# CFD modeling of incinerator to increase PCBs removal from outlet gas

- Kamyar Yaghmaeian
^{1, 2}, - Nematallah Jaafarzadeh
^{3}, - Ramin Nabizadeh
^{1, 2}, - Golbarg Dastforoushan
^{4}and - Jalil Jaafari
^{5}Email author

**13**:60

https://doi.org/10.1186/s40201-015-0212-0

© Yaghmaeian et al. 2015

**Received: **3 March 2015

**Accepted: **21 July 2015

**Published: **12 August 2015

## Abstract

Incineration of persistent organic pollutants (POPs) is an important alternative way for disposal of this type of hazardous waste. PCBs are very stable compounds and do not decompose readily. Individuals can be exposed to PCBs through several ways and damaged by their effects. A well design of a waste incinerator will convert these components to unharmfull materials. In this paper we have studied the design parameters of an incinerator with numerical approaches. The CFD software Fluent 6.3 is used for modelling of an incinerator. The effects of several baffles inside the incinerator on flow distribution and heat is investigated. The results show that baffles can reduce eddy flows, increase retaining times, and efficiencies. The baffles reduced cool areas and increased efficiencies of heat as maximum temperature in two and three baffle embedded incinerator were 100 and 200 °C higher than the non-baffle case, respectively. Also the gas emission leaves the incinerator with a lower speed across a longer path and the turbulent flow in the incinerator is stronger.

## Keywords

## Introduction

Waste incineration is a well-established treatment technology for municipal, industrial, hospital and hazardous wastes [1–3]. It is also one of the most frequently selected method of waste management for no-longer reusable or recyclable industrial products and materials [4–8]. In the some part of waste incinerators, persistent organic pollutants (POPs) are formed due to the presence of products of incomplete combustion, oxygen and chlorine at temperatures between 200 and 800 °C [9, 10]. The final solution for persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs) that cannot be recycled or landfilled is to use an incinerator [11]. The minimum residence time suggested for removal of PCBs in an incinerator is about 2 s at 1200 °C or 1.5 s at 1600 °C [11]. This can be achieved only through increasing the residence time or improved heat distribution. Measurements indicate that most PCBs incinerators are not able to provide these conditions due to the presence of inefficient cold zone with low efficiency in terms of mixing and heat distribution [12, 13]. Furthermore, in the cold zone of waste incinerators polychlorinated biphenyls (PCBs) are formed [10, 14–16].

In recent years, many attempts have been made to model processes in incinerators. San José et al., investigated the effect of incinerator efficiency on the emissions in an industrial area with the help of MM5, CMAQ and EMIMO [17]. The results showed that the effect of emissions from incinerator is insignificant compared to the surrounding industries and highways. The effect was comparable just in the case of ozone. Stanmore et al., modeled the formation of PCDD/F in municipal and hospital incinerators and proposed a general empirical model to calculate the level of gaseous and solid PCDD/F [18].

In another study, Goh et al., modeled the combustion bed of a municipal incinerator and proposed a comprehensive flexible model [19]. The results of the model were used as boundary conditions for modeling upper gases in CFD models. The model results can also be used to optimize the incinerator and reduce the production of waste sludge and waste mixing [19]. In a similar study, the waste mixing was modeled before burning in an incinerator. In this study, a mathematical model was proposed for simulating waste mixing in the incinerator and the model results were compared with experimental results. Huang, used a kinetic model of reaction to model the formation PCDD/F in an industrial incinerator [20]. The model variables include the formation and removal rates of PCDD/F, carbon gasification, partial pressure of oxygen and equations for temperature and time. A good agreement was obtained between the experimental and model results. Khiari et al., proposed a mathematical model for dynamic simulation of an incinerator [21]. The lower part of the incinerator and waste pyrolysis were modeled. The model results were compared with the results of similar studies. Thomas offered an one-dimensional model to simulate the incineration of emissions in an incinerator [22]. Taking into account radiation, convection and conduction heat transfer processes, and the gas flow was simulated. The heat capacity of gases, thermal conductivity and viscosity effects were included considering the temperature dependence of the reaction.

The main objective of this study was to investigate the explores ways to optimize the efficiency of the PCBs removal in incinerators in the presence of baffles embedded in the combustion chamber. Modeling was performed first with 2 and then with 3 baffles.

## Materials and methods

### Incinerator specification

_{12}H

_{9}Cl, 13 % C

_{12}H

_{8}Cl

_{2}, 45 % C

_{12}H

_{7}C

_{l3}, 31 % C

_{12}H

_{6}C

_{l4}and 10 % C

_{12}H

_{5}Cl

_{5}with average Cl content of 42 %. Arochlorine-1242 is among the most commonly used types of PCBs, especially in power transformers in great plants. The specification of the incinerator was selected according to Theodore and Reynolds [23]. The Arochlorine-1242 incinerator was designed manually and with the help of software. The design principles in different parts of incinerator such as primary and secondary combustion chambers, furnaces, boilers, suppressor devices and air pollution control devices were similar to the incinerator modeled in [23]. Design calculations were performed according to Charles’s law and Dulong’s equation. Table 1 summarizes the specification calculated for the incinerator. The technical specifications presented in Table 1 were used as initial inputs to Fluent model for simulating the incinerator.

The technical specification of the incinerator

Value | Parameter |
---|---|

2270 (kg/h) | Maximum capacity |

C | Input pollutant |

20,277 (KJ/Kg) | Net thermal value |

50 % | Excess air |

2.76 m | Initial diameter |

11 m | Initial height |

0.5 m | Inlet and outlet diameter |

2.7 s | Initial residence time |

### The equations governing the pollutant flow in the incinerator

Given the air flow velocity and the dimensions of the incinerator as well as the high temperatures, the flow regime is turbulent. Neglecting the net rotating flows, since all changes along the flow and in vertical direction are important, the k-ε turbulence model is a good model for analyzing this problem. The equations required to solve the isothermal gas flow in the incinerator include time-averaged mass and momentum conservation equations [24]:

Mass conservation \( \frac{\partial {U}_i}{\partial {X}_i}=0 \) (1)

_{i}is velocity along i, i = 1, 2, 3, X

_{i}is x, y, z coordinates along i, Y mass fraction of gas emissions, ρ air density, υ kinematic viscosity, U

_{i}turbulent velocity component along i’ and S

_{Mi}is the momentum source along i’.As mentioned previously, since the Reynolds removal process and time-averaged equations will lead to unknown relationships for fluctuating velocity components, so a turbulent model is also needed. Thus, the k-ε model was used. This model requires the solution of two additional transport equations, one for turbulent kinetic energy, k and the other for its dissipation rate or ε [24]:

Enthalpy conservation: \( \frac{\partial }{\partial {x}_i}\left(\rho {u}_ih\right)=\frac{\partial }{\partial {x}_i}\left[\left(\frac{\mu +{\mu}_t}{\delta_h}\right)\left(\frac{\partial h}{\partial {x}_i}\right)\right]+{S}_h \) (5)

Chemical species conservation: \( \frac{\partial }{\partial {x}_i}\left(\rho {u}_i{m}_s\right)=\frac{\partial }{\partial {x}_i}\left[\left(\frac{\mu +{\mu}_t}{\delta_h}\right)\left(\frac{\partial {m}_s}{\partial {x}_i}\right)\right]+{S}_s \) (6)

Equation of State: \( \rho =\frac{P}{RT\sigma {m}_j/{M}_j} \) (7)

### Incinerator simulation

The inputs to the Fluent model

Value | Parameter |
---|---|

3 % | Turbulent Intensity |

Stationary | Wall Motion |

No slip | Shear Condition |

0.00004 (m) | Roughness Height |

2.76 (m) | Hydraulic Diameter |

0.5 | Roughness Constant |

## Results and discussion

### Efficiency Optimization

## Conclusions

The present paper modeled a PCBs incinerator by computational fluid dynamics with the help of Fluent. First, the technical specifications of the incinerator were calculated considering the type of the pollutant. The calculated specifications were used as Fluent model inputs. Numerical modeling was performed in three different modes. In the normal mode, a cylindrical incinerator without any baffle was modeled. The results indicated the presence of vortices and cold zones in the incinerator which reduced the efficiency. Then, the impact of baffles on the heat distribution and mixing efficiency of gaseous emissions in the incinerator was studied. The baffles reduced eddies and improved the heat distribution in the incinerator. In the third case, the maximum residence time, temperature and turbulence of gaseous emissions were greater than the previous two cases. In this case, the reduced cold zones by adding baffles increased the thermal efficiency and pollutant removal by 10 % and 16 %, respectively.

## Declarations

### Acknowledgements

The authors are most grateful to the Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences, Iran, for their collaboration in this research.

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## Authors’ Affiliations

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