Simulation Research on 40t/d Counter-ow Rotary Kiln Incineration System

In this paper, a counter-flow rotary kiln incineration system with a processing capacity of 40 t/d of an environmental protection company in Jiangsu Province was used as the research object, and the mixed pyrolysis model and computational fluid dynamics （ CFD ） model of the counter-flow rotary kiln incineration system were established using Aspen plus and Fluent software. The influence of the operating temperature of the rotary kiln, the heating value of solid waste, the operating load of the rotary kiln, and the primary air volume and primary air speed on the operating effect of the rotary kiln were explored. Operating temperature, primary air volume and wind speed can all have a greater impact on the incineration performance of the counterflow rotary kiln. When the operating temperature of the counter-flow rotary kiln is greater than 800 o C, the hazardous waste heat value is not less than 1500 kcal/kg, and the primary wind speed is 1.5 m/s, the operating effect of the counter-flow rotary kiln incineration system is the best. The simulation results can provide theoretical guidance for the design and optimal operation of the counterflow rotary kiln incineration system.


Introduction
Since the reform and opening up, the rapid development of various industrial fields in China has led to a significant increase in social and economic standards, but at the same time, the problem of environmental pollution has also become more serious (Nanda and Berruti 2021). Global warming is becoming an increasingly serious problem as CO 2 emissions increase year on year. As a result, a global ' carbon peak and carbon neutral' goal has been proposed to improve global warming. Solid waste is one of the main pollutants juxtaposed with waste gas and liquid waste, which can cause serious harm to the atmosphere, soil and water (Visvanathan 1996). Solid waste is a major source of carbon emissions (Wang and Geng 2015), so, it is necessary to explore a reasonable and effective way of solid waste disposal, which is one of the key tasks to implement the concept of sustainable development and solve the environmental * Corresponding author. Email: ruixiao@seu.edu.cn pollution problem in the new era.
The existing solid waste treatment methods are mainly divided into two categories: resource utilization (Makarichi, et al. 2018;Singhabhandhu and Tezuka 2010) and harmless disposal (Paula Ottoboni, et al. 1998). The resource utilization of solid waste is represented by waste incineration for power generation, which not only realizes solid waste treatment but also saves energy, and meets the current development requirements of energy saving and environmental protection. However, the complexity and variety of solid waste types make the resource utilization of solid waste more difficult (Xin-gang, et al. 2016). By contrast, harmless disposal is a much simpler way of treating solid waste. Incineration and safe landfills are the two main means of harmless disposal of solid waste (Yang, et al. 2003). In terms of incineration, existing solid waste incineration technologies include rotary kiln incineration, cement kiln coincineration (Zhang, et al. 2011) and pyrolysis incineration (Qureshi, et al. 2020), of which the most widely used incineration method is rotary kiln incineration. Studies have shown that rotary kiln incineration is the most efficient way to treat solid waste (Yang, et al. 2007). Depending on whether the flow direction of solid waste and flue gas in the rotary kiln is the same, rotary kilns can be divided into two categories: down-flow rotary kilns (Eriksson, et al. 2014;Rahman, et al. 2013) and counter-flow rotary kilns (Acharya and Novak 1991;Lemieux and Pershing 1989), and most of the existing rotary kilns in China are of the down-flow type, which has the advantages of simple air and material intake and simple operation compared to counter-flow rotary kilns (Sharifah, et al. 2008). However, in actual operation, the down-flow rotary kiln suffers from uneven gas-solid mixing, easy coking and high gaseous pollutant emissions, while the counter-flow rotary kiln has more adequate gas-solid mixing and higher heat transfer efficiency in actual operation, which can better control coking and gaseous pollutant emissions (Li, et al. 2014;Pershing, et al. 1993). Counter-flow rotary kilns have been used in relatively few engineering projects at present. Therefore, the study of counter-flow rotary kiln incineration systems can help to complement this aspect and provide guidance for the future development and application of counter-flow rotary kilns.
In this paper, the mixed pyrolysis incineration process of solid waste in a counter-flow rotary kiln with a processing capacity of 40 t/d was simulated using Aspen plus software (Ismail, et al. 2020b), the temperature distribution and concentration distribution of various substances in the rotary kiln were simulated using Fluent software (Ismail, et al. 2020a;Liu and Yang 2015;Siripaiboon, et al. 2020). The factors affecting the incineration performance of counterflow rotary kilns were investigated in depth.   Table 1.

Fluent modeling
In this paper, a CFD model of the counter-flow rotary kiln incineration system was developed using

Geometric model and boundary conditions
The geometry of the counter-flow rotary kiln is shown in Figure 3, from which it can be seen that the number of inlets and outlets of the rotary kiln is four, divided into the following four categories: material inlet, primary air inlet, mixed gas outlet and slag outlet.
The boundary conditions for each inlet and outlet are given in Table 2.  (2) Where，t is time, ρ d is the density of mixture, v d is the speed of mixture, t d is the turbulent viscosity of mixture, G kd is the turbulent energy,C 1e ,C 2e , σ k and σ ε are turbulence model coefficients.
Where, ρ is the density, ρ' is the average density, 3 Simulation results and analysis

Model validation
In the actual operation of a counter-flow

Effect of solid waste calorific value
When the calorific value of the solid waste is low, there is a risk that the counter-flow rotary kiln incineration system will not be able to meet the operating temperature of 800 o C or more, and that the high level of gaseous pollutants generated by the operation of the system will indicate that the material cannot be incinerated by the counter-flow rotary kiln incineration system. For this reason, the actual operation of the counter-flow rotary kiln incineration system was simulated using a mixed pyrolysis model with solid waste of different calorific values. Figure 6 shows  (1) As the operating temperature increases, the content of SOx and NO x gradually decreases, and when the operating temperature of the counter-flow rotary kiln incineration system is greater than 800 o C, the content of the two gaseous products tends to stable, indicating that the incineration system should meet the condition of an operating temperature greater than 800 o C.
(2) At an operating temperature of 850 o C, when the calorific value of the solid waste is below 1500 kcal/kg, the incineration system is unable to meet its own heat balance through the combustion of solid waste. With a solid waste material of 1000 kcal/kg, for example, the operating temperature of the incineration system is only 768 o C, and the treatment of solid waste is not effective. Therefore, when using this incineration system for the incineration of solid waste materials, the solid waste should be simply screened according to the different calorific values.
(3) The active air supply method helps to stabilize the operating conditions of the incineration system, but the primary air speed and the size of the primary air volume affect the actual operating effect of the incineration system. When the primary air flow and primary air speed are too small, the mixed pyrolysis of the solid waste is not complete and the thermal reduction rate of the solid waste is not up to standard.
When the primary air volume and velocity are too high, the temperature field in the kiln is not uniform and the temperature at the kiln head is too high. The simulation shows that the incineration system operates best at a wind speed of 1.5 m/s, which corresponds to a primary air flow of 5700 Nm 3 /s.

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