Modelling of the Fuel Stream and Combustion in a Rotary-kiln Hazardous Waste Incinerator

Hazardous wastes from various industrial processes are normally incinerated with rotary kilns in order to comply with current environmental regulations. They have often very complicated chemical compositions in a variety of physical forms and difficult to characterise. The complex transport phenomena within the incinerator are not well understood, and the incineration process expects large uncertainties in process chemistry and thermal/ emission control. For better understanding of the incineration process, process simulation was conducted using Computational Fluid-dynamics (CFD) code Phoenics to characterise temperature and species distribution in the incinerator. To include all the waste streams in a single CFD model is difficult, and how to define the different waste streams with different calorific values and chemical compositions is a challenge to the CFD modelling. In the current paper, hazardous waste in various forms is firstly converted to a hydrocarbon-based virtual fuel mixture. The combustion of the simplified waste was then simulated with a 7-gas combustion model. The distribution of temperature and chemical species is broadly investigated. Distribution of CO concentration, as a good indicator of emission level for the incineration process, could be used to evaluate the emission control. The predicted temperature distribution has been validated with available measurement data from the operating rotary kiln waste incinerator AVR-Chemie in the Netherlands. New statistical post-processing of the standard CFDoutput has been developed to give an overview of the average temperature profile and overall reactor behaviour for process control. INTRODUCTION Rotary kiln incinerators are widely used in the incineration of various hazardous wastes such as liquid, sludge, and solids in bulk or in packages. The benefits lie in the drastic volume reduction and the substantial energy recovery. The main objectives of the incineration are the complete combustion of all the waste materials, and efficient recovery of the thermal energy from the off-gases after the waste combustion. Emission control of certain remaining species in the off-gases such as CO and dioxins is an important criterion for the operation. The complete destruction of hazardous compounds depends very much on gas mixing extent of air and various waste streams, the distribution of gas temperature and residence time within the kiln and the secondary combustion chamber (SCC). Due to large variations of waste types and difficulties in feed characterization of physical, chemical and thermal properties, the complex transport and chemical processes within the kiln system are not well understood, and thus the incineration process often anticipates substantial but unpredictable fluctuations of gas temperatures within the system. The temperature fluctuations lead to uncertainties in the process chemistry and difficulties in emission control. AVR-Chemie, a business unit of the AVR Business Group Industry located in the Rotterdam harbor area of the Netherlands, is specialized in hazardous waste incineration. It operates two rotary kilns at their plant, each with a waste processing capacity of 50,000 tons a year. The newly enforced directive from the European Union (L332) [EU directive, 2000] with increasingly strict emission control requires a better understanding of the incineration process and improved process control for low emissions and less environmental impact. The current European threshold value for carbon monoxide is not easy to comply with. Instead of hourly averaged values, monitoring of half-hour and ten-minute averaging is often required. The formation of carbon monoxide is caused by poor mixing of air and waste streams and insufficient residence time and temperature. A minimum incineration temperature of 1100°C during 2 seconds applies to chlorine bearing waste (> 1% Cl) and a minimum temperature of 850°C for non-chlorine bearing waste [EU directive, 2000]. On one hand, the operators have to comply with strict European guidelines and legislation. On the other hand, the waste supply for incineration is declining and the composition of the waste is frequently fluctuating. The high calorific wastes can be used in energy intensive industries to replace primary fuels, and thus only the most difficult types of waste are delivered for incineration in rotary kilns. In order to get better understanding of the overall incineration process, research has been carried out in authors’ group in close cooperation with AVR-Chemie. Computational Fluid-dynamics has been used to predict more insights of the gas flow, heat transfer and waste combustion within the incineration rotary kiln. CFD is a very convenient and flexible tool to simulate the flow related transport phenomena for large scale industrial processes. For rotary kiln waste incineration a number of modeling attempts concerning flow and heat transfer in the incinerators have been reported [Jenkins et al. 1980; Clark et al. 1984; Wolbach and Garman 1984; Williams et al. 1988; Chen and Lee 1995; Leger et al. 1993; Khan et al. 1993; Jakway et al. 1996; Veranth et al. 1996., 1997; Wardenier and Van den Bluck 1997; Ficarella and Copyright © 2003 CSIRO Australia 25 Laforgia 2000], and CFD simulation has been used in a couple cases for hazardous waste incineration [Leger et al. 1993; Khan et al. 1993; Jakway et al. 1996; Veranth et al. 1996., 1997; Wardenier and Van den Bluck 1997; Ficarella and Laforgia 2000]. However, in all the previous modeling work combustion of different types of wastes in one process has not been investigated, and a lot of questions need to be further answered. The early work from the current project focused more on the thermal contribution to the temperature distribution [Yang et al. 2001, 2002], by using a global combustion model of Spalding (3-gas model). All the waste was averaged and modeled as a global fuel. In general, it proved to be a useful approach, however, it is not able to model the chemical species distribution within the incinerator. In order to model the distribution of major chemical species in the waste combustion system, an extended global combustion model (7-gas model) was applied. Since the diversity of the waste types and large difference in heating value and chemical compositions, the simulation program could not handle directly the multi-fuel system. Therefore conversion of various waste streams to a general fuel which can be used in the CFD model becomes an important step. This has been carried out through the definition of artificial fuel mixture and waste stream optimization. Then the subsequent combustion modelling has been conducted by defining each individual waste stream by using the artificial fuel mixture. In this paper, the latest model developments and simulation results with the 7-gas combustion model are presented. The prediction of species distribution, especially the CO concentration, gives good indication for the emission level after the incineration process. New post-processing approach was illustrated to present large data output from CFD simulation in a condensed and engineering format. Temperature measurements at a number of accessible locations of the operating incinerator at AVR-Chemie in Netherlands was used to validate the combustion model, and the further needs to validate the model from both thermal and chemical aspects of the waste combustion process are emphasized. THE ROTARY KILN WASTE INCINERATOR The incineration system is a standard type of industrial scale rotary kiln waste-incinerator operated at AVRChemie. It consists of a rotary kiln and a secondary combustion chamber (SCC). The rotary kiln is 4.2 m in diameter and 11.7 m in length, mounted at a 1-2o angle and it rotates at a speed of 0.07 rpm. The SCC is 6.3 m in width, 5.5 m in depth, and 18 m in height. The thermal capacity of the incinerator ranges from 30 to 40 MW, and the waste processing rate is about 7 tons per hour. A wide range of hazardous wastes with heating value of about 5 to 30 MJ/kg is incinerated in the system. The waste enters the kiln and SCC in a variety of ways, as is shown in Figure 1: main burner, burner for shredded solid waste, sludge burner, SCC burner, and a couple of lances for various liquid or sludge wastes. A load-chute is used to supply combustion air and previously for supplying containerised solid waste, 14 air lances are installed in SCC to supply additional combustion air. Air is known to leak into the kiln through the front and end seals of the kiln, viewports, and the ash sump. The off-gas leaves the SCC to the waste-heat boiler (WHB) for energy recovery and preliminary dust separation. Molten slag formed in the kiln flows down to the lower end of the rotary kiln and falls into the ash sump located at the bottom of the secondary combustion chamber, which also causes water vapour entering the SCC. 26 Rotary Kiln Secondary Combustion Chamber (SCC) Ash Sump Burner flame Solid-waste flame Load chute Main burner Sludge burner SCC burner Air lances (x 7) Air lances (x 7) Off-gas