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

#### **4.1. Introduction**

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

where: C0 - the concentration of NOx in the flue gases normalized at 0% O2, when the

Creb - the concentration of NOx in the flue gases normalized at 0% O2, when the reburning

**Figure 6.** Degree of reduction of NOx concentration – *k,* depending on air excess coefficient in reburning zone, related

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


0 0 *reb c c <sup>k</sup> c*

relationship [30]:

process is carried on.

reburning process is not carried on,

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to comparable conditions – 0% O2 in flue gases

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 and clean flue gases reach the atmosphere.

In the designing process of the third combustion zone two main factors must be balanced. The first is residence time of combusting gases in this zone. It should ensure a significant reduction in the concentration of CO and VOCs, preferably to a value less than required by local emission standards. The second is the amount of air supplied to the zone. Further increasing the air flow supplied to this zone should reduce the concentration of CO and VOCs in the flue gases. In order to determine the optimal parameters for the task of effective lowering the concentration of main impurities of the flue gases, the simulation calculations were done.

The starting point of the calculations of flue gas composition leaving the third combustion zone was the composition of the exhaust fumes leaving the reburning zone. The measured composition was converted into wet gases composition, the amount of components present in flue gases was reduced because of requirements of used kinetic model. It was assumed that in the inlet stream there are presented O2, H2O, CO, CO2, N2, NO, CH4, C2H2, C2H4, C2H6, HCHO and CH3OH. The amount of water carried by the exhaust fumes was determined on the basis of the balance calculations. The share of remaining components, not taken into consideration was negligibly small.

Next, the temperature and composition of the inlet stream was corrected in accordance to addition of overflame air stream introduced into the third combustion zone. Addition of varying amounts of air (from 0 to 12%) results in changes in the temperature (from 930 °C to 820 °C) and content of O2 (from 0.12 to 1.93%), N2 (from 62.54 to 63.97%), H2O (from 21.48 to 19.62%) and VOCs (from 0.42 to 0.40%) in diluted exhaust fumes. As the temperature of the gas entering the third zone it was assumed temperature calculated from the heat balance, taking into account the temperature of flue gases measured in the reburning zone and room temperature of added air. Overfire air stream was given as a volume fraction of the primary air introduced into the reactor. Simulation of the combustion process was carried out assuming plug flow, isothermal conditions and mass balance for any chosen component i of this system in the form of differential equation:

$$\frac{dm\_i}{dt} = V \cdot \mathbf{M\_i} \cdot \stackrel{\bullet}{\mathbf{v}\_i} \tag{31}$$

conditions, when in the reaction environment there are present higher (than methane) hydrocarbons and their derivatives. The Marinov model turned out to be an appropriate to apply in a case of propane combustion. The latter describes by 638 equations of chemical reaction together with their kinetic parameters, the combustion process which consists of reactions of 126 particles involved therein. Knowing the concentration of the components of the reaction mixture introduced into the third combustion zone and the temperature at the

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initial instant, the mixture composition after any time can be calculated numerically.

**Figure 7.** Schematic representation of combustion zones in reactor during conduction of the reburning process

For purposes of modeling it was assumed that the residence time for the reactants in the model reactor, so in the third combustion zone, is equal to 2 seconds. From the combustion reaction kinetics point of view of, this time is sufficient for degradation of many complex hydrocarbons and their combustion to CO2. Legal requirements in the European Union [3] provides that in the case of waste incineration, flue gases holding time at 850 °C cannot be less than 2 seconds. By a certain velocity of gas flow through the reactor, two-second time determines the reactor area that will have to be occupied by the third-combustion zone. Calculation results are

**4.2. Results of simulation**

where: νi - the molar production rate of the ith species by all elementary reactions taking into account,

Mi - the molecular weight of the ith species,

V - the volume element of the plug flow system.

Specified species can participate in a variety of elementary reactions, and their set can describe even complex processes. Simulation of the combustion process was carried out by solving set of equations (10) using kinetic model of elementary reactions provided by Marinov [40]. Tests of different kinetic models, performed during the preparation of other work of authors [41], revealed that in the case of propane combustion application of a different model e.g. Konnov model [42] - did not give satisfactory results. Konnov model is not appropriate in this procesal conditions, when in the reaction environment there are present higher (than methane) hydrocarbons and their derivatives. The Marinov model turned out to be an appropriate to apply in a case of propane combustion. The latter describes by 638 equations of chemical reaction together with their kinetic parameters, the combustion process which consists of reactions of 126 particles involved therein. Knowing the concentration of the components of the reaction mixture introduced into the third combustion zone and the temperature at the initial instant, the mixture composition after any time can be calculated numerically.

**Figure 7.** Schematic representation of combustion zones in reactor during conduction of the reburning process

#### **4.2. Results of simulation**

In the designing process of the third combustion zone two main factors must be balanced. The first is residence time of combusting gases in this zone. It should ensure a significant reduction in the concentration of CO and VOCs, preferably to a value less than required by local emission standards. The second is the amount of air supplied to the zone. Further increasing the air flow supplied to this zone should reduce the concentration of CO and VOCs in the flue gases. In order to determine the optimal parameters for the task of effective lowering the concentration

The starting point of the calculations of flue gas composition leaving the third combustion zone was the composition of the exhaust fumes leaving the reburning zone. The measured composition was converted into wet gases composition, the amount of components present in flue gases was reduced because of requirements of used kinetic model. It was assumed that in the inlet stream there are presented O2, H2O, CO, CO2, N2, NO, CH4, C2H2, C2H4, C2H6, HCHO and CH3OH. The amount of water carried by the exhaust fumes was determined on the basis of the balance calculations. The share of remaining components, not taken into consideration

Next, the temperature and composition of the inlet stream was corrected in accordance to addition of overflame air stream introduced into the third combustion zone. Addition of varying amounts of air (from 0 to 12%) results in changes in the temperature (from 930 °C to 820 °C) and content of O2 (from 0.12 to 1.93%), N2 (from 62.54 to 63.97%), H2O (from 21.48 to 19.62%) and VOCs (from 0.42 to 0.40%) in diluted exhaust fumes. As the temperature of the gas entering the third zone it was assumed temperature calculated from the heat balance, taking into account the temperature of flue gases measured in the reburning zone and room temperature of added air. Overfire air stream was given as a volume fraction of the primary air introduced into the reactor. Simulation of the combustion process was carried out assuming plug flow, isothermal conditions and mass balance for any chosen component i of this system

> *<sup>i</sup> <sup>i</sup> <sup>i</sup> dm V M*

where: νi - the molar production rate of the ith species by all elementary reactions taking into

Specified species can participate in a variety of elementary reactions, and their set can describe even complex processes. Simulation of the combustion process was carried out by solving set of equations (10) using kinetic model of elementary reactions provided by Marinov [40]. Tests of different kinetic models, performed during the preparation of other work of authors [41], revealed that in the case of propane combustion application of a different model e.g. Konnov model [42] - did not give satisfactory results. Konnov model is not appropriate in this procesal

n

=× × (31)

·

*dt*

of main impurities of the flue gases, the simulation calculations were done.

266 Advances in Internal Combustion Engines and Fuel Technologies

was negligibly small.

in the form of differential equation:


V - the volume element of the plug flow system.

account,

Mi

For purposes of modeling it was assumed that the residence time for the reactants in the model reactor, so in the third combustion zone, is equal to 2 seconds. From the combustion reaction kinetics point of view of, this time is sufficient for degradation of many complex hydrocarbons and their combustion to CO2. Legal requirements in the European Union [3] provides that in the case of waste incineration, flue gases holding time at 850 °C cannot be less than 2 seconds. By a certain velocity of gas flow through the reactor, two-second time determines the reactor area that will have to be occupied by the third-combustion zone. Calculation results are concentrations of individual compounds in selected cross-section of this zone. Calculations were performed for an additional air stream in the size of 1-12% of air used for combustion of fuel. In Figure 8a,b,c there are shown changes in concentrations of VOC, CO and O2. With the increase in the amount of air supplied to the III combustion zone VOCs concentration in the flue gases leaving this zone decreased sharply in a short time. Only by addition of 1% of air VOC concentration changed mild. Where air stream of the size 4% or more (in relation to the air used for combustion of the fuel) is used the concentration of VOCs is reduced about 300 times in less than 0.1 s. The result is that speed of the process of hydrocarbons oxidation reaches almost zero value and model calculations indicate no changes in the concentration of VOCs.

combustion reactions, which main product is CO [43]. Complete oxidation of hydrocarbons is associated with a sizable decrease in the CO concentration, this phenomena is greater as larger is the stream of additional air. The calculations showed that at the output of the third com‐ bustion zone the concentration of CO is equal to 20 ppm after about 0.2 seconds with a stream

**Figure 9.** VOC, CO, and O2 concentrations in flue gases from III combustion zone depending on amount of overfire air

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Changes in the concentration of oxygen in the gases leaving the third combustion zone strongly depend on the size of the air stream introduced into this zone, which determines the quantity of available oxidant. The oxidation of VOCs and CO causes that at the beginning more or less dynamic decrease in the concentration of oxygen occurs. The fastest decrease in this concen‐ tration is observed when the amount of the additional air stream was 4-6% of the amount of primary air. For these values, the concentration of oxygen at the end of the zone is the smallest. This means that virtually all supplied oxygen is consumed in the process. When the size of stream added to the third combustion zone is 1-2% amount of the primary air, a low concen‐ tration of oxygen is a factor limiting the VOC and CO oxidation reaction speed and at the end of this zone its concentration is comparable to the concentration which is obtained by intro‐ ducing into the third combustion zone stream of the size 10% of the primary air stream. Figure 8 a and 8b shows that for the smaller air streams introduced into the third zone changes in the

In Figure 9 there are summarized the results of calculations of concentrations of volatile organic compounds and carbon monoxide in the flue gases from the third combustion zone, converted to mg/m3 and standardized at 6% of oxygen in the flue gases and the molar fraction of oxygen in the flue gases, obtained for different amounts of air stream introduced into the third combustion zone. Amount of hydrocarbons in the flue gases decreases rapidly with increasing amount of additional air. These changes are followed by changes in oxygen concentration. The addition of air stream which constitutes 4% of the primary air caused decrease in the VOCs

of additional air constituting 12% of the air supplied the first zone.

concentration of VOCs and CO are small.

in this zone

**Figure 8.** Time profiles of model of VOC, CO and O2 in third combustion zone

Change in CO concentration in the flue gases occurs slightly different. After entering the air at first CO concentration increases slightly. This is due to taking place parallel hydrocarbon Low-Emission Combustion of Alternative Solid Fuel in Fluidized Bed Reactor http://dx.doi.org/10.5772/54158 269

concentrations of individual compounds in selected cross-section of this zone. Calculations were performed for an additional air stream in the size of 1-12% of air used for combustion of fuel. In Figure 8a,b,c there are shown changes in concentrations of VOC, CO and O2. With the increase in the amount of air supplied to the III combustion zone VOCs concentration in the flue gases leaving this zone decreased sharply in a short time. Only by addition of 1% of air VOC concentration changed mild. Where air stream of the size 4% or more (in relation to the air used for combustion of the fuel) is used the concentration of VOCs is reduced about 300 times in less than 0.1 s. The result is that speed of the process of hydrocarbons oxidation reaches almost zero value and model calculations indicate no changes in the concentration of VOCs.

268 Advances in Internal Combustion Engines and Fuel Technologies

**Figure 8.** Time profiles of model of VOC, CO and O2 in third combustion zone

Change in CO concentration in the flue gases occurs slightly different. After entering the air at first CO concentration increases slightly. This is due to taking place parallel hydrocarbon

(a) (b)

(c)

**Figure 9.** VOC, CO, and O2 concentrations in flue gases from III combustion zone depending on amount of overfire air in this zone

combustion reactions, which main product is CO [43]. Complete oxidation of hydrocarbons is associated with a sizable decrease in the CO concentration, this phenomena is greater as larger is the stream of additional air. The calculations showed that at the output of the third com‐ bustion zone the concentration of CO is equal to 20 ppm after about 0.2 seconds with a stream of additional air constituting 12% of the air supplied the first zone.

Changes in the concentration of oxygen in the gases leaving the third combustion zone strongly depend on the size of the air stream introduced into this zone, which determines the quantity of available oxidant. The oxidation of VOCs and CO causes that at the beginning more or less dynamic decrease in the concentration of oxygen occurs. The fastest decrease in this concen‐ tration is observed when the amount of the additional air stream was 4-6% of the amount of primary air. For these values, the concentration of oxygen at the end of the zone is the smallest. This means that virtually all supplied oxygen is consumed in the process. When the size of stream added to the third combustion zone is 1-2% amount of the primary air, a low concen‐ tration of oxygen is a factor limiting the VOC and CO oxidation reaction speed and at the end of this zone its concentration is comparable to the concentration which is obtained by intro‐ ducing into the third combustion zone stream of the size 10% of the primary air stream. Figure 8 a and 8b shows that for the smaller air streams introduced into the third zone changes in the concentration of VOCs and CO are small.

In Figure 9 there are summarized the results of calculations of concentrations of volatile organic compounds and carbon monoxide in the flue gases from the third combustion zone, converted to mg/m3 and standardized at 6% of oxygen in the flue gases and the molar fraction of oxygen in the flue gases, obtained for different amounts of air stream introduced into the third combustion zone. Amount of hydrocarbons in the flue gases decreases rapidly with increasing amount of additional air. These changes are followed by changes in oxygen concentration. The addition of air stream which constitutes 4% of the primary air caused decrease in the VOCs concentration practically to 20 mg/m3. Under the same conditions, the concentration of CO in the third zone is reduced slightly. Only increase the amount of supplied air to this zone caused a significant decrease in the concentration of CO and the simultaneous increase in the con‐ centration of O2.

CFB - circulating fluidized bed

EC - electrochemical sensors

FCC - fluid catalytic cracking

FID - flame ionization detector

IR - infra red detection

λ- air excess coefficient

compounds, oxygen

SS - sewage sludge

t - time

Mi

process is carried on

CLA - chemiluminescence technique

DIR - dispersive infrared spectroscopy

FTIR - infrared spectroscopy with Fourier transform

K - degree of reduction of NOx concentration

λr - air excess coefficient in reburning area


NDIR - non dispersive infrared spectroscopy

NiCr-Ni - nickel chrome-nickel thermocouples

SCR - Selective Catalytic Reduction

VOCs - volatile organic compounds

SNCR - Selective Non-Catalytic Reduction

V - the volume element of the plug flow system

N - number of components present in the modeled reaction zone

nss - sum o moles of all compounds in flue gases, dry conditions

νi - the molar production rate of the ith species by elementary reaction,

nCO, nCO2, nVOCs, nO2 - number of moles of carbon monoxide, carbon dioxide, volatile organic

m - mass of reacting gases mixture

γ- stoichiometric coefficient for volatile organic compounds in products

κ - stoichiometric coefficient for oxygen delivered to the reaction zone as substrate

Creb - the concentration of NOx in the flue gases normalized at 0% O2, when the reburning

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The simulation of the third combustion zone, as a necessary element in the combustion process, reveals that the adjustment of the composition of the flue gases in this way is possible and does not require building of special installations. Most of the oxidation reactions take place at high speed and the combustion process is almost completed in less than 0.5 seconds.
