**1. Introduction**

Incineration of conventional fuels is hitherto the main method of worldwide energy obtain‐ ment. In view of this fact researches on finding alternative fuels, which may successfully replace fossil fuels, are carried out continually. Both environmental criteria and economic viability are taken into consideration. Materials which fulfill these criteria are different types of waste (industrial, municipal and agricultural) and biomass. These fuels can be significantly less expensive than conventional ones. Moreover, they sources seem to be unfailing at present considering their excessive production by human. Deposited, not processed wastes are even more serious threat to the environment. However, to have possibility of carrying out thermal utilization of this type of material, they must have adequately high calorific value. The material meets this and other above-mentioned criteria, selected to researches which are presented in this paper, is the sewage sludge. Its gross calorific value can exceed 20 MJ [1]. It can successfully be proecological alternative to fossil fuels. Depending on the material used and applied combustion technology, the process may entail various complications.

#### **1.1. Fluidized bed reactors**

In processes involving solid-phase fluidized beds show several valuable properties. In fluidization conditions mass and heat transfer is very good and mixing of the components of the reaction mixture is excellent. Therefore, fluidization of solid particles has a number of industrial applications such as combustion of coal and other combustible materials, fluid catalytic cracking (FCC) of heavy oil into gasoline, spray drying of aqueous solutions, drying of solids like cement and limestone, obtaining very pure silicon by decomposition of silane, separation of fine dust of solids and many others. The possibility of obtaining state of fluidi‐ zation depends strongly on the particle size and density [2]. With the increase in the flow rate

© 2013 Baron et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Baron et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of gas or liquid through the solid layer several operating regimes of bed are distinguished: packed bed, minimum fluidization, bubbling fluidization, turbulent fluidization and pneu‐ matic transport (with lean phase fluidization). In industrial practice fluidized-bed processes are performed in each of these regimes.

**Total rated thermal input (MW)** **Coal and lignite and other solid fuels**

\*) 450 in case of pulverized lignite combustion

**1.2. Nitrogen oxides formation**

metric conditions applied in the process.

present in the air.

**Table 1.** SO2, NOx and dust emission standards in UE

**Biomass Peat**

study was conducted in a laboratory scale fluidized bed reactor.

**Coal and lignite and other solid fuels**

SO2, mg/Nm3 NOx, mg/Nm3 dust, mg/Nm3

In the case of thermal utilization of sewage sludge, biomass, and other wastes, the problem is in very high emission of nitrogen oxides into the atmosphere. It is well known that nitrogen oxides creating and penetrating into the atmosphere from combustion processes carried out both in industrial processes, power industry and in households, pose a serious threat to the environment and health. Sludge contains nitrogen bound in organic and inorganic com‐ pounds. According to Petersen [4], the high nitrogen content in the sludge results in nitrogen oxides emission up to 2500 mg/m3, during its combustion. Reduction of nitrogen oxides concentration in flue gases, produced during thermal utilization of solid alternative fuel, is the main topic of this paper. A technique that has been used for this purpose is the reburning. The

Due to the fact of availability, costs and the properties the most commonly used oxidizer in combustion processes is oxygen from the air. However, its use in the thermal utilization processes has also disadvantages. One of these is that under certain conditions, reacts with nitrogen leads to unavoidable formation of nitrogen oxides irrespectively of type of fuel which is used and combustion technology which is applied. Nitrogen oxides can be formed in the combustion processes by a number of mechanisms from atmospheric or fuel nitrogen. During the synthesis of NO, which consists of a number of complex processes, multiplicity of radicals are created, which quantity can be influenced depending on thermodynamic and stoichio‐

Formation of nitrogen oxides from atmospheric nitrogen is explained by means of ther‐ mal mechanism (Zeldovich mechanism) [5]. The beginning of the processes leading to the formation of NO here is the thermal dissociation of oxygen and nitrogen molecules

*O N NO N* <sup>2</sup> +® + (1)

50-100 400 200 300 300\*) 300 30 30 100-300 250 200 300 200 250 25 22 >300 200 200 200 200 200 20 20

**Biomass and peat**

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

**Coal and lignite and other solid fuels**

http://dx.doi.org/10.5772/54158

**Biomass and peat**

247

Combustion of fuels in a fluidized bed has the advantage that during vigorous stirring oxygen is supplied to the particulate fuel. This virtually eliminates the occurrence of oxygen-poor zones in the reactor and this results in relatively even emission of heat from the combustion process. Processes of drying and degassing which always accompany the burning of solid fuels take place intensely in the whole volume of fluidized bed (on all the grains put into the fuel) and not, as in the case of a constant fuel layer only in a narrow zone of high temperature. In the power industry stationary fluidized bed boilers (usually bubbling), fluidized bed with a thermal capacity of less than 150-200 MW, often less than 15 MWe and fluidized bed boilers with circulating fluidized bed of high power - up to about 500 MWe are used. Currently units with a capacity of 600-800 MW, which will fullfil in the near future high requirements of municipal operators, are designed.

In the power plant Łagisza situated in Bedzin (Poland) in the year 2009, first Supercritical CFB (circulating fluidized bed) boiler also being the world's largest CFB unit with a capacity of 460 Mwe, started to work. Capital expenditures for the construction of the CFB boiler was more than 15% lower than expected expenditures for the construction of dust boiler of the same power along with the necessary installation of flue gases desulphurization. The boiler produces 361 kg/s steam at 560 °C and a pressure of 27.5 MPa. For heating the boiler maximum 187 t/h of coal is consumed in the presence of SO2 and NO<sup>x</sup> emissions less than 200 mg/Nm3 , and dust emission below 30 mg/Nm3 . An additional advantage of such boiler is its fuel flexibility. Circulating Fluidized Bed combustion has given boiler and power plant operators a greater flexibility in burning a wide range of coal and other fuels. All this without compro‐ mising efficiency and with reduced pollution.

The necessity of transformation of a large mass of particulate solid into a fluidized state causes that air have to be used to this aim, in an amount much greater than required for combustion of the fuel. That is the reason why gas production in fluidized bed boilers is on considerable amount. Since combustion takes place at a temperature of 750-900 °C, too low to be able to play an important role in the processes of synthesis of NO from nitrogen and oxygen contained in the air, the content of NOx in the exhaust gases is significantly lower than during the combustion of fuel using other methods. Intense mixing of solid particles, however, causes that exhaust gases leaving the boiler contain more dust than those from other types of boilers. Installation for fluidized bed combustion requires much more efficient dust collection systems.

In the European Union obtains the directive [3] in which emission values limits for selected air pollutants are defined for power equipment of specified size. Crucial data for existing installations (i.e. combustion plants which have been granted a permit before 7January 2013) are summarized in Table 1.


\*) 450 in case of pulverized lignite combustion

of gas or liquid through the solid layer several operating regimes of bed are distinguished: packed bed, minimum fluidization, bubbling fluidization, turbulent fluidization and pneu‐ matic transport (with lean phase fluidization). In industrial practice fluidized-bed processes

Combustion of fuels in a fluidized bed has the advantage that during vigorous stirring oxygen is supplied to the particulate fuel. This virtually eliminates the occurrence of oxygen-poor zones in the reactor and this results in relatively even emission of heat from the combustion process. Processes of drying and degassing which always accompany the burning of solid fuels take place intensely in the whole volume of fluidized bed (on all the grains put into the fuel) and not, as in the case of a constant fuel layer only in a narrow zone of high temperature. In the power industry stationary fluidized bed boilers (usually bubbling), fluidized bed with a thermal capacity of less than 150-200 MW, often less than 15 MWe and fluidized bed boilers with circulating fluidized bed of high power - up to about 500 MWe are used. Currently units with a capacity of 600-800 MW, which will fullfil in the near future high requirements of

In the power plant Łagisza situated in Bedzin (Poland) in the year 2009, first Supercritical CFB (circulating fluidized bed) boiler also being the world's largest CFB unit with a capacity of 460 Mwe, started to work. Capital expenditures for the construction of the CFB boiler was more than 15% lower than expected expenditures for the construction of dust boiler of the same power along with the necessary installation of flue gases desulphurization. The boiler produces 361 kg/s steam at 560 °C and a pressure of 27.5 MPa. For heating the boiler maximum 187 t/h of coal is consumed in the presence of SO2 and NO<sup>x</sup> emissions less than 200 mg/Nm3

flexibility. Circulating Fluidized Bed combustion has given boiler and power plant operators a greater flexibility in burning a wide range of coal and other fuels. All this without compro‐

The necessity of transformation of a large mass of particulate solid into a fluidized state causes that air have to be used to this aim, in an amount much greater than required for combustion of the fuel. That is the reason why gas production in fluidized bed boilers is on considerable amount. Since combustion takes place at a temperature of 750-900 °C, too low to be able to play an important role in the processes of synthesis of NO from nitrogen and oxygen contained in the air, the content of NOx in the exhaust gases is significantly lower than during the combustion of fuel using other methods. Intense mixing of solid particles, however, causes that exhaust gases leaving the boiler contain more dust than those from other types of boilers. Installation for fluidized bed combustion requires much more efficient dust collection systems.

In the European Union obtains the directive [3] in which emission values limits for selected air pollutants are defined for power equipment of specified size. Crucial data for existing installations (i.e. combustion plants which have been granted a permit before 7January 2013)

. An additional advantage of such boiler is its fuel

are performed in each of these regimes.

246 Advances in Internal Combustion Engines and Fuel Technologies

municipal operators, are designed.

and dust emission below 30 mg/Nm3

are summarized in Table 1.

mising efficiency and with reduced pollution.

**Table 1.** SO2, NOx and dust emission standards in UE

In the case of thermal utilization of sewage sludge, biomass, and other wastes, the problem is in very high emission of nitrogen oxides into the atmosphere. It is well known that nitrogen oxides creating and penetrating into the atmosphere from combustion processes carried out both in industrial processes, power industry and in households, pose a serious threat to the environment and health. Sludge contains nitrogen bound in organic and inorganic com‐ pounds. According to Petersen [4], the high nitrogen content in the sludge results in nitrogen oxides emission up to 2500 mg/m3, during its combustion. Reduction of nitrogen oxides concentration in flue gases, produced during thermal utilization of solid alternative fuel, is the main topic of this paper. A technique that has been used for this purpose is the reburning. The study was conducted in a laboratory scale fluidized bed reactor.

#### **1.2. Nitrogen oxides formation**

,

Due to the fact of availability, costs and the properties the most commonly used oxidizer in combustion processes is oxygen from the air. However, its use in the thermal utilization processes has also disadvantages. One of these is that under certain conditions, reacts with nitrogen leads to unavoidable formation of nitrogen oxides irrespectively of type of fuel which is used and combustion technology which is applied. Nitrogen oxides can be formed in the combustion processes by a number of mechanisms from atmospheric or fuel nitrogen. During the synthesis of NO, which consists of a number of complex processes, multiplicity of radicals are created, which quantity can be influenced depending on thermodynamic and stoichio‐ metric conditions applied in the process.

Formation of nitrogen oxides from atmospheric nitrogen is explained by means of ther‐ mal mechanism (Zeldovich mechanism) [5]. The beginning of the processes leading to the formation of NO here is the thermal dissociation of oxygen and nitrogen molecules present in the air.

$$\rm{NO} + \rm{N}\_2 \rightarrow \rm{NO} + \rm{N} \tag{1}$$

$$\rm{NO} + \rm{O}\_2 \rightarrow \rm{NO} + \rm{O} \tag{2}$$

*HCN H CN H*<sup>2</sup> +® + (10)

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

http://dx.doi.org/10.5772/54158

249

*CN OH HCO N* +® + (11)

*CN O CO N* +® + (12)

*CN O NCO O* <sup>2</sup> +® + (13)

*NCO O NO CO* +® + (14)

*NH O NO H* +® + (15)

*NH OH HNO H* +® + (16)

*NH O HNO H* <sup>2</sup> +® + (17)

*HNO H NO H*<sup>2</sup> +® + (18)

*HNO OH NO H O*<sup>2</sup> +®+ (19)

*HNO O NO OH* +® + (20)

Due to the requirements of environmental protection and associated regulations, NOx produced during combustion processes must be removed from the flue gases which enter the atmosphere. In practice, the NOx reduction processes are carried out directly in the boiler (primary methods –Selective Non-Catalytic Reduction (SNCR), reburning) or in separate

The essence of the SNCR method is addition to the combustion zone stoichiometrically selected amounts of ammonia or urea. These substances are transformed and as results NHi radicals are created, which in turn react with NO reducing it to N2 [12]. In case of use of ammonia process is carried out at a temperature of 770 - 1000 °C. In the case of urea, the first stage of the process (the decomposition of urea) it is carried out in the temperature range 300-620 °C. In

installations (secondary methods – Selective Catalytic Reduction (SCR)).

**1.3. Reduction methods of nitrogen oxides**

*1.3.1. SNCR and SCR metod*

$$\text{N} + \text{OH} \rightarrow \text{NO} + \text{H} \tag{3}$$

Dissociation processes of those molecules take place efficiently in a high temperature (over 1400 °C) which means that in practice, during the combustion processes in a fluidized bed reactor it can be omitted part NO formed in accordance with a thermal mechanism.

Formation of NO in the flue gases during the combustion of hydrocarbon fuels at a temperature lower than 1000 °C describes proposed by Fenimore "prompt".mechanism [6]. Crucial role plays in it CH radicals, which undergoes transformation in reactions with nitrogen and oxygen from the air, they are source of the NO formation at a high level [6-9].

$$\rm{CH} + \rm{N}\_2 \rightarrow \rm{HCN} + \rm{N} \tag{4}$$

$$\text{CH}\_2 + \text{N}\_2 \rightarrow \text{HCN} + \text{NH} \tag{5}$$

$$H\text{CN} + O \rightarrow \text{NCO} + H \tag{6}$$

$$\text{NCO} + \text{O} \rightarrow \text{NO} + \text{CO} \tag{7}$$

Under increased pressure significant path for NO formation is through N2:

$$N\_2 + O + M \to N\_2O + M \tag{8}$$

Such conditions are not met often in fluidized installations, usually working under atmos‐ pheric pressure.

The most important role in the creation of NO plays mechanism whereby to the formation of nitrogen oxides, nitrogen bound in fuel is used. Nitrogen - usually bounded in the organic matter in a form of cyclic compounds or amines - reacts easier at elevated temperatures. In the combustion processes nitrogen occurs in hydrogen cyanide and radicals CN, HNO and NHi. As a result of transformation of these radicals in the reaction with oxygen and OH radicals NO is produced [10,11]. OH radical which plays a central role in the oxidation of carbon, associated in organic matter, to CO also plays an important role in the oxidation of nitrogen bounded in fuel to nitrogen oxide.

$$H\text{CN} + M \rightarrow \text{CN} + H + M \tag{9}$$

Low-Emission Combustion of Alternative Solid Fuel in Fluidized Bed Reactor http://dx.doi.org/10.5772/54158 249

$$H\text{CN} + H \to \text{CN} + H\_2 \tag{10}$$

$$\text{CH} + \text{OH} \rightarrow \text{HCO} + \text{N} \tag{11}$$

$$\text{CN} + \text{O} \rightarrow \text{CO} + \text{N} \tag{12}$$

$$\text{CN} + \text{O}\_2 \rightarrow \text{NCO} + \text{O} \tag{13}$$

$$\text{NCO} + \text{O} \rightarrow \text{NO} + \text{CO} \tag{14}$$

$$\text{NH} + \text{O} \rightarrow \text{NO} + \text{H} \tag{15}$$

$$\text{NH} + \text{OH} \rightarrow \text{HNO} + \text{H} \tag{16}$$

$$\rm NH\_2 + O \rightarrow HNO + H \tag{17}$$

$$H\text{NO} + H \rightarrow \text{NO} + H\_2 \tag{18}$$

$$H\text{NO} + \text{OH} \rightarrow \text{NO} + \text{H}\_2\text{O} \tag{19}$$

$$\text{HNO} + \text{O} \rightarrow \text{NO} + \text{OH} \tag{20}$$

#### **1.3. Reduction methods of nitrogen oxides**

#### *1.3.1. SNCR and SCR metod*

*N O NO O* <sup>2</sup> +® + (2)

*N OH NO H* +®+ (3)

*CH N HCN N* <sup>2</sup> +® + (4)

*CH N HCN NH* 2 2 +® + (5)

*HCN O NCO H* +® + (6)

*NCO O NO CO* +® + (7)

*N O M NO M* 2 2 ++ ® + (8)

*HCN M CN H M* + ® ++ (9)

Dissociation processes of those molecules take place efficiently in a high temperature (over 1400 °C) which means that in practice, during the combustion processes in a fluidized bed

Formation of NO in the flue gases during the combustion of hydrocarbon fuels at a temperature lower than 1000 °C describes proposed by Fenimore "prompt".mechanism [6]. Crucial role plays in it CH radicals, which undergoes transformation in reactions with nitrogen and oxygen

reactor it can be omitted part NO formed in accordance with a thermal mechanism.

from the air, they are source of the NO formation at a high level [6-9].

248 Advances in Internal Combustion Engines and Fuel Technologies

Under increased pressure significant path for NO formation is through N2:

pheric pressure.

fuel to nitrogen oxide.

Such conditions are not met often in fluidized installations, usually working under atmos‐

The most important role in the creation of NO plays mechanism whereby to the formation of nitrogen oxides, nitrogen bound in fuel is used. Nitrogen - usually bounded in the organic matter in a form of cyclic compounds or amines - reacts easier at elevated temperatures. In the combustion processes nitrogen occurs in hydrogen cyanide and radicals CN, HNO and NHi. As a result of transformation of these radicals in the reaction with oxygen and OH radicals NO is produced [10,11]. OH radical which plays a central role in the oxidation of carbon, associated in organic matter, to CO also plays an important role in the oxidation of nitrogen bounded in

Due to the requirements of environmental protection and associated regulations, NOx produced during combustion processes must be removed from the flue gases which enter the atmosphere. In practice, the NOx reduction processes are carried out directly in the boiler (primary methods –Selective Non-Catalytic Reduction (SNCR), reburning) or in separate installations (secondary methods – Selective Catalytic Reduction (SCR)).

The essence of the SNCR method is addition to the combustion zone stoichiometrically selected amounts of ammonia or urea. These substances are transformed and as results NHi radicals are created, which in turn react with NO reducing it to N2 [12]. In case of use of ammonia process is carried out at a temperature of 770 - 1000 °C. In the case of urea, the first stage of the process (the decomposition of urea) it is carried out in the temperature range 300-620 °C. In this method it is important to maintain a suitable temperature, because at temperatures above 1093 °C the process of ammonia oxidation, with oxygen from the air which results in consid‐ erable amounts of NO, becomes extremely important. Applying the SNCR method a 70% reduction of NO can be achieved [12,13] and in practice urea is used more often because of its safety.

used as a reburning fuel, what is practice until today. Combustion installations are one of the main sources of nitrogen oxides emissions. Many of them do not have technical solutions, which provide simultaneous combustion of two fuels - solid as a primary fuel and gas or liquid as reburning fuel. In connection with this fact continual researches of the two zone combustion process where coal is the reburning fuel are carried on [17]. Efficiency of those processes is at

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

http://dx.doi.org/10.5772/54158

251

Other commonly used reburning fuels are gaseous fuels [18,22-25,27-29]. Studies have shown that by using such reburning fuel, with a suitably selected residence time of the reactants in the combustion chamber in the installation of 350 MW power, it can be achieved the degree of

Increasingly, there are made attempts to use biomass and other wastes, such as meat or waste tires as reburning fuel [30-34]. The obtained results indicate that a reduction of the NO concentration which can be achieved here is about 70% [34] and even up to 80% [32,33].

In the simplest version of the process organization, in the reactor with stationary (bubbling) fluidized bed combustion is carrying on exclusively in one zone. However conducting processes this way causes that thermal utilization of the materials with a high fuel-nitrogen content, such as sewage sludge, becomes impossible due to the fact of the emission of nitrogen oxides. On the other hand the effective waste management taking into account i.a. costs of transport may suggest, application of a scattered spatially, small scale devices where a gaseous fuel is used as a reburning factor and location of NOx reduction zone is in the rare zone of the fluidized bed. In the literature, there is a lack of reliable information about carrying on reburning processes using this configuration. Aim of this study is to examine the reburning process, achieved by introducing additional gaseous fuel - propane - to rare zone of the bed

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

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

level about 60 - 70%, when air excess coefficient in the reburning area is 0.7 - 1.1.

NO conversion of more than 60% [18].

during combustion of alternative solid fuel.

**1.4. Aim of the researches**

**2. Experimental**

alternative fuel.

**2.1. Experimental equipment**

In the SCR method [14] - NOx reduction is carried out outside the combustion chamber after thorough dedusting of flue gases. Reactions are taking place there on properly selected catalyst and the most commonly reducing reagent used is ammonia. Application of catalyst in this case decreases the activation energy for the reduction reaction of nitrogen oxides leading to N2. Catalysts, which are used in the SCR technology are: platinum, tungsten-vanadium supported on TiO2, ZrO2, SiO2, Al2O3 and zeolite carriers [14,15].

#### *1.3.2. Reburning method*

Reburning is one of the primary methods of NOx reduction. This technology involves the introduction of additional hydrocarbon fuel into the zone of flue gases, which means the creation in this area of the second combustion zone. The authors of this method - Wendt and collaborators [16] called it reburning. They proposed it in 1973 to reduction of SO2 to SO3 and NOx to N2. A sine qua non for the occurrence of the reduction process in this second com‐ bustion zone is creation of a reducing environment there. In such environment CHx, OH, CN and other radicals are present, which are involved in the complex mechanism of NO reduction summary described by the equation:

$$\text{2NO} + \text{C}\_3\text{H}\_8 + 4\text{O}\_2 \rightarrow \text{N}\_2 + \text{3CO}\_2 + 4\text{H}\_2\text{O} \tag{21}$$

Reburningu method since the beginning of its implementation was and still is widely used in the technics [17-23]. Its main advantages are: the reduction of NOx at a satisfactory level (up to 70% in industrial installations), the economic viability (it is cheaper method compared to SCR), technological simplicity and safety compared to the SCR and SNCR where ammonia is used. The reducing reagent in reburning method is a hydrocarbon fuel, often the same which is used in the first combustion zone. Broad applicability criterion of reburning process causes that continual research on its modification are carried on, the reason of that is obtaining the flue gases of the best composition. The practical significance of this method is evident when there is taken into consideration the power industry which is one of the main producers of NOx. Reburning is used there as nitrogen oxides removal method from flue gases [20, 24-26]. NOx concentration reduction degree achieved in industrial installations of this type exceeds 70% [26]. First who showed this technology on such large scale - the reactor MW-power coalfired, at the end of 80's was The Babcock & Wilcox Company [20]. They obtained then the degree of nitrogen oxide concentration reduction of about 50%.

There are carried out researches to optimize the process of combustion in the second zone. They are usually focused on the selection and use of various reburning fuels. Initially coal was used as a reburning fuel, what is practice until today. Combustion installations are one of the main sources of nitrogen oxides emissions. Many of them do not have technical solutions, which provide simultaneous combustion of two fuels - solid as a primary fuel and gas or liquid as reburning fuel. In connection with this fact continual researches of the two zone combustion process where coal is the reburning fuel are carried on [17]. Efficiency of those processes is at level about 60 - 70%, when air excess coefficient in the reburning area is 0.7 - 1.1.

Other commonly used reburning fuels are gaseous fuels [18,22-25,27-29]. Studies have shown that by using such reburning fuel, with a suitably selected residence time of the reactants in the combustion chamber in the installation of 350 MW power, it can be achieved the degree of NO conversion of more than 60% [18].

Increasingly, there are made attempts to use biomass and other wastes, such as meat or waste tires as reburning fuel [30-34]. The obtained results indicate that a reduction of the NO concentration which can be achieved here is about 70% [34] and even up to 80% [32,33].

## **1.4. Aim of the researches**

this method it is important to maintain a suitable temperature, because at temperatures above 1093 °C the process of ammonia oxidation, with oxygen from the air which results in consid‐ erable amounts of NO, becomes extremely important. Applying the SNCR method a 70% reduction of NO can be achieved [12,13] and in practice urea is used more often because of its

In the SCR method [14] - NOx reduction is carried out outside the combustion chamber after thorough dedusting of flue gases. Reactions are taking place there on properly selected catalyst and the most commonly reducing reagent used is ammonia. Application of catalyst in this case decreases the activation energy for the reduction reaction of nitrogen oxides leading to N2. Catalysts, which are used in the SCR technology are: platinum, tungsten-vanadium supported

Reburning is one of the primary methods of NOx reduction. This technology involves the introduction of additional hydrocarbon fuel into the zone of flue gases, which means the creation in this area of the second combustion zone. The authors of this method - Wendt and collaborators [16] called it reburning. They proposed it in 1973 to reduction of SO2 to SO3 and NOx to N2. A sine qua non for the occurrence of the reduction process in this second com‐ bustion zone is creation of a reducing environment there. In such environment CHx, OH, CN and other radicals are present, which are involved in the complex mechanism of NO reduction

2*NO* + *C*3*H*<sup>8</sup> + 4*O*2→ *N*<sup>2</sup> + 3*CO*<sup>2</sup> + 4*H*2*O*

degree of nitrogen oxide concentration reduction of about 50%.

Reburningu method since the beginning of its implementation was and still is widely used in the technics [17-23]. Its main advantages are: the reduction of NOx at a satisfactory level (up to 70% in industrial installations), the economic viability (it is cheaper method compared to SCR), technological simplicity and safety compared to the SCR and SNCR where ammonia is used. The reducing reagent in reburning method is a hydrocarbon fuel, often the same which is used in the first combustion zone. Broad applicability criterion of reburning process causes that continual research on its modification are carried on, the reason of that is obtaining the flue gases of the best composition. The practical significance of this method is evident when there is taken into consideration the power industry which is one of the main producers of NOx. Reburning is used there as nitrogen oxides removal method from flue gases [20, 24-26]. NOx concentration reduction degree achieved in industrial installations of this type exceeds 70% [26]. First who showed this technology on such large scale - the reactor MW-power coalfired, at the end of 80's was The Babcock & Wilcox Company [20]. They obtained then the

There are carried out researches to optimize the process of combustion in the second zone. They are usually focused on the selection and use of various reburning fuels. Initially coal was

on TiO2, ZrO2, SiO2, Al2O3 and zeolite carriers [14,15].

250 Advances in Internal Combustion Engines and Fuel Technologies

safety.

*1.3.2. Reburning method*

summary described by the equation:

In the simplest version of the process organization, in the reactor with stationary (bubbling) fluidized bed combustion is carrying on exclusively in one zone. However conducting processes this way causes that thermal utilization of the materials with a high fuel-nitrogen content, such as sewage sludge, becomes impossible due to the fact of the emission of nitrogen oxides. On the other hand the effective waste management taking into account i.a. costs of transport may suggest, application of a scattered spatially, small scale devices where a gaseous fuel is used as a reburning factor and location of NOx reduction zone is in the rare zone of the fluidized bed. In the literature, there is a lack of reliable information about carrying on reburning processes using this configuration. Aim of this study is to examine the reburning process, achieved by introducing additional gaseous fuel - propane - to rare zone of the bed during combustion of alternative solid fuel.
