constant distance from each other and limiting number of additional elements that could influence the process through hydrodynamics or catalytic effects. This enabled also the exact determination of each measurement point relative to the distributor and reburning burner nozzles.

Analyzers applied to measure the concentration of individual chemical components in the flue gases were divided into two analytical blocks (Figure 1). For measuring the flue gases composition, the standard analytical methods were used. They allow for the direct processing of the measured physical quantities into electric signals, wherein with reference to some components the duplicate measurements, based on the different physical properties of the measured components, were done. This procedure helped to eliminate the cross-effects on obtained data and to verify them. On the base of those data values of air excess coefficient and the degree of reduction of nitrogen oxides concentration were calculated. Analyzers used in researches use the following methods for the detection of chemical compounds. MRU Vario Plus analyzer (Figure 1, item F) measures the concentration of O2, CO, NO, NO2, SO2 using electrochemical sensors (EC), CO2 and volatile organic compounds (marked in the case of the analyzer as CxHy) are measured using IR detection (non-dispersive infrared NDIR). The analyzer of volatile organic compounds VOCs JUM Model 3-200 (Figure 1, item A) makes measurements using flame ionization detector (FID). Analyzer ECOM Plus SG (Figure 1, item B) measures the concentration of O2, CO, NO, NO2, SO2 using electrochemical sensors (EC). Analyzer Horiba PG250 (Figure 1, item C) consists of three kinds of sensors. O2 concentration is measured by electrochemical sensor (EC), for determining the amount of gases such as CO, CO2, SO2 analyzer uses in the IR detectors (non-dispersive infrared NDIR), concentration of nitrogen oxides (II) and (IV) - NOx is measured using a chemiluminescence technique (CLA). Analyzer Gasmet DX-4000 (Figure 1, item E) measures the concentration of inorganic and organic compounds based on the method of infrared spectroscopy with Fourier transform (FTIR).

rotating frequency. The amount of material dosed from the plate to the hopper and then to the reactor was regulated by scraper setting. It was verified that such a construction allows a

As an inert fluidized bed, sand was used with a mass of 250 g and particle size of 0.375-0.430 mm. This material does not wear out during the process, has an adequate mechanical strength, has a high softening temperature (about 1050 °C) and does not react with the compounds

The temperature measurement system was organized in the way to be able to measuring temperature both in bed and in the area above the fluidized bed. The temperature in the bed was measured using two NiCr-Ni thermocouples which connectors were located at a height of 20 and 50 mm above the distributor (Figure 1, item 17). In the zone above the bed temperature measurements were made using a specially designed set of eight thermocouples (Figure 1, item 2). There are made of wires with a diameter of 1 mm. One part of each thermocouple consisted of nickel wire which was joint for every thermocouple, second part consisted of eight wires made of chrome-nickel alloy. Application of one joint wire gave opportunity of setting thermocouples connectors at a constant distance from each other and limiting number of additional elements that could influence the process through hydrodynamics or catalytic effects. This enabled also the exact determination of each measurement point relative to the

Analyzers applied to measure the concentration of individual chemical components in the flue gases were divided into two analytical blocks (Figure 1). For measuring the flue gases composition, the standard analytical methods were used. They allow for the direct processing of the measured physical quantities into electric signals, wherein with reference to some components the duplicate measurements, based on the different physical properties of the measured components, were done. This procedure helped to eliminate the cross-effects on obtained data and to verify them. On the base of those data values of air excess coefficient and the degree of reduction of nitrogen oxides concentration were calculated. Analyzers used in researches use the following methods for the detection of chemical compounds. MRU Vario Plus analyzer (Figure 1, item F) measures the concentration of O2, CO, NO, NO2, SO2 using electrochemical sensors (EC), CO2 and volatile organic compounds (marked in the case of the analyzer as CxHy) are measured using IR detection (non-dispersive infrared NDIR). The analyzer of volatile organic compounds VOCs JUM Model 3-200 (Figure 1, item A) makes measurements using flame ionization detector (FID). Analyzer ECOM Plus SG (Figure 1, item B) measures the concentration of O2, CO, NO, NO2, SO2 using electrochemical sensors (EC). Analyzer Horiba PG250 (Figure 1, item C) consists of three kinds of sensors. O2 concentration is measured by electrochemical sensor (EC), for determining the amount of gases such as CO, CO2, SO2 analyzer uses in the IR detectors (non-dispersive infrared NDIR), concentration of nitrogen oxides (II) and (IV) - NOx is measured using a chemiluminescence technique (CLA). Analyzer Gasmet DX-4000 (Figure 1, item E) measures the concentration of inorganic and

batcher to dose a steady stream of fuel mass to the reactor.

254 Advances in Internal Combustion Engines and Fuel Technologies

**2.2. Analytical and measuring equipment and methodology**

distributor and reburning burner nozzles.

present in the reaction environment during the process of combustion.

Application of analyzer Gasmet DX-4000 in measurements, resulted in necessity to adjust the sensivity of the analyzer to the expected range of concentrations of the components analyzed by it. Therefore, analyzers used for the determination of chemical compounds in the flue gases were divided into two separate blocks (Figure 1). Part of the exhaust fumes was taken from area above second combustion zone by heated probe, mounted 475 mm above the distributor (collection point I) and led to the analytical block I (Figure 1). The second part of the flue gases was directed to the analytical block II. It was taken from the reactor after passing through first cross-section measurement, quickly cooled and mixed with secondary air in 1:3 ratio. Then from flue gases stream, partially dedusted in ash trap for coarser particles, sample was collected (collection point II) and passed to analytical block II. The concentration values obtained by the FTIR analyzer should be verified, because in the case of the complex compo‐ sition of the gas sample optimization method can also generate results contain errors. There‐ fore, in the analytical block II was mounted analyzer MRU Vario Plus, which allowed for doubling the measurements of CO2, CO, NOx, SO2 and to measure O2 in this measurement point. The flue gases to be analyzed first had to be diluted. Data obtained from those analyzes were calculated into values before dilution, the aim of this was having values of concentration as they were passing the first measuring point and in turns data from all the analyzers were studied. The dilution degree of flue gases between the first and the second measuring point, necessary for this calculation, was determined on the basis of the mass balance of the two components: CO2 and CO, assuming that the diluting air is practically free from these com‐ ponents (in comparison to their concentrations in the exhaust fumes). This structure of measuring blocks allowed for execution of quantitative determinations of chosen organic substances in the flue gases, when the concentration of these compounds in reburning zone significantly exceeded the measuring range of the analyzer Gasmet DX-4000.

In the case of flue gases from the thermal utilization of alternative fuels, we have to deal with the presence of organic and inorganic compounds in it, derived from the first and second combustion zone. Their presence is the result of a complex chemical composition of solid fuel (e.g. presence of HCl and HF), the complexity of the combustion of solid alternative fuels and the interaction between the various components of the flue gases. It can be expected that, compared to the combustion of gaseous fuel, in the flue gases from this processes there will be more organic compounds. Analyzer, which makes possible measuring the composition of such a complex mixture is the Gasmet DX-4000 - FTIR. Applied detection method in this analyzer, utilizes the phenomenon of the absorption of infrared radiation by the analyzed components. An important modification of the method is to replace the spectrometer by interferometer. Unit of the equipment, on which method is basing, the does not generate an infrared absorption spectrum (as in the dispersive infra-red spectroscopy - DIR) does not generate directly the absorbance values for the selected wavelength (as in the non-dispersive infra-red spectroscopy - NDIR). In this method, the answer is obtained in the form of a complex relationship between the position of the interferometer mirrors and the size of the measured signal [35]. This relationship is interferogram. Interferogram has the form of implicit informa‐ tion about the absorbance of the analyzed gases in the entire wavelength range of electromag‐ netic radiation from a source. This relationship is unraveling after a Fourier transform on the data forming the interferogram [35]. The advantage of FTIR over other methods based on absorption of infrared radiation by analyzed components is that as the measurement data absorbance spectrum in a wide wavelength range of infrared radiation is obtained, not for the narrow range or at a point. On the basis of a single measurement obtained with a single gas chamber, information on concentration of a number of components is gained. Amount and type of components need not be imposed in advance. The individual concentrations are matched by comparing the spectrum of the sample with the reference spectrums for typed, as present in the sample, components. In such a situation, significant becomes appropriate selection of the spectra library which is used by optimization method described above. If the list of the compounds is too short, in the residual spectrum remain the signals from components which are not included in the analysis, and the accuracy of the determination of concentrations of the analyzed compounds will be small. Too long list of compounds in turn leads to numerical errors that reduce the accuracy of the calculations. Changes to the list of compounds for analysis can be made based on the knowledge of the combustion processes occurring in certain conditions, resulting from the review of the literature data and own preliminary experiments. For the DX-4000 analyzer manufacturer has set the standard method of calculation takes into account the following compounds: H2O, CO2, CO, NO, NO2, N2O, SO2, HCl, HF, NH3, CH4, C2H6, C3H8, C2H4, C6H14, HCHO, acetaldehyde, acrolein and HCN. As a result of tests this list was supplemented about ethin, propene, butane, isobutane, pentane, benzene, toluene, xylenes, styrene, ethyl benzene, ethanol, methanol, acetone, formic acid and acetic acid. Not all substances added to the database of compounds included in the analysis of FTIR were detected in the analyzed gases, but the extension of the standard library of compounds allowed for the determination in the flue gases components specific for the combustion process carrying on in lack of oxygen conditions. It has also increased the accuracy of the determinations of components which are relevant to the assessment of NxOy reduction process and residual IR absorbance spectrum decreased compared to the standard library.

method. Mineral content was determined by incineration of the sample and then calcination of the residue to constant weight in a chamber furnace at a temperature of 815 oC. The heat of

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This material was dosed into the reactor in the form of particles of suitable size and form. Essential shape of the material was obtained by realization of several operations. The first stage was crushing pieces of municipal sewage sludge into grains with a smaller diameter and separation from it fraction with a grain diameter of 0,3-4,0 mm. Then a measured amount of wasted bleaching earth, originating from the bleaching of paraffin waxes, was melted in a water bath. To liquefied bleaching earth was gradually added, prepared in advance, municipal sewage sludge and calcium carbonate. The prepared material after cooling down for the most part was in the form of granules, the rest of the material easily give up granulation. The resulting granulate was sieved to obtain grains agglomerates having a diameter 0.3-4.0 mm.

> C, %mass. 31.32 H, % mass. 5.60 N, % mass. 1.19 O + S, % mass. 6.10 Mineral parts, % mass. 52.46 Humidity, % mass. 3.33 Heat of Combustion, MJ/kg 16.22

**Table 2.** Alternative solid fuel: SS (30%)+BE (62%)+CaCO3 (8%) - composition and heat of combustion

ison purposes. Process parameters are summarized in Table 3.

The combustion of alternative fuels in a fluidized bed reactor consisted of a series of stages. The first is the start of the fluidization process at ambient temperature – cold fluidization. When fluidization of the bed is reached, the ignition and warm up the bed by combustion a mixture of propane (0.056 ± 0.001 dm3/s) with air (1.66 ± 0.08 dm3/s) in it took place, it was the initial phase of the experiment. After the bed temperature reached approximately 900 °C dosage of alternative solid fuel was started, with simultaneously closing the flow of propane to the reactor - the only source of heat was now combustion of solid fuel. Alternative fuel was dosed into the fluidized bed reactor at a rate of approximately 17 g/min. The thermal utilization of the fuel was carried on at this stage only in one zone. The purpose of this was to obtain comparative data for the main study process. In the next stage, dosage of the reburning gas – propane to the rare zone of the reactor, zone above the bed, was initiated. Its flow during the entire two zone combustion was maintained at level 0.018-0.026 dm3/s. During this step combustion of solid fuel in the bed was carried out with simultaneous reduction of nitrogen oxides in the second combustion zone. In the final stage of experiment, the flow of reburning fuel was closed while combustion of solid fuel in a fluidized was continued, also for compar‐

**2.4. Course of the combustion process**

combustion was determined by calorimetric method.

#### **2.3. Alternative solid fuel**

In the fluidized bed reactor, that has been discussed above, thermal utilization of alternative solid fuel has been carried on. Fuel selected to researches consisted of municipal sewage sludge (30%mass) – SS, wasted bleaching earth (62%mass) – BE, and lime - consisting almost exclu‐ sively of CaCO3 (8%mass). This last component allows usage of fluidized bed for the absorp‐ tion of SO2. This fuel composition was caused by the assumptions in the direction of researches lead to obtain a solid alternative fuel with a high, known and fixed nitrogen content bound in it. The base of the fuel was sludge with a high content of nitrogen and sulfur. By controlled addition of wasted bleaching earth it was yielded a fuel with a high nitrogen content, but less than in the case of sludge. This composition allowed for thermal utilization of two wasted materials deposited in landfills, and gave ability of controlling amount of nitrogen bounded in the fuel. Detailed elemental composition of the fuel, mineral content, and the heat of combustion are presented in Table 2. C, H and N content in the sample was determined using analyzer PerkinElmer 2400 Series II CHNS/O Elemental Analyzer based on Pregl-Dumas's method. Mineral content was determined by incineration of the sample and then calcination of the residue to constant weight in a chamber furnace at a temperature of 815 oC. The heat of combustion was determined by calorimetric method.

This material was dosed into the reactor in the form of particles of suitable size and form. Essential shape of the material was obtained by realization of several operations. The first stage was crushing pieces of municipal sewage sludge into grains with a smaller diameter and separation from it fraction with a grain diameter of 0,3-4,0 mm. Then a measured amount of wasted bleaching earth, originating from the bleaching of paraffin waxes, was melted in a water bath. To liquefied bleaching earth was gradually added, prepared in advance, municipal sewage sludge and calcium carbonate. The prepared material after cooling down for the most part was in the form of granules, the rest of the material easily give up granulation. The resulting granulate was sieved to obtain grains agglomerates having a diameter 0.3-4.0 mm.


**Table 2.** Alternative solid fuel: SS (30%)+BE (62%)+CaCO3 (8%) - composition and heat of combustion

#### **2.4. Course of the combustion process**

tion about the absorbance of the analyzed gases in the entire wavelength range of electromag‐ netic radiation from a source. This relationship is unraveling after a Fourier transform on the data forming the interferogram [35]. The advantage of FTIR over other methods based on absorption of infrared radiation by analyzed components is that as the measurement data absorbance spectrum in a wide wavelength range of infrared radiation is obtained, not for the narrow range or at a point. On the basis of a single measurement obtained with a single gas chamber, information on concentration of a number of components is gained. Amount and type of components need not be imposed in advance. The individual concentrations are matched by comparing the spectrum of the sample with the reference spectrums for typed, as present in the sample, components. In such a situation, significant becomes appropriate selection of the spectra library which is used by optimization method described above. If the list of the compounds is too short, in the residual spectrum remain the signals from components which are not included in the analysis, and the accuracy of the determination of concentrations of the analyzed compounds will be small. Too long list of compounds in turn leads to numerical errors that reduce the accuracy of the calculations. Changes to the list of compounds for analysis can be made based on the knowledge of the combustion processes occurring in certain conditions, resulting from the review of the literature data and own preliminary experiments. For the DX-4000 analyzer manufacturer has set the standard method of calculation takes into account the following compounds: H2O, CO2, CO, NO, NO2, N2O, SO2, HCl, HF, NH3, CH4, C2H6, C3H8, C2H4, C6H14, HCHO, acetaldehyde, acrolein and HCN. As a result of tests this list was supplemented about ethin, propene, butane, isobutane, pentane, benzene, toluene, xylenes, styrene, ethyl benzene, ethanol, methanol, acetone, formic acid and acetic acid. Not all substances added to the database of compounds included in the analysis of FTIR were detected in the analyzed gases, but the extension of the standard library of compounds allowed for the determination in the flue gases components specific for the combustion process carrying on in lack of oxygen conditions. It has also increased the accuracy of the determinations of components which are relevant to the assessment of NxOy reduction process and residual IR

256 Advances in Internal Combustion Engines and Fuel Technologies

absorbance spectrum decreased compared to the standard library.

In the fluidized bed reactor, that has been discussed above, thermal utilization of alternative solid fuel has been carried on. Fuel selected to researches consisted of municipal sewage sludge (30%mass) – SS, wasted bleaching earth (62%mass) – BE, and lime - consisting almost exclu‐ sively of CaCO3 (8%mass). This last component allows usage of fluidized bed for the absorp‐ tion of SO2. This fuel composition was caused by the assumptions in the direction of researches lead to obtain a solid alternative fuel with a high, known and fixed nitrogen content bound in it. The base of the fuel was sludge with a high content of nitrogen and sulfur. By controlled addition of wasted bleaching earth it was yielded a fuel with a high nitrogen content, but less than in the case of sludge. This composition allowed for thermal utilization of two wasted materials deposited in landfills, and gave ability of controlling amount of nitrogen bounded in the fuel. Detailed elemental composition of the fuel, mineral content, and the heat of combustion are presented in Table 2. C, H and N content in the sample was determined using analyzer PerkinElmer 2400 Series II CHNS/O Elemental Analyzer based on Pregl-Dumas's

**2.3. Alternative solid fuel**

The combustion of alternative fuels in a fluidized bed reactor consisted of a series of stages. The first is the start of the fluidization process at ambient temperature – cold fluidization. When fluidization of the bed is reached, the ignition and warm up the bed by combustion a mixture of propane (0.056 ± 0.001 dm3/s) with air (1.66 ± 0.08 dm3/s) in it took place, it was the initial phase of the experiment. After the bed temperature reached approximately 900 °C dosage of alternative solid fuel was started, with simultaneously closing the flow of propane to the reactor - the only source of heat was now combustion of solid fuel. Alternative fuel was dosed into the fluidized bed reactor at a rate of approximately 17 g/min. The thermal utilization of the fuel was carried on at this stage only in one zone. The purpose of this was to obtain comparative data for the main study process. In the next stage, dosage of the reburning gas – propane to the rare zone of the reactor, zone above the bed, was initiated. Its flow during the entire two zone combustion was maintained at level 0.018-0.026 dm3/s. During this step combustion of solid fuel in the bed was carried out with simultaneous reduction of nitrogen oxides in the second combustion zone. In the final stage of experiment, the flow of reburning fuel was closed while combustion of solid fuel in a fluidized was continued, also for compar‐ ison purposes. Process parameters are summarized in Table 3.


*κ* =3*α* +

lations:

were:

1

<sup>2</sup> <sup>⋅</sup>3(1−*α*) <sup>+</sup> <sup>2</sup> <sup>+</sup> *<sup>β</sup>* <sup>=</sup> <sup>3</sup>

a

**•** nVOCs is an amount of unburnt compounds

concentrations (*y CO2 , y CO , y O2 , y VOCs*):

organic compounds)

<sup>2</sup> *<sup>α</sup>* <sup>+</sup> *<sup>β</sup>* <sup>+</sup>

æ ö

b

g

*r*

l

**•** nss is a sum o moles of all compounds in flue gases, dry conditions,

7 2 ;

2

*CO CO n n*

*nVOCs nCO*<sup>2</sup>

= *γ* <sup>3</sup>*α* ;

3(1 ) 3

a

those equations after modifications are in form which allows to use molar fractions in calcu‐

2 2 2

*CO CO ss CO CO CO CO CO CO CO ss ss*

2 2

3 3 *O O CO CO CO CO*

3 3 *VOCs VOCs CO CO CO CO*

*n y nn yy*

*n y nn yy*

2 2

2 2

Equations [23-28] give possibility of calculate of air excess coefficient based on measured

2 2

*CO CO O*

*y yy*

æ ö ç ÷ + +

2

è ø

*CO CO VOCs*

where: y – molar fractions of individual compounds present in flue gases (VOCs-volatile

Values of the air excess coefficient in the first combustion zone during the thermal utilization were maintained at level 1.3-1.6, as shown in Figure 3, while periods when the reburning was not carried out. Air excess coefficient in reburning area during two zone combustion was

<sup>7</sup> 5 3 <sup>1</sup> <sup>2</sup> 5 3

<sup>=</sup> + +

*yy y*

= = + + (27)

= = + + (28)

*n n y n n n n n y y n n*

= == ç ÷ ç ÷ + + è ø <sup>+</sup>

2 2 2

a


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Low-Emission Combustion of Alternative Solid Fuel in Fluidized Bed Reactor

(26)

259

(29)

**Table 3.** Parameters of solid alternative fuel combustion process

#### **3. Discussion of two zone combustion results**

The temperature and the concentrations of the individual compounds in the flue gases were recorded by the analytical and measuring equipment for the whole time of the process.

On the basis of the experimental data temporary and average values of air excess coefficient in reburning area - λr were calculated. Reduction degree of nitrogen oxides in reburning area depends on air excess coefficient in this zone. Air excess coefficient was calculated on base of stechiometric model of reburning fuel combustion, which is shown below. Presence in exhaust fumes of CO2, CO, remaining O2, and uncombusted fuel are taken into consideration in this model:

$$\kappa \left( 1+\gamma \right) \mathbf{C}\_3 \mathbf{H}\_8 + \kappa \mathbf{C}\_2 + \frac{79}{21} \kappa \mathbf{N}\_2 \to 3\kappa \mathbf{C} \mathbf{O}\_2 + 3(1-a)\mathbf{C} \mathbf{O} + 4\mathbf{H}\_2 \mathbf{O} + \beta \mathbf{O}\_2 + \frac{79}{21} \kappa \mathbf{N}\_2 + \gamma \mathbf{C}\_3 \mathbf{H}\_8 \tag{22}$$

Air excess coefficient λ is defined as quotient of amount of oxygen delivered to reaction zone and amount of oxygen used in combustion processes. For combustion reaction [22] this dependence can be written as:

$$
\lambda = \frac{\kappa}{5\left(1 + \gamma\right)}\tag{23}
$$

where coefficient *κ* was calculated from stoichiometric equation [22]:

$$
\kappa = 3a + \frac{1}{2} \cdot 3\left(1 - a\right) + 2 + \beta = \frac{3}{2}a + \beta + \frac{7}{2} \tag{24}
$$

and coefficients *α, β, γ* can be calculated from dependences:

$$n\_{\text{AC}} = 3\alpha + \frac{1}{2} \cdot 3(1 - \alpha) + 2 + \beta = \frac{3}{2}\alpha + \beta + \frac{7}{2} \vdots \frac{n\_{\text{VOCs}}}{n\_{\text{CO}\_2}} = \frac{\mathcal{V}}{3\alpha} \; : \; \begin{aligned} n\_{\text{VOCs}} &= \frac{\mathcal{V}}{3\alpha} \; : \\\\ n\_{\text{CO}\_2} &= \frac{3(1 - \alpha)}{3\alpha} \end{aligned} \tag{25}$$

those equations after modifications are in form which allows to use molar fractions in calcu‐ lations:

$$\alpha = \left(\frac{n\_{CO\_2}}{n\_{CO} + n\_{CO\_2}}\right) = \frac{\frac{n\_{CO\_2}}{n\_{ss}}}{\frac{n\_{CO}}{n\_{ss}} + \frac{n\_{CO\_2}}{n\_{ss}}} = \frac{y\_{CO\_2}}{y\_{CO} + y\_{CO\_2}}\tag{26}$$

$$\beta = \frac{3\eta\_{O\_2}}{\eta\_{CO} + \eta\_{CO\_2}} = \frac{3y\_{O\_2}}{y\_{CO} + y\_{CO\_2}} \tag{27}$$

$$\gamma = \frac{\mathfrak{Im}\_{\rm VOCs}}{\mathfrak{n}\_{\rm CO} + \mathfrak{n}\_{\rm CO\_2}} = \frac{\mathfrak{Z}y\_{\rm VOCs}}{y\_{\rm CO} + y\_{\rm CO\_2}} \tag{28}$$

were:

**Material of the Bed** sand

model:

g

 k

dependence can be written as:

 k

k a

and coefficients *α, β, γ* can be calculated from dependences:

**Solid fuel** alternative fuel

258 Advances in Internal Combustion Engines and Fuel Technologies

**Table 3.** Parameters of solid alternative fuel combustion process

**3. Discussion of two zone combustion results**

mass 250.60 g

dosage 17 g/min fraction 0.3-4.0 mm

**Reburning fuel** propan flow 0.018-0.026 dm3/s

The temperature and the concentrations of the individual compounds in the flue gases were recorded by the analytical and measuring equipment for the whole time of the process.

On the basis of the experimental data temporary and average values of air excess coefficient in reburning area - λr were calculated. Reduction degree of nitrogen oxides in reburning area depends on air excess coefficient in this zone. Air excess coefficient was calculated on base of stechiometric model of reburning fuel combustion, which is shown below. Presence in exhaust fumes of CO2, CO, remaining O2, and uncombusted fuel are taken into consideration in this

( ) ( ) 38 2 2 2 2 2 2 38

+ + + ® +- + + + +

21 21

Air excess coefficient λ is defined as quotient of amount of oxygen delivered to reaction zone and amount of oxygen used in combustion processes. For combustion reaction [22] this

> 5 1( ) k

( ) 1 37 3 31 2 2 22

 b

 ab= +× - ++ = + + (24)

 a g

 a

*C H O N CO CO H O O N C H* (22)

<sup>79</sup> <sup>79</sup> <sup>1</sup> 3 31 4

l

where coefficient *κ* was calculated from stoichiometric equation [22]:

 a

**Fluidization** air flow 1.66 dm3/s

fraction 0.375-0.430 mm

 b

<sup>=</sup> <sup>+</sup> (23)

 kg


Equations [23-28] give possibility of calculate of air excess coefficient based on measured concentrations (*y CO2 , y CO , y O2 , y VOCs*):

$$\mathcal{A}\_r = \frac{1}{5} \left( \frac{5y\_{CO\_2} + \frac{7}{2}y\_{CO} + 3y\_{O\_2}}{y\_{CO} + y\_{CO\_2} + 3y\_{VOCs}} \right) \tag{29}$$

where: y – molar fractions of individual compounds present in flue gases (VOCs-volatile organic compounds)

Values of the air excess coefficient in the first combustion zone during the thermal utilization were maintained at level 1.3-1.6, as shown in Figure 3, while periods when the reburning was not carried out. Air excess coefficient in reburning area during two zone combustion was maintained at a level similar to 1.0 or slightly lower. It is a sine qua non for carrying out the process of NOx reduction and obtaining the optimal effect.

temperature in the second combustion zone is higher about 100 °C than in the bed. It is a natural phenomena because with starting the reburning additional combustion process appears and is source of heat. It is noteworthy that, during the combustion in the second zone the temper‐ ature in the bed increases about 20-30 oC. This is due to the transport of heat from reburning zone to the bed. This effect is desirable since it allows to maintain the proper temperature in

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261

With the beginning of propane dosage to zone above the fluidized bed and the creation of the second combustion zone, an increase in the concentration of CO2 in the flue gases from about

During the combustion of alternative solid fuel exclusively in the fluidized bed, the carbon monoxide concentration was about 3000 ppm (Figure 3). Along with the initiation of the process of nitrogen oxides reduction in the second combustion zone, the CO concentration increased, reaching a maximum value exceeding 20000 ppm. For most time of the reburning process CO concentration was maintained at a level higher than 14000 ppm. The reason for this is that in the second zone during the combustion process the oxygen concentration is stoichiometric and in certain periods there are even reductive conditions – for this reason part

A similar situation occurs in the case of volatile organic compounds - VOCs. When the combustion was carried on only in one zone – fluidized bed, VOCs level did not exceed 300 mg/m3. During conduction of the reburning process an increase in concentrations of these

A detailed analysis of the composition of the hydrocarbons in the flue gases during two-zone combustion was done. As a result, it was found absence of propane in the flue gases. The analyzer indicated the presence of ethane in them. His concentration did not exceed 220 ppm (Figure 4). Compounds identified in larger quantities approximately up to 10-fold higher than the concentration of ethane were methane, ethene and ethine. Their concentration was in the range of about from 500 to 2000 ppm (Figure 4). Their quantities are similar, with the highest concentrations found in the case of ethine. The presence of those hydrocarbons in the flue gases is desired. These compounds are the products of propane structural transition in thermal processes. They are source of CHx radicals and those radicals play a leading role in the

Compounds, which may also be created as a result of thermal degradation of propane are organic compounds containing oxygen, such as formic acid, methanol and formaldehyde. The concentration of formic acid showed by the FTIR analyzer during the research is negligible (Figure 4). Methanol is present in the flue gases, but in small amounts, with a peak concen‐ tration of 48 ppm (Figure 4). The highest concentration in the flue gases during reburning process, received formaldehyde. Its maximum value was at a level slightly greater than 300

The presence of the identified organic compounds in the exhaust fumes (containing or not oxygen in their structure) at high concentrations, in the absence of a propane which was reburning fuel, provides advance of the reburning process carrying on in the second zone. It

compounds up to a maximum value of 6000 mg/m3 was observed (Figure 3).

the bed, and hence appropriate autothermic conditions of solid fuel combustion.

14% to over 17% was registered (Figure 3).

of the CO was not oxidized to CO2.

reburning process [7].

ppm (Figure 4).

**Figure 3.** Temperature, air excess coefficient in reburning area and concentrations of chosen gaseous compounds, containing carbon in their structure, in exhaust fumes. (Indexes: digits - height (in mm) of thermocouple above distrib‐ utor: *20-*in fluidized bed*, 200-*in second combustion zone; index *r* – in reburning zone; index –*r-av* – average value in reburning zone)

Comparing the temperature changes occurring in the fluidized bed with those in the second combustion zone during the periods when only alternative fuel combustion in a fluidized bed was carried on, it can be seen that recorded values were similar in both cases (Figure 3). However, it notes fact that set of eight thermocouples located in the area above the bed recorded the larger fluctuations. The reasons for this can lay in the properties of the applied solid fuel. It contains in its composition readily volatile paraffins, behave good cohesiveness thanks to that and most of it burned in the bed. The intense mixing that occurs in the fluidized bed resulting in the intensification of heat transfer and hence a more uniform temperature distribution in the area. In the rare zone of the reactor periodically appeared uncombusted dust of sewage sludge dumped from the bed during the process. It underwent complete combustion in rare zone and this resulted in temporary increases in temperature increasing fluctuations in the area of carrying on the reburning process.

When such a comparison will be made for a period of time when reburning process in the reactor was carried on (Figure 3), it can be seen that with the beginning of reburning the temperature in the second combustion zone is higher about 100 °C than in the bed. It is a natural phenomena because with starting the reburning additional combustion process appears and is source of heat. It is noteworthy that, during the combustion in the second zone the temper‐ ature in the bed increases about 20-30 oC. This is due to the transport of heat from reburning zone to the bed. This effect is desirable since it allows to maintain the proper temperature in the bed, and hence appropriate autothermic conditions of solid fuel combustion.

maintained at a level similar to 1.0 or slightly lower. It is a sine qua non for carrying out the

**Figure 3.** Temperature, air excess coefficient in reburning area and concentrations of chosen gaseous compounds, containing carbon in their structure, in exhaust fumes. (Indexes: digits - height (in mm) of thermocouple above distrib‐ utor: *20-*in fluidized bed*, 200-*in second combustion zone; index *r* – in reburning zone; index –*r-av* – average value in

Comparing the temperature changes occurring in the fluidized bed with those in the second combustion zone during the periods when only alternative fuel combustion in a fluidized bed was carried on, it can be seen that recorded values were similar in both cases (Figure 3). However, it notes fact that set of eight thermocouples located in the area above the bed recorded the larger fluctuations. The reasons for this can lay in the properties of the applied solid fuel. It contains in its composition readily volatile paraffins, behave good cohesiveness thanks to that and most of it burned in the bed. The intense mixing that occurs in the fluidized bed resulting in the intensification of heat transfer and hence a more uniform temperature distribution in the area. In the rare zone of the reactor periodically appeared uncombusted dust of sewage sludge dumped from the bed during the process. It underwent complete combustion in rare zone and this resulted in temporary increases in temperature increasing

When such a comparison will be made for a period of time when reburning process in the reactor was carried on (Figure 3), it can be seen that with the beginning of reburning the

fluctuations in the area of carrying on the reburning process.

process of NOx reduction and obtaining the optimal effect.

260 Advances in Internal Combustion Engines and Fuel Technologies

reburning zone)

With the beginning of propane dosage to zone above the fluidized bed and the creation of the second combustion zone, an increase in the concentration of CO2 in the flue gases from about 14% to over 17% was registered (Figure 3).

During the combustion of alternative solid fuel exclusively in the fluidized bed, the carbon monoxide concentration was about 3000 ppm (Figure 3). Along with the initiation of the process of nitrogen oxides reduction in the second combustion zone, the CO concentration increased, reaching a maximum value exceeding 20000 ppm. For most time of the reburning process CO concentration was maintained at a level higher than 14000 ppm. The reason for this is that in the second zone during the combustion process the oxygen concentration is stoichiometric and in certain periods there are even reductive conditions – for this reason part of the CO was not oxidized to CO2.

A similar situation occurs in the case of volatile organic compounds - VOCs. When the combustion was carried on only in one zone – fluidized bed, VOCs level did not exceed 300 mg/m3. During conduction of the reburning process an increase in concentrations of these compounds up to a maximum value of 6000 mg/m3 was observed (Figure 3).

A detailed analysis of the composition of the hydrocarbons in the flue gases during two-zone combustion was done. As a result, it was found absence of propane in the flue gases. The analyzer indicated the presence of ethane in them. His concentration did not exceed 220 ppm (Figure 4). Compounds identified in larger quantities approximately up to 10-fold higher than the concentration of ethane were methane, ethene and ethine. Their concentration was in the range of about from 500 to 2000 ppm (Figure 4). Their quantities are similar, with the highest concentrations found in the case of ethine. The presence of those hydrocarbons in the flue gases is desired. These compounds are the products of propane structural transition in thermal processes. They are source of CHx radicals and those radicals play a leading role in the reburning process [7].

Compounds, which may also be created as a result of thermal degradation of propane are organic compounds containing oxygen, such as formic acid, methanol and formaldehyde. The concentration of formic acid showed by the FTIR analyzer during the research is negligible (Figure 4). Methanol is present in the flue gases, but in small amounts, with a peak concen‐ tration of 48 ppm (Figure 4). The highest concentration in the flue gases during reburning process, received formaldehyde. Its maximum value was at a level slightly greater than 300 ppm (Figure 4).

The presence of the identified organic compounds in the exhaust fumes (containing or not oxygen in their structure) at high concentrations, in the absence of a propane which was reburning fuel, provides advance of the reburning process carrying on in the second zone. It

the IR method as a reliable for determining the SO2 concentration values. The concentration of sulfur dioxide can be correctly determined in those phases of the experiment, in which the

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**Figure 5.** Concentrations of SO2 and NOx in exhaust fumes. (Index –*r-av* – average value in reburning zone).

The composition of utilized fuel (Table 2) was selected in a manner to bound SO2 present in the flue gases by CaO formed from the CaCO3 contained in the fuel. Analysis of the concen‐ tration of sulfur dioxide was carried out only at a time of one-zone combustion. They showed that its concentration did not exceed 250 mg/m3 then (Figure 5). The obtained value is satisfactory, which means that this desulphurization method is fully sufficient during such

During combustion of alternative fuels with high fuel-nitrogen content, as in the case of the researched material (Table 2) NOx emission level is of about 1100-1400 mgNO2/m3 (normalized at 6% of O2 in the flue gases) with a highest value more than 1500 mgNO2/m3 (Figure 5). These values are very high, often exceeding the emission stand‐ ards in Europe [3]. With the beginning of reburning fuel dosage into the rare zone of the reactor and creation of the second combustion zone, NOx concentration in the flue gases leaving this zone decreased to approximately 400 mgNO2/m3. By more than half of time when reburning process was carried on the NOx concentration was lower than 400 mgNO2/m3. These results are entirely satisfactory, however, an important conclusion as‐ sociated with the use in the process of the fuel which consisted of 8% of calcium oxide brings up. Hayhurst and Lawrence indicate [37] that even 2% addition of calcium oxide to the process environment may contribute up to twentyfold increase in the rate of NOx formation. The reason of that, is catalytic activity of CaO in the oxidation reaction of CN radical to CNO and further oxidation of CNO to CO and NO. Therefore it can be con‐ cluded that decreasing the amount of CaO in the combustion environment will result in reduction in the concentration of nitrogen oxides after reduction in the second combus‐

reburning fuel was not dosed to the reactor.

processes.

tion zone.

**Figure 4.** Concentrations of chosen organic compounds, registered by Gasmet DX-4000 analyzer, in flue gases. (Index –*r-av* – average value in reburning zone).

should be remembered, however, that it is unacceptable that flue gases with such a high content of carbon compounds to get into atmosphere. These compounds must be combusted in the next - third combustion zone.

The fuel used in the tests characterize a high sulfur content (Table 2). One of the products of thermal utilization of such alternative fuel is sulfur dioxide, which amount in the exhaust gases is strictly regulated by law. The problem that was encountered during the study was that at this stage it is not possible to correct analyze of the SO2 concentration values in the flue gases, during the period when the reburning process was carried on. The reason for this is the application for the detection of this gas electrochemical methods or IR radiation absorption. In devices where to the SO2 determination electrochemical sensors are used, the results are questionable because these sensors have a cross-sensivity to C2H4 and others [36]. During researches when reburning process was conducted, conditions of oxygen insufficiency were present in second combustion zone, resulting in that these compounds are present in signifi‐ cant quantities in the flue gases (Figure 3,4). Moreover, the products of propane combustion are unsaturated hydrocarbons and aldehydes. They characterize an intense absorption of IR radiation in the range 1000 - 1800 cm-1, overlap the typical range for SO2 absorption (1150-1450 cm-1). Their high concentration during carrying on combustion in reburning zone also reduces the IR method as a reliable for determining the SO2 concentration values. The concentration of sulfur dioxide can be correctly determined in those phases of the experiment, in which the reburning fuel was not dosed to the reactor.

**Figure 5.** Concentrations of SO2 and NOx in exhaust fumes. (Index –*r-av* – average value in reburning zone).

should be remembered, however, that it is unacceptable that flue gases with such a high content of carbon compounds to get into atmosphere. These compounds must be combusted in the

**Figure 4.** Concentrations of chosen organic compounds, registered by Gasmet DX-4000 analyzer, in flue gases. (Index

The fuel used in the tests characterize a high sulfur content (Table 2). One of the products of thermal utilization of such alternative fuel is sulfur dioxide, which amount in the exhaust gases is strictly regulated by law. The problem that was encountered during the study was that at this stage it is not possible to correct analyze of the SO2 concentration values in the flue gases, during the period when the reburning process was carried on. The reason for this is the application for the detection of this gas electrochemical methods or IR radiation absorption. In devices where to the SO2 determination electrochemical sensors are used, the results are questionable because these sensors have a cross-sensivity to C2H4 and others [36]. During researches when reburning process was conducted, conditions of oxygen insufficiency were present in second combustion zone, resulting in that these compounds are present in signifi‐ cant quantities in the flue gases (Figure 3,4). Moreover, the products of propane combustion are unsaturated hydrocarbons and aldehydes. They characterize an intense absorption of IR radiation in the range 1000 - 1800 cm-1, overlap the typical range for SO2 absorption (1150-1450 cm-1). Their high concentration during carrying on combustion in reburning zone also reduces

next - third combustion zone.

–*r-av* – average value in reburning zone).

262 Advances in Internal Combustion Engines and Fuel Technologies

The composition of utilized fuel (Table 2) was selected in a manner to bound SO2 present in the flue gases by CaO formed from the CaCO3 contained in the fuel. Analysis of the concen‐ tration of sulfur dioxide was carried out only at a time of one-zone combustion. They showed that its concentration did not exceed 250 mg/m3 then (Figure 5). The obtained value is satisfactory, which means that this desulphurization method is fully sufficient during such processes.

During combustion of alternative fuels with high fuel-nitrogen content, as in the case of the researched material (Table 2) NOx emission level is of about 1100-1400 mgNO2/m3 (normalized at 6% of O2 in the flue gases) with a highest value more than 1500 mgNO2/m3 (Figure 5). These values are very high, often exceeding the emission stand‐ ards in Europe [3]. With the beginning of reburning fuel dosage into the rare zone of the reactor and creation of the second combustion zone, NOx concentration in the flue gases leaving this zone decreased to approximately 400 mgNO2/m3. By more than half of time when reburning process was carried on the NOx concentration was lower than 400 mgNO2/m3. These results are entirely satisfactory, however, an important conclusion as‐ sociated with the use in the process of the fuel which consisted of 8% of calcium oxide brings up. Hayhurst and Lawrence indicate [37] that even 2% addition of calcium oxide to the process environment may contribute up to twentyfold increase in the rate of NOx formation. The reason of that, is catalytic activity of CaO in the oxidation reaction of CN radical to CNO and further oxidation of CNO to CO and NO. Therefore it can be con‐ cluded that decreasing the amount of CaO in the combustion environment will result in reduction in the concentration of nitrogen oxides after reduction in the second combus‐ tion zone.

Evaluation of effectiveness of nitrogen oxides reduction process was done by calculating of degree of reduction of NOx concentration and presenting it as a function of air excess coeffi‐ cient in reburning zone (Figure 6). The degree of reduction - k was determined from the relationship [30]:

$$k = \frac{c\_0 - c\_{reb}}{c\_0} \tag{30}$$

77% degree of reduction of NOx concentration at λr≈0.9. Further increasing of the amount of

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The obtained values of degree of reduction of NOx concentration were compared with literature data for industrial boiler [38] and the laboratory scale boiler [39]. None of them worked in the fluidized bed technology. In the case of industrial scale reactor [38], where residence times of the particles in the second combustion zone are larger (about 1 s) than in the reactor of about 10 kilowatts power used in the tests (residence times 0.2-0.3 s), the maximum degree of reduction achieved was about 69% with a air excess coefficient - λr = 0.9. It also may be noted that in the case of the literature data, despite longer residence times of compounds in reaction zone - air excess coefficient - λr have to be more reduced to achieve the same degree of reduction of nitrogen oxides concentration than in case of presented studies. The results submitted by Wendt et al. [39] for reactor of similar scale to that which was used in the presented study, the maximum degree of NOx reduction that has been achieved was 70% for λr=0.8. In presented in this paper experiment for λr > 1.1 and thus for much less amount of fuel added into the reburning zone, the degree of reduction of the nitrogen oxide concen‐ tration was over 50%, which for ecological and economic reasons is very important. Lowering of the air excess coefficient below 1.0 is associated with an increasing in dosage of fuel to second combustion zone, thus resulting in increase of the procesal cost. Simultaneously, the deficit of oxygen in the reburning zone increases the concentration of CO and hydrocarbons in the flue gases which for environmental reasons is unacceptable. In this case, flue gases from second combustion zone have to be combusted which increases the cost of the process. It could signify that obtained results presage well about legitimacy of using proposed method of nitrogen oxides reduction in small-scale systems, where installation costs play an important role. The achieved results from the reburning process are better than those obtained in industrial reactors and significantly better than those obtained in laboratory scale reactors operating in

High concentrations of carbon monoxide and volatile organic compounds in the flue gases from the reburning zone causes, from legal and ecological point of view, that they should not be emitted into the atmosphere. It is necessary that in the device, where the combustion using reburning to reduce NOx concentration is carried out, it have to be prepared an extra space in which the carbon-containing compounds (other than CO2) will be combusted. This can be achieved by introducing an additional air stream above the zone of nitrogen oxides reduction (Figure 7) [20]. The amount of additional air should be suitably selected in aim not to increase the losses at the outlet of the reactor and obtain gases of the proper quality. The flue gases leaving this zone are directed to the heat exchanger, purified from the ashes, and the cold, dry

dosed reburning fuel did not have an effect on increase of the reduction of NOx.

other than a fluidized bed technique.

**4.1. Introduction**

**4. Modeling of the third combustion zone**

and clean flue gases reach the atmosphere.

where: C0 - the concentration of NOx in the flue gases normalized at 0% O2, when the reburning process is not carried on,

Creb - the concentration of NOx in the flue gases normalized at 0% O2, when the reburning process is carried on.

**Figure 6.** Degree of reduction of NOx concentration – *k,* depending on air excess coefficient in reburning zone, related to comparable conditions – 0% O2 in flue gases

The values obtained were normalized to the concentration of oxygen in the flue gases equal to 0%, in order to be able to compare them with the literature data. When the values of the air excess coefficient in the reburning zone were equal 1.1, the degree of reduction of nitrogen oxides concentration reached about 55% (Figure 6). Increasing the amount of propane dosed to the reburning zone causes a decrease in the amount of oxygen in this zone, so decrease of λr with increase of degree of NOx reduction. For stoichiometric conditions λr=1, k value exceeded 60%. Reducing of air excess coefficient to the value of λr <1 allows to obtain even 77% degree of reduction of NOx concentration at λr≈0.9. Further increasing of the amount of dosed reburning fuel did not have an effect on increase of the reduction of NOx.

The obtained values of degree of reduction of NOx concentration were compared with literature data for industrial boiler [38] and the laboratory scale boiler [39]. None of them worked in the fluidized bed technology. In the case of industrial scale reactor [38], where residence times of the particles in the second combustion zone are larger (about 1 s) than in the reactor of about 10 kilowatts power used in the tests (residence times 0.2-0.3 s), the maximum degree of reduction achieved was about 69% with a air excess coefficient - λr = 0.9. It also may be noted that in the case of the literature data, despite longer residence times of compounds in reaction zone - air excess coefficient - λr have to be more reduced to achieve the same degree of reduction of nitrogen oxides concentration than in case of presented studies. The results submitted by Wendt et al. [39] for reactor of similar scale to that which was used in the presented study, the maximum degree of NOx reduction that has been achieved was 70% for λr=0.8. In presented in this paper experiment for λr > 1.1 and thus for much less amount of fuel added into the reburning zone, the degree of reduction of the nitrogen oxide concen‐ tration was over 50%, which for ecological and economic reasons is very important. Lowering of the air excess coefficient below 1.0 is associated with an increasing in dosage of fuel to second combustion zone, thus resulting in increase of the procesal cost. Simultaneously, the deficit of oxygen in the reburning zone increases the concentration of CO and hydrocarbons in the flue gases which for environmental reasons is unacceptable. In this case, flue gases from second combustion zone have to be combusted which increases the cost of the process. It could signify that obtained results presage well about legitimacy of using proposed method of nitrogen oxides reduction in small-scale systems, where installation costs play an important role. The achieved results from the reburning process are better than those obtained in industrial reactors and significantly better than those obtained in laboratory scale reactors operating in other than a fluidized bed technique.
