1.1 Waste Incineration

Modern incinerators burn wastes in high – efficiency furnaces/ boilers to produce steam and/or electricity and incorporates modern air pollution control systems and continuous emissions monitor [2]. An incinerator is a furnace for burning refuse. Modern incinerators include pollution mitigation equipment such as flue-gas cleaning. Waste incineration has the following advantages:

• Volume reduction, especially for solids with a high combustible content
• Detoxification, especially for combustible carcinogens, pathologically contaminated material, toxic organic compounds, etc.
• Environmental impact mitigation, by destruction of all undesired secondary effluents or byproducts which create further significant pollution problems.
• Energy recovery, which is the most important advantage, especially when large quantities of waste are available, which is a good practice of waste to energy (WtE)

There are various types of incinerator plant design:

• Simple
• Fixed or moving grate combustion
• Rotary kiln
• Multiple stepped hearth
• Fluidized bed

Below is a brief description of the above mentioned incinerator:

1.1.2 Fixed or moving grate combustion incinerator

These are large fixed hearth incinerators, with a moving grate. The moving grate enables the movement of waste through the combustion chamber to be optimized to allow a more efficient burn. These incinerators are typically used for combustion of municipal wastes [3].

Figure 1: Moving Grate Incinerator, (council of the European Union directives 96/61/EC: Article 16 92).

1.1.3 Rotary –kiln incinerator

Rotary-kiln incinerator is used by large municipalities and industrial plants. The design of incinerators has two chambers: a primary chamber and secondary chamber. The primary chamber in a rotary kiln incinerator consists of an inclined refractory lined cylindrical tube. Movement of the cylinder on its axis facilitates movement of waste. In the primary chamber, there is conversion of solid fractions to gases, through volatilization, destructive distillation and partial combustion reactions.

The secondary chamber is necessary to complete gas phase combustion reactions. The clinkers spill out at the end of the cylinder. A tall gas stack, fan, or steam jet supplies the needed draft. Ash drops through the grate, but many particles are carried along with the hot gases. The particles and any combustible gases may be combusted in an afterburner. To control air pollution, the combustion product gases are further treated with acid gas scrubbers to remove sulfuric acid and nitric acid emissions, and then routed through bag houses to remove particulates before the gases are released into the atmosphere [4].

Figure 2: Rotary Kiln Incinerator (Rovaglio et al., 1998).

1.1.5 Fluidized bed incinerator

A strong airflow is forced through a sand bed where the air seeps through the sand until a point is reached where the sand particles separate to let the air through and mixing and churning occurs; thus, a fluidized bed is created and fuel and waste can now be introduced [5,6].

The sand with the pre-treated waste and/or fuel is kept suspended on pumped air currents and takes on a fluid like characteristic. The bed is thereby violently mixed and agitated keeping small inert particles and air in a fluid like state. This allows all of the mass of waste, fuel and sand to be fully circulated through the furnace.

Figure 3: Fluidized Bed Incinerator.

1.2 Polychlorinated biphenyls (PCBs)

Polychlorinated biphenyls (PCBs) are a class of organic compounds. PCBs classified as persistent organic pollutants. PCBs contain 1 to 10 chlorine atoms attached to biphenyl and a general chemical formula of C12H10-xClx (Figure 4 shows the structure of the PCBs), most PCBs were manufactured as:

• Cooling and insulating fluids for industrial transformers and capacitors.
• Stabilizing additives in flexible PVC coatings of electrical wiring and electronic components.

Figure 4: PCBs Chemical Structure.

PCBs mixtures have been used for a variety of applications. These applications include dielectric fluids for capacitors and transformers, heat transfer fluids, hydraulic fluids, lubricating and cutting oils, and as additives in pesticides, paints, carbonless copy (NCR) paper, adhesives, sealants, plastics, reactive flame retardants, capacitors, insulating fluids in transformers, vacuum pump fluids, hydraulic fluids, and as a fixative for microscopy [7,8].

They were also used in surgical implants. However, PCBs production was banned in the 1970s due to the high toxicity of most PCBs congeners and mixtures. Research indicating they were likely carcinogens having the potential to adversely impact the environment and therefore undesirable as commercial products.

Most of the 209 different PCB solutions are colorless, odorless crystals. Commercial PCB mixtures are clear viscous liquids (the more highly chlorinated mixtures are more viscous), for example its viscosity may reach that of sticky resins. PCBs have low water solubility and low vapor pressures at room temperature, but they have high solubility in most organic solvents, oils and fats. Other physical and chemical properties vary widely across the class [9,10].

Examples of PCBs are PCB-resistant materials include viton, polyethylene, polyvinyl acetate (PVA), polytetraflouroethylene (PTFE), bytyl rubber, nitrile rubber, and neoprene.

Table 1: Examples of PCBs component with identifying case number and number of chlorine atoms present (Rahuman et al., 2000).

1.3 Destruction methods of PCBs

PCBs are very stable compounds and are not easily degradable. They can be destroyed by chemical, thermal, and biochemical processes [11]. Though, it is extremely difficult to achieve full destruction. Also, the risk of creating extremely toxic dibenzodioxins and dibenzofurans is high through partial oxidation particularly due to high thermodynamic stability of PCBs.

All degradation mechanisms are difficult to sustain. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis. Environmental and metabolic degradation generally proceeds quite slowly relative to many other compounds.

Microbial and chemical treatment of PCBs is possible; but in this work, incineration process will be discussed. Even PCBs do not ignite themselves; they can be combusted under extreme and carefully controlled conditions. The current regulations require that PCBs are burnt at a temperature of 1200°C for at least two seconds, in the presence of fuel oil and excess oxygen. A lack of oxygen can result in the formation of dioxins, or the incomplete destruction of the PCBs. Such specific conditions mean that it is extremely expensive to destroy on a tonnage scale, and it can only be used on PCB containing equipment and contaminated liquid. This method is not suitable for the decontamination of affected soils.

This work concentrate on modeling and simulating the incineration of hazardous including the emissions in both gas and solid phases and the combustion process and environmental issues. Here, we mainly study the kinetics and rates of species production from combustion process. Thus, the current work summarizes as follows:

• Theoretical mechanism for the incineration of hazardous substances (difficult to anticipate-wide variations in the constituents) PCBs include the number of chlorine atoms varies from (1 to 10). This model was developed to PCBs by Rovaglio et al. [12]. The results of the model were compared with experimental values reported by Rovaglio et al. [12].
• The physical properties of PCBs will be estimated if they were not available.
• The variation of temperature with time inside the incinerator will be evaluated.
• MATLAB code was developed to estimate the above mentioned items and explained in the second part of this paper.

2.1 Chemical equilibrium reaction of PCBs

Thus, the general stoichiometric relationship can be used to describe the structure of the feed stock to the incinerator [12].

C_mH_nO_pS_qN_xCl_y + vO₂O₂→mCO₂ + n−y 2 H₂O + qSO₂ + x−α 2 N₂ + αNO + yHCl (1)

However, the following general expression and chemical reaction for PCBs incineration is given by:

C 12 H 10−n C l n + 29−n 2 O 2 →12C O 2 + ( 5−n  )  H 2 O + n HCl (2)

As in the case of the heterogeneous combustion, carbon mono-oxide (CO) is assumed to be the main oxidation product of carbon. The total combustion to CO₂ takes place in the gas phase. Furthermore, waste compounds containing nitrogen are decomposed to give N₂ and NO according to the following equation [12]:

N₂ + O₂→2NO (3)

Which evaluates α (in equation 1) fraction of kiln temperature and excess oxygen.

The theoretical model [12] is used throughout this work and is solved using MATLAB. Mass and energy balances for both solid and gas phases are presented. The resulting equations is solved simultaneously to get the required temperatures, flow rates, energy produced, and gas phase concentrations. In addition, the amount of different persistent organic pollutants (POPs) in the input of waste incinerators is compared to that in the output [16].

2.1.1 Solid waste material balance

The solid waste is undergoing the heterogeneous combustion, passing through the rotary kiln incinerator, for a certain contact time θ (retention time), while the effect of the mechanical agitation induced by rotation consists of the renewal of the surface area exposed to hot gases containing oxygen. On this area, both volatilization and combustion processes take place. The bulk solid phase movement along the rotary kiln is not different in nature from that of similar units. The solid movement in the rotary kiln in the transit conditions is related to both gas and solid flows are assumed to be fast with respect to mass and energy dynamic balances. In agreement with the aforementioned assumptions the mass balance of the solid phase written as follows:

d M solid −n dt = W waste (1− ω H 2 O )− R dr − W out s (4)

The evaluation of A, the interface area of the solid oxygen contact which is important sometimes to evaluate the rate of mass destruction, requires the knowledge of both gas and solid phase motions inside the rotary kiln. This area is function of kiln geometry and the operating conditions [15,16]. The maximum possible interface area depends also on particle diameter and solid angle of repose; if it is possible to estimate the average renewal surface velocity [Ar(m²/s)] then:

A= A r θ (5)

θ = 1.77.F.L.Φ0.5/S.D.N is the residence time for the solid material inside the rotary kiln [12]. The residence time varies depending on the parameters shown in the defining equation such as the square root of the dynamic angle of repose for the solid material inside the rotary kiln Φ, the type of the rotary kiln whether damped or not damped, F parameter which generally its value changes from 1or greater, internal diameter and length of the combustion chamber D and L respectively, kiln slope S, and the rotation speed N. Generally, the value of the retention time of the solids inside the rotary kiln is 15-60 minutes depending on the parameters above. Figure 5 shows the control volume where conservation equations are applied.

Figure 5: Control volume where conservation equations are applied, (European Council Directives, 96/61).

The destruction rate of R_dr can be derived based on the assumption of complete waste destruction:

W waste (1− ω H 2 O )− R dr − W out s =0 (6)

W out s = W waste (1− ω H 2 O )(1− ω bf ) (7)

In which, the destruction rate can be expressed as

R dr = W out s * ω bf /(1− ω bf ) (8)

,where the destruction rate can be related to the solid mass holdup inside the rotary kiln as follows:

R dr = M solid θ * ω bf (1− ω bf ) (9)

2.2 Energy Balance inside incinerator

The three modes of heat transfer, convection, condition, and radiation are present during the incineration process. Inside incinerators, the heat is generally transferred to the walls or to the upper surface of the solid beds by radiation and convection and to the lower (covered) surface by the regenerative action of the rotating kiln walls. The radiating gases in the kiln (not occupied by the wastefree volume) are also present within the boundaries of the burner and/or waste flames so that they may be considered as covering the entire free volume. Thus, due to the high gas temperature, solids and exposed walls receive heat primarily by radiation from the gas volume while both convective and regenerative heat flow play only a minor role in the overall heat transfer process. On the basis of the following assumptions [12]:

1. Perfectly mixed conditions (outlet temperature of both the gas and solid is the same as that temperature inside the rotary kiln)
2. Equilibrium condition between gas and solid phase (if present)
3. Complete combustion (for both waste and auxiliary fuel)

Therefore, based on the above assumptions, the following energy balance around the control volume shown in figure 4 can be derived [12]:

d E tot dt = W air in H air in + W waste Q waste + W C H 4 Q C H 4 − W out gas H out −S GS ¯ ( T g 4 − T w,in 4 )− h in πDL( T gas − T in )− W out s C p s ¯ ( T gas − T ref ) (14)

Where

E tot = M s C V s ( T gas − T ref )+ M gas ∑ i Mi ∑ i M i ∫ T ref T gas C v i (T)dT (15)

H air in = ∫ T ref T gas C p i (T)dT (16)

H out = ∑ i Mi ∑ i M i ∫ T ref T gas C p i (T)dT (17)

Equation (15) implies that waste and fuel are fed at the reference temperature. It can be clearly seen that E is a strong function of temperature. Equation (15) is used to express E as function of temperature in the next section in details. The general radiation shape factor ( GS ¯ ) can be evaluated by means of the well-stirred combustion chamber in which it is evaluated as [12]:

GS ¯ = ∫ v . ∫ s . kdvdScosϑτ(r) π r 2 = A s 1 c g ε p + 1 ε G (18)

The evaluation of the gas emissivity εG can be estimated graphically and/or experimentally referring to the suitable diagram concerned with radiating heat transfer, and as water and carbon dioxide are the main components in the gaseous product, the emission band that will be considered here is directly related to the combustion products: water and carbon dioxide. The emissivity of such components can be calculated through a polynomial regression of the experimental data as a function of the sum of the single partial pressures (P_CW=P_CO2+P_H2O) and of the beam length (LW):

log 10 (T ε G )= ∑ n=1 11 C n x n−1 (19)

Here, x is x=log₁₀ (P_CW*L_W) and the coefficients C_n , which are summarized in the research by Rovaglio et al. [12] when analyzing the effect of radiation energy, are given as follows.

This procedure will lead to an expression of emissivity as function of temperature that will be solved as independent variable with time in the dynamic modeling. The heat transfer by radiation cannot be neglected in deriving the model even this will lead to a source of high nonlinearity and so to a very stiff system, however, the radiation term consists of the following parameters:

E rad =S. GS ¯ ( T g 4 − T w,in 4 ) (20)

, which is equivalent to the following:

E rad =εσA( T g 4 − T w,in 4 ) (21)

Where:

S. GS ¯ accounts for the emissivity and Stefan-Boltzman value is temperature dependent value and can derived depending on equations 18 and 19 and the values presented in table 2. T_g is the value of the effluent gas temperature from the rotary kiln, and is assumed to be as that inside the combustion chamber. T_w,in is the wall temperature where the gas volume emits its energy to. σ is the Stefan-Boltzmann constant.

Table 2: Polynomial coefficients for gas emissivity evaluation (Rovaglio et al., 1998).

The definition of the radiation energy requires the presence of the emissivity value which should be found in a form making it possible to be inserted in the model to account for the temperature variation. The total pressure inside the rotary kiln is given by:

P_CW=P_CO2+P_H2O

which shows that all of the pressure produced comes from water vapor and carbon dioxide, the beam length LW can be found in the literature for different geometries, the next theoretical calculations is performed to derive the emissivity ε as function of temperature [12]:

log 10 (T ε G )= ∑ n−1 11 C n x n−1 (22)

where X=log₁₀ (P_CW*L_W) and the coefficients C_n are summarized in table 2 where the final result is an expression of the form:

ε= θ T (23)

where θ is a theoretical constant.

An important feature of the radiation heat transfer is the shape factor. Equation 21 should be multiplied by the value of the shape factor. Referring to the literature figures is presented to account for the shape factor.

In the case of circular cylinder emitting to all its surfaces, the dimensions are important in the case of the rotary kiln which is used for PCBs incineration. The ratio between its length and radius is large and the ratio between outer and inner radius is zero (only one cylinder is used), the value of the shape factor for these conditions is 1.0.

The second major heat transfer mode is the convection heat transfer. The convective heat transfer coefficient is computed by a Dittus-Boelter relationship:

Nu=0.023 Re 0.8 Pr 1/3 (24)

, where Nu is the Nusselt number and it can be calculated as hD/k, and so it can be corrected to take into account the effect of the entry length as follows [12]:

h in =h(1+ 1.4D L ) (25)

As for the transient heat condition within the furnace walls, the problem can be easily modeled by means of the classical equation of heat diffusion in one dimension to be solved for adjacent layers of different materials and with proper boundary conditions ensuring the continuity of heat fluxes. The real problem lies in choosing which discretization method is to be adopted and which numerical algorithm to use for integration purposes.

2.3 Energy balance in air heater

In the rotary kiln incineration plant configuration, there is a heat recovery section consisting of a sequence of heat exchangers placed after the burning system. These devices have a double function:

Figure 6: Heaters used to heat air fuel mixture before entering the rotary kiln (Rovaglio et al., 1998).

• Air combustion pre-heating to improve the thermal yield of the plant.
• Gas cooling to reduce water consumption inside the scrubbing section.

These apparatus consist of a metal shell which covers a cylinder of refractory and insulating material where, inside the enclosure, a coil of special alloy is placed. The cold air flows inside the coils while the hot gas moves along the refractory lined chamber.

Inside the heat exchangers, the hot gas temperature can be considered uniform, that is, a continuous stirred tank reactor (CSTR) scheme is assumed because of high turbulence due to the gas velocity and presence of coils and devices in the heaters [12]:

d E tot dt = W f H in − W f H out − Q losses − Q exit (26)

Here, Hin and Hout can be evaluated through equations (16 and 17) while Wf represents the hot gas flow rate which is assumed to be equal for inlet and outlet flows. The heat losses Qlosses through the walls can be determined as follows [12]:

Q losses = GS ¯ ( 1− k a 1− k 4 + A wall H in 4σ T a )σ( T f 4 − T w 4 ) (27)

k= T w / T f (28)

The term inside the parenthesis defines the global heat transfer coefficient which takes into account both the convective and the radiative terms. In a similar manner, the heat exchanged between smokes and coils can be determined on the basis of the following relationship [12]:

Q exit = ∑ n=1 NS ( GS ¯ σ( T f 4 − T wall,n 4 )+ h e A n ( T f − T wall,n 4 )) (29)

Where the external convective coefficient he can be evaluated through the Nu number given by (John, 1997):

Nu=0.11 Re 0.675 Pr 1/3 (30)

This refers to a system with cross-flow geometry.

The model will be solved for the geometry and characteristics that are briefly summarized in table 3 which describes the pilot project designed [12] to investigate experimentally the incineration process. The model is based only on a limited number of variables measured and available from the plant as follows:

• Outlet gas temperature from the rotary kiln.
• Outlet gas temperature from the post combustion chamber.
• External skin temperature for the rotary kiln.
• External skin temperature for the post combustion chamber.
• Oxygen mole fraction in the post combustion gas outlet.
• Air temperature from heat exchangers.

Table 3: Geometry and characteristics of rotary kiln.

However, the comparison is completed by the knowledge of the transient evolution of some input variables, such as: waste flow rate to the rotary kiln, Fuel flow rates to kiln and post combustion chamber, and air flow rate of the spiral heat exchanger [19,20].