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Latin American applied research

versión impresa ISSN 0327-0793

Lat. Am. appl. res. vol.42 no.4 Bahía Blanca oct. 2012

 

Combustion of dry sewage sludge particle in a fluidized bed reactor

R.A. Rodriguez and S.M. Udaquiola

Chemical Eng. Institute, San Juan University, San Juan, CA 5400, Argentina.
rrodri@unsj.edu.ar, estelaudaquiola@unsj.edu.ar

Abstract — This paper presents a mathematical model of the cylindrical particle combustion in a fluidized bed reactor. This particle is composed of sewage sludge. The model performs a mathematical description of the physical and chemical phenomena that occur during particle combustion. The uniform particle model without ash accumulation is proposed. Sewage sludge is considered composed of three fractions. Two fractions pyrolyzed forming char. Then, this char is burned. The other fraction is burned without pyrolysis. In order to describe the mechanism and kinetics reaction, data from thermogravimetric analysis were used. The reaction kinetics was proposed as a function of mass loss of each fraction. To validate the model, the particle combustion time obtained by the simulation was compared with the experimental combustion time.
Also, this work presents the combustion atmosphere temperature and the particle size influence in the combustion phenomena. The model predicts acceptably the combustion time, according to experimental results.

Keywords — Sewage Sludge Particle; Combustion; Fluidized Bed Reactor; Mathematical Model

I. INTRODUCTION

In order to describe the reaction between a solid particle and surrounding fluid, there are several kinetic models. Amongst them, the models of homogeneous and heterogeneous particle can be mentioned. In the first case, the particle is considered as a homogeneous medium. The solid is not porous, the reaction occurs on the external surface of the particle, therefore, during the time, the particle size is reduced. The limiting factors of the global reaction rate can be: the chemical reaction rate, diffusion, or both. There are several variants of this approach: the unreacted core model without release of their products, the unreacted core model with release of their products and model homogeneous porous particle (Ogada and Werther, 1996). In the latter approach, there are two cases: the solid produce or not a residue (ash).

Dümplemann et al. (1991) used the homogeneous particle approach for modeling the sewage sludge particle pyrolysis. The phenomenon is controlled by external heat transfer until aconversion equal to 90% and then, it is produced under kinetic control. These authors considered that the solid density varied with the time. Dennis et al. (2005) also used this approach in order to describe the sludge char combustion phenomena, but they assumed that the controlling step was the oxygen transfer to the surface particle reaction and its density does not change. The ash does not release, it forms a skeleton, and no attrition or fragmentation. Khiari (2006) worked assuming homogeneous particle, but its combustion was controlled by both, mass and heat transfer and the reactions rate. This author considered that the density varied in terms of bound water and volatiles release. The model presented by Cano et al. (2007) supposed a uniform particle and the ash remains attached to it during the combustion phenomenon. These authors considered the attrition of the particle.

In the heterogeneous particle model, a particle discretization in elementary control multiple domains is performed. Each domain is characterized by a set of physico-chemical properties. The mass, heat and quantity of movement conservation equations must be resolved in each one. The macroscopic properties of heterogeneous material to the local level, the initial conditions and limits must be known. Bruch et al. (2003) applied this model to a large wood particle during combustion.

According to the reviewed literature, there are multiple models that describe the sewage sludge particles combustion of different sizes and different operation conditions. In this work, a mathematical model was proposed in order to describe the phenomena that occurred during the large cylindrical particle sewage sludge combustion. The simulations were carried out at different temperatures and particle diameter in order to observe these influences in the studied combustion phenomena.

II. Model of a single sewage sludge particle combustion in a fluidized bed reactor

A. Main assumptions of the model

- The sludge particle is cylindrical. Its size, mass, density and temperature are known. It is considered homogeneous, its properties are identical in any spatial position.

- The particle is not porous. This hypothesis is justified by an estimate of its porosity:

(1)

where ε, ρp are the porosity and density of the particle, Vpores is the pore volume determined experimentally (Rodriguez et al., 2008). The calculated porosity is equal to 0.0036, confirming the hypothesis.

- The particle is considered composing by organic matter and ash. The decomposition kinetic of each fraction, and their residues have been described by Arrhenius law (Rodriguez et al., 2008). Let suppose the sludge is composed of three parts or fractions Fi (i = 1, 2, 3). These parts or fractions are decomposed in oxidative atmosphere according to (Eq. (2) to (6)):

(2)
(3)
(4)
(5)
(6)

where Vi and Si are the gases and volatiles produced and the solid residue formed by the decomposition, res-pectively, si the yield coefficient of the solid residue.

It was supposed that the second fraction is oxidized without previous decomposition (oxidative pyrolysis), and the solid residues resulting of reactions (I) and (III) are oxidized (Font et al., 2005).

- Only the solid particle is considered, the reactions between gaseous products of pyrolysis are not taken into account.

- The external mass transfer (to the particle surface) during the various stages of the combustion, is negligible comparing with the reaction kinetics (Khiari, 2007). The mass release from the particles is totally controlled by this kinetics.

- All produced gases are released immediately.

- The ash is chemically inert.

- The surrounding atmosphere of the particle is homogeneous taking into account the temperature and composition, its physico-chemical properties are known.

- The particle radius varies with the different reactions occurring during combustion phenomenon. In order to verify this hypothesis, sewage sludge particles are burning in a muffle furnace at 850°C. They did not lose their way after this treatment but, they broke with a gentle touch. Considering these results, the ash formed during the combustion in fluidized bed is loosed due to the bed motion. According to this hypothesis, the proposed model is an unreacted core model with release of its products.

- The fluidization gas temperature is considered equals to the reactor wall temperature (stagnant atmosphere).

- Particle is considered isothermal. In order to check this hypothesis, the thermal Biot number was calculated (Bi):

(7)

where h*T includes the heat transfer by convection and radiation, dp is the particle diameter and λp is the thermal conductivity of sewage sludge. In order to calculate the dimensionless number, the thermal sewage sludge conductivity is considered constant (Gratias, 2002). The calculated Bi value is smaller than unity. The Fig. 1 shows the Bi variation with the particle diameter during the combustion:


Figure 1: Bi variation with the particle diameter during the combustion.

Considering the obtained results, the particle temperature is supposed uniform during the combustion.

B. Energy balance

The kinetics of each reaction depends strongly on the particles temperature. In order to know this temperature, it is necessary to perform the energy balance. The enthalpy variation of the particle during the combustion can be described by the following equation:

(8)

where Hp is the particle enthalpy, Qext(t) is the external heat flow, Qpir(t) is the heat flow due to pyrolysis reactions and Qcomb(t) is the heat flow due to combustion reactions. The particle enthalpy can be described:

(9)

where Tref is the reference temperature, Tp(t) is the particle temperature, cp(t) is the solid heat capacity and Wp(t), the solid total mass at any instant. The external heat flow can be written as:

(10)

where Sp(t) is the particle external surface, hT is the coefficient of heat transfer by convection, Tg the gas temperature, σ and ε are the Bolztman constant and the emissivity respectively. Tpared is the reactor wall temperature. Tg and Tpared are considered constant, assuming that the reactor has reached thermal equilibrium.

The pyrolysis reaction energy can be written as:

(11)

where Fmpir(t) is the mass flow of released volatiles leaving the particle and ΔHpir is the pyrolysis enthalpy. Its value is considered equal to -255kJ/kg (Babu and Chaurasia, 2004) and it is identical for all pyrolysis reactions. The energy of combustion reaction can be written as:

(12)

where Fmcomb(t) is the mass flow of consumed char and ΔHcomb is the combustion enthalpy considered identical for all combustion reactions. This value is assumed equal to the average low heating value, 9976.5kJ/kg.

C. Material balance

Pyrolysis and combustion: The mechanism adopted for the combustion phenomena is shown in Eq. (2) to (6). The total mass can be expressed according to:

(13)

where Wp(t), WFi(t) and WSi(t) are the particle mass, different fractions and residues solids mass at time t, respectively. The mass of different fractions and char at time t can be described by the following equations:

(14)
(15)
(16)
(17)
(18)

where W0 is the initial mass of the particle, wi are the weight ratios of each fraction, wi is the weight ratio at infinite time, n and m are the corresponding reaction orders, and ksi and ki are constants reaction, expressed as a function particle temperature, using Arrhenius law. According to the proximate analysis (content of ash between 48-51 %), w can be expressed (Rodriguez et al., 2008):

(19)

The mass evolution of the different fractions and chars in time are obtained from mass balances:

(20)
(21)
(22)
(23)
(24)

The variation of the particle radius is due to the mass loss in each reaction; such variation can be described by the following Eq.:

(25)

where ρp is the particle density, it is considered constant and Sp is the particle external surface (all cylindrical particle surface).

D. Model parameters

The physical properties of fluidization air have been calculated using the correlations shown in Table 1, where Tg is the gas temperature into the reactor. The kinetic parameters of the different reactions are (Rodriguez et al., 2008):

Table 1: Correlations used to calculate the physical properties of fluidization air.

k'1= 9.36 *1012 min-1 ; E1 = 100000 J/mol; w01 = 0.275; w1= 0.100; n1 = 3.6

k'2= 3.3*1014 min1; E2 = 104000 J/mol; w02 = 0.335; w2= 0.180 n2= 4.6

k'3= 5.4*1017 min-1; E3 = 210000 J/mol; w03 = 0.360; w3 = 0.200 n3 = 5.1

kS1'= 4.8*1010 min-1; ES1 = 99500 J/mol; w01 = 0.000; wS1 = 0.000; m1= 3.4

kS3'= 5.1*109 min-1; ES3 = 110000 J/mol; w03 = 0.000; wS3= 0.000; m3= 3.8

It is important to consider that the thermogravimetric technique measures the sample weight loss with time and temperature. Under controlled conditions the limitations of heat and mass transfers between the sample, sample tray holder and carrier gas can be considered negligible, therefore this technique is used in order to follow only the chemical kinetics and other processes (Menis et al., 1980; González et al., 2001).

The simulation was carried out with a initial diameter and height of particle equal to 10 mm and 5 mm, respectively. Its density is considered equal to 1200 kgm-3 (Dümpelmann et al., 1991) and conductivity, 0.8 Wm-1K-1) (Gratias, 2002).

The heat capacity (Jkg-1K), was calculated using the following expression (Babu and Chaurasia 2004):

(26)

where Tp is the particle temperature. The conditions of external heat transfer around the cylindrical particle are calculated based on the correlation shown in Table 2.

Table 2: Operation conditions of fluidized bed.

h*T cannot be compared directly with the convective transfer coefficient (hT) due to h*T includes the heat transfer by convection and radiation.

The differential equations were expressed as finite differences and solved simultaneously using the software Mathcad 11.

III. MODEL VALIDATION

Experiences of sewage sludge combustion in a fluidized bed were carried out in order to validate the proposed model. Pellets of dried sewage sludge to 10 mm diameter and height equal to 5 mm were combusted in a fluidized bed reactor at two temperatures (650 and 850°C), under air atmosphere, determining the time of CO2 emissions in the exit flow gas.

The experimental setup is composed of a high temperature, electrically-heated, fluidized bed reactor, 0.105 m internal diameter, coupled to Quadruple Mass Spectrometer, Model QMG 511 (Fig. 2). The 0.7 mm diameter sand bed is fluidized by a preheated gas (air or mixture) and maintained at the desired temperature by 2 half-cylinder radiative shells. K-thermocouples measure the temperature at several depths in the bed, as well as at the gas inlet and outlet. When at steady state, the cylindrical sludge particles were injected into the bed, thanks to a compressed air device. Then, the emitted CO2 in the outlet gas was measured by mass spectrometry .


Figure 2: Experimental setup

The test is discontinuous about the combustion process. Indeed, although the bed worked continuously in terms of flue gas and feeding. Two experiments were conducted for each temperature. The results are shown in Table 3.

Table 3: Experimental time of CO2 emission during the combustion of sewage sludge particle.

The experimental behavior of CO2 emission is similar in all tests. After the moment the measured gas starts to be detected, there is a rapid increase in the emission, which is more abrupt for higher temperature (850°C).

Then, a plateau is verified in the emission (independent of specific disturbances of records the detection system): The plateau duration is between 10-12 s to 850° C and 18-20 s to 650 ° C. The next step is a gradual reduction of CO2 emission, forming a "tail" where the emission rate goes down. The detected CO2 may have originated in the pyrolysis processes, pyrolysis products combustion and generated char combustion. The results of the proposed model show that an important step is particles heating to reach volatile release temperatures and then, the char combust.

The volatilization and combustion processes in the particle occur after reaching a certain temperature (about 250°C). This period may be associated with the experimental observed increase in CO2 emissions and the plateau. Homogeneous phase reactions of pyrolysis products and the dispersion caused by fluid dynamic characteristics of the fluidized bed, can lead to the "tail" of CO2 emission observed experimentally.

IV. RESULTS AND DISCUSSION

In this section, the proposed model results are presented. Figure 3 shows the mass ratio evolution (mass / initial mass) and the particle temperature, as well as the mass ratio of different fractions during the combustion phenomenon in fluidized bed at 850 °C. The particle diameter is equal to 10mm.


Figure 3: Mass ratio of the sewage sludge organic fractions during the combustion. Variation of particle temperature in the time.

The all fraction weights fall around 250°C due to their decomposition. These results show that during the combustion of a dried sewage sludge particle, the volatiles release is fast, which has also been observed by Harris et al. (2004).

There is a significant increase of the char mass (S1 + S2), from the 41 s (Tp = 275 ° C) to 45s (Tp = 728 ° C). Then, it decline quickly. A slow increase in particle temperature is observed at the phenomenon beginning, but then, it increases rapidly at the moment the char reacts with the oxygen.

The ash is the only solid waste and its temperature is equal to the surrounding atmosphere temperature. The reactions are fast due to the external heat transfer is the controlling step of the overall reaction rate (Harris et al., 2004).

Effect of fludization gas temperature: Various simulations were performed at different fluidization gas temperatures in order to evaluate their influence on the combustion time, h*T value and final particle temperature. An important influence of fluidisation gas temperature in the combustion time was observed. When this temperature increases, the external heat transfer coefficient also augments, increasing the heat flow from the gas to the particle, the reaction rate and reducing the combustion time.

The difference between the combustion time corresponding to lower gas fluidization temperatures (between 650 and 750 ° C, this difference is equal to 14 s) is higher than for higher temperatures (between 950 and 1050 °C, it is equal to 4s). This phenomenon occurs due to the reduction of the fluidizing gas temperature decreases the reaction rate of each fractions and solids involved in combustion, resulting in a global combustion time much higher. Moreover, increasing fluidization gas temperature, the maximum amount of char formed decreases (S1+S3). This phenomenon occurs due to increased reaction rate, resulting immediate consumption without accumulation of these solids.

Effect of particle size: Several simulations were performed in order to evaluate the particle size effect in the global time of the sewage sludge combustion. The gas temperature was equal to 850°C. The diameter: height ratio was equal to 2. There is a clear dependence of combustion time with the particle size. The particle size variation has the following effects:

1. Decreasing particle diameter, the convective heat transfer coefficient increases and also, the particle quickly absorbs the radiation heat (Yang et al., 2005), so small particles are heated more rapidly.

2. The heat exchange surface increases when the particle diameter rises, resulting in a reduction of the overall combustion time.

3. A decrease in different fractions mass (smaller particle diameters) results in a less time required for each decomposition stages, reducing the overall combustion time.

The particle mass increases in greater proportion than the transfer area due to consider the particle density is constant. Larger particles have a smaller convective heat transfer coefficient, requiring more heat to increase its internal energy. All these combined effects result in increase combustion time, when the particle size augments. Smaller particles rapidly reach the temperature to begin the reaction as well as the maximum temperature, so we can conclude that the influence of heat transfer on the overall combustion time is more important comparing with the particle exchange surface. This phenomenon was also observed by Khiari (2006).

These results have not been validated experimentally and they are only obtained by using the simulation model for the sewage sludge particles combustion in fluidized bed.

V. CONCLUSIONS

This paper presents a mathematical model to describe the combustion process of a dried sewage sludge particle, determining the influence of the fluidization gas temperature and the particle size in this phenomenon. The main assumptions of this model are: the particle is homogeneous and isothermal and; the sludge is formed by three fractions that react during the combustion independently.

According to the validation experiences, the model describes the essential aspects of the dry sewage sludge particle combustion in a fluidized bed. The predict overall combustion time is acceptable comparing with the experimental results. Therefore, the proposed model can be used to analyze the particle combustion behavior and the influence of various operational variables on it.

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Received: July 26, 2011.
Accepted: February 9, 2012
Recommended by subject editor: Mariano Martín Martín