Enhancing the Performance of a Hazardous Waste Incineration Facility through the Usage of a Dedicated Application

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Enhancing the Performance of a Hazardous Waste Incineration Facility through the Usage of a Dedicated Application


1. Introduction

Rotary kilns are widely used in the cement, chemical, metallurgical and waste management industries. The processes carried out in the device are diverse, ranging from drying, clinker burning, iron ore processing, production of titanium compounds, to thermal processing of waste [1,2,3]. Its structure can be characterized as a horizontally arranged, cylindrical drum made of steel with a specific diameter. The kiln revolves on its axis at a speed that ranges from 0.25 to 4.5 rpm. Due to the high temperatures at which the device operates, the interior of the kiln is equipped with a fire-resistant lining [3,4]. Protection against improper movement of the input material is provided by tilting the furnace to the ground at a slight angle. This forces a clear direction of waste mass transport in this device. The charge material is fed into the kiln and then moves down the kiln lining according to its inclination [3,4,5].
When it comes to the management of waste through thermal processes, the rotary kiln primarily incinerates hazardous waste, often medical, veterinary, chemical and industrial waste [6,7,8]. According to the EU regulations [9], hazardous waste has one or more of the hazardous properties, among others, flammable, infectious, and corrosive. Rotary kilns used in the cement industry and metallurgy can also be fed with waste in the so-called form of alternative fuel. A significant difference in the design of the kiln, dedicated only to hazardous waste, is the associated after-combustion chamber, which is a kind of “fuse” for the proper implementation of the combustion process [10,11]. In this chamber, the final combustion of incompletely burned products takes place, it also ensures the appropriate exhaust gas temperature (e.g., 1100 °C for hazardous waste containing more than 1% Cl) and the appropriate residence time (min. 2 s) required by law for these types of facilities [12].
In accordance with the principles of circular economy, as much waste as possible should be prepared for re-use, or recycled. Due to the imposition of municipal waste recycling levels and the limitation of landfilling, the EU authorities referred to the incinerator element of the waste management system. Regarding the installation of thermal waste conversion, the suggestions included in the studies of the Circular Economy program concern the role of the incinerator as a device neutralizing waste, the material recovery of which is practically impossible [13,14,15], and the storage of which is prohibited in accordance with [16]. Basically, the place of the thermal installation in the waste management hierarchy has remained unchanged, but the waste stream directed to it will potentially decrease, although this largely depends on the waste management strategy in force in a given country, as well as the type of waste.
The gradual reduction in incineration applies primarily to municipal waste, which can be recycled. Hazardous waste, including medical waste, for which there is often no other alternative method of management, is thermally processed in a rotary kiln. Additionally, there is the aspect of neutralizing harmful substances contained in this waste. Moreover, it may be necessary to build new installations of this type, among others, due to the projected increase in the volume of hazardous waste [17,18]. It is worth noting that in addition to the basic goal of this type of process, which is the neutralization of harmful substances or pathogenic microorganisms, hazardous waste incineration plants generate heat and/or electricity. If the thermal process is skillfully carried out, they are able to meet some of their own energy needs, as well as supply these utilities to nearby industrial plants or networks.
In such a case, it is possible to determine the coexistence of the idea of circular economy with hazardous waste incinerators. Attempts to optimize the process of thermal transformation of hazardous waste promote, among other things, more effective energy recovery and reducing the use of an additional fuel in the process, mostly gas. Activities like these contribute to sustainability. Optimization of the installation for the thermal treatment of hazardous waste may concern various aspects of its operation. These may include study on the movement of the feed material in a rotary kiln [19,20,21,22], development of a numerical model for the thermal transformation of waste [8,23,24,25] or environmental assessment of the impact of industrial waste incineration in a rotary kiln [11]. An example of considerations devoted to modeling the thermal transformation of contaminated waste can be found in the study of Lemieux et al. [25]. Vermeulen et al. [11] made an environmental evaluation of incineration industrial waste of high calorific value as a substitute for fossil fuels in the rotary kiln—waste incineration plant and cement kiln.

The operational aspect also seems to be important here and has not been widely described in the literature so far. This article presents a tool which is a new approach to this issue—a desktop application supporting waste incineration plant operators by selecting components (different types of waste) for individual portions of the feed mixture. In the next stage, one can consider coupling the described tool with other models. Possible extension of the application can be a module for energy and mass calculations.

2. Materials and Methods

An important area of optimization of the performance of the incineration plant with a rotary kiln is the appropriate composition of subsequent portions of the feed material. This situation applies especially to incineration facilities for hazardous waste that handle diverse types of waste. These wastes are characterized by different fuel properties, which complicates the process of their thermal transformation and contributes to operational problems.

2.1. Selected Issues of Conducting the Thermal Process in a Rotary Kiln

Selecting the appropriate batch of the input material is a tedious task. Proficiency, knowledge and effective collaboration of employees of an incineration facility are essential prerequisites for this task. However, despite meeting all of the necessary conditions, unforeseen issues may arise that can impact the functionality of the entire system. These complications are associated with the characteristics of specific waste types, which exert a substantial adverse influence on the overall functioning of the facility. It is important to ensure that the input material remains consistent in terms of physicochemical and fuel properties during a thermal process. Furthermore, these properties are required to fulfill the necessary criteria, enabling the process to be conducted in compliance with the relevant legislation.

The parameters of the input material that are particularly taken into account are:

  • calorific value and elemental composition,

  • content of chlorine and other halogens,

  • content of alkaline salts and other compounds of this nature,

  • pH, flammability and reactivity.

Improper selection of the batch of feed material can result in a range of consequences. They can include a short-term increase in the emission of a specific gas, a temporary halt in the movement of material within the kiln, or even the requirement to halt the entire installation for necessary repairs and renovations. Three of the most common problematic situations can be distinguished here:

  • usage of the kiln lining,

  • emissions exceeding standards,

  • temporary retention of the material in the kiln.

As part of a literature review, it is worth mentioning a study by Ramanenka et al. [26], in which the authors examine the bricks used in the lining, indicating that the main reason for their usage is the chipping of its fragments due to the mismatch of the thermal expansion coefficients. The degradation of the lining is highlighted by Stjernberg et al. [27] as a result of alkaline corrosion taking place at elevated temperatures. The study conducted by Gan et al. [28] provides an insight into how the operating conditions of the kiln are influenced by the thickness of the agglomerated material within the kiln.
The primary objective of the study conducted by Slovikovskii et al. [29] was to assess the influence of incinerating municipal waste in a cement rotary kiln on the integrity of the lining. The outcome suggests that corrosion is the primary mechanism responsible for the deterioration of bricks. Chemically, they distinguished the reactions taking place between gases containing alkalis, sulfur and chlorine emitted during the thermal process, and the refractory material, resulting in the formation of chloride, sulfate and complex salts. They possess a low melting point and have the capability to transform the lining material, causing it to adopt a loose structure that is prone to chipping.

In the basic chemical approach to the phenomena related to the degradation of the lining in a rotary kiln, it can be described by the following reactions.

R-COOH + 2NaOH → RH + Na2CO3 + H2O↑

R-CH2-OH + NaOH → R-CH2-ONa +H2O↑

Al2O3 + 2NaOH → 2NaAlO2 + H2O↑

xR-CH2-Cl + (2x + 3)O2→ (x + 1)CO2 + (x + 2)/2H2O + x/2Cl2

R-CH2-CH2-Cl → R-CH=CH2 + HCl↑

Al2O3 + 6HCl → 2AlCl3 + 3H2O↑

Reactions (1)–(3) take place at high temperatures, corresponding to the operating regime of a rotary kiln. Reactions (1) and (2) are characterized by the combination of examples of compounds potentially present in hazardous waste—carboxylic acids and alcohols—with basic compounds, here: sodium. These reactions produce solid sodium salts that can form a layer of material in the furnace. As the thickness of this layer increases, a temporary agglomeration of the charge material takes place in the furnace. Reaction (3) is a probable illustration of the reaction of alkaline compounds with the lining material, here defined as aluminum oxide—the basic building block of fireclay bricks in a furnace in a waste incineration plant. Aluminum and its compounds are characterized by amphoteric properties, hence the possibility of forming a basic salt. The resulting product, compared to durable ceramic aluminum (III) oxide, is characterized by susceptibility to crumbling, which increases susceptibility to chipping.

In the case of equations with acidic compounds, here: chlorine, the first two Reactions (4) and (5)—describe the breakdown of an organic compound containing chlorine. The first one (4) is combustion taking place in an oxygen-rich atmosphere, which produces chlorine gas. In the case of interaction with the lining, chlorine gas is not considered as a factor that reacts with aluminum oxide, but gaseous components containing chlorine e.g., HCl, are important. It is produced in a situation of oxygen deficiency, which may occur in the rotary kiln chamber of a hazardous waste incinerator. The formation of hydrogen chloride is described by Reaction (5). Then it can react with amphoteric aluminum oxide, which is a component of fireclay brick, to form a salt—aluminum chloride. This salt, like NaAlO2, is susceptible to crushing. This is related to the chemical structure of the compound—the ionic bonds present there. For comparison, aluminum oxide is described as a durable compound due to its strong interatomic bonds—polar covalent. Therefore, after the formation of salts with chlorine, the brick will tend to chip—erode.

2.2. Proposed Optimization Solution

Considering the information mentioned above, the authors suggest a desktop application as a viable solution to assist waste incineration plant users in their work. The objective of the mentioned application is to facilitate the incineration plant operator in the automatic selection of the components for the mixtures. These mixtures are the subsequent portions that will be fed into the kiln. The C# programming language was used to develop the tool using .NET technology. The ant algorithm, which is extensively explained in various papers including [30,31], served as the basis for its creation.
As a result of the insufficient information available in the literature regarding the operational parameters of a rotary kiln in an incinerator, data obtained from this type of installation were used. On this basis, the most advantageous conditions, here understood as ranges, of the thermal process carried out in the rotary kiln were determined. In the subsequent phase, reference values were established for each parameter within the specified range. The ranges of values for a particular parameter were acquired through various means, including study visits to facilities of this nature. Table 1 provides an overview of the parameters and reference values.
The average value was chosen as the reference value for calorific value and pH. The reference value representing 10% of the predetermined maximum value was specified for parameters with a limit value. The choice of a reference value offers an added benefit, namely the resilience of the model to specific deviations in the performance of the algorithm. There exists a considerable margin of error, which additionally enables the acquisition of output values that align with the specified parameter ranges. Figure 1 shows the general scheme of the application model.

The experimental determination of these values primarily focuses on ensuring the proper execution of the thermal process within the kiln, and on what influences the financial aspect of the incineration plant performance. Specifically, the goal of achieving a consistent temperature distribution in an autothermal process can be accomplished by utilizing a batch mixture that possesses a consistent calorific value. As a result of this, there is no need to provide additional fuel to the rotary kiln, thereby minimizing costs in this particular domain. Furthermore, the prolonged presence of waste with excessive levels of acidic or alkaline substances leads to accelerated lining usage and deterioration. This involves more frequent renovations, which is expensive. The installation then does not conduct a thermal process, and therefore does not generate revenues.

Moreover, the application should meet several conditions for intuitive and effective use, namely:

  • enable compatibility with systems used in waste incineration plants,

  • ensure flexibility in selecting the mixture,

  • ensure the stability of the solution, consistent with the adopted assumptions,

  • enable different modes of use.

It is assumed that the proposed solution will, among other things, accelerate the decision-making process and create a feeding schedule.

2.3. Algorithm for Selecting Components for the Mixture

The aim of choosing the elements of the waste mixture with the use of the developed application is to optimally create an input batch to the rotary kiln. The term “optimally” in this context refers to guaranteeing the preservation of specific operational parameters of the facility while minimizing any operational issues that may arise in the installation for thermal treatment of hazardous waste.

The application is limited in scope as it considers only five specific parameters, namely calorific value, pH, contents of: chlorine (Cl), alkaline salts (Na, K, Ca salts) and other halogens (F, I, Br). Each parameter is allocated a specific range of values in which the acquired outcomes are expected to be located. This information can be observed within the application window.

The algorithm presented in this work includes functionality that allows assigning weights to individual parameters. The mass composition of a mixture can be determined by calculating the weighted average, which takes into account the mass of each component. In the application, however, there are no considerations on specific, chemical reactions taking place at elevated temperatures. It is true that the implementation of information regarding these relationships is not subject to testing and might be extremely difficult to determine, but it is expected that the mixture properties may be adjusted by the installation operator. In practical use of the application, this is manifested by the possibility of assigning a weight to each parameter, through which the algorithm will determine the composition of the mixture, taking into account in particular the value of the selected parameter or parameters. Using this function is optional, but it may be helpful in emergency situations.

Therefore, the objective function was established in the following manner:

f m 1 , m 2 , , m n = i = 1 5 w i p ¯ i p w z i 2

where:

  • m 1 ,   m 2 , ,   m n   —mass of particular components,

  • n—number of components,

  • p w z i —reference value of the parameter,

  • w i —weight of individual components,

  • p ¯ i —weighted average of the parameter with index i, which is defined as:

p ¯ i = j = 1 n s j · p i j s j

  • s j —mass of the j-th component of the mixture,

  • p i j —value of the parameter i for mixture component j.

The function is dependent on n-variables, where n represents the quantity of components that can be incorporated into the mixture. Then it is minimized in relation to the weight of individual components. The quality of the mixture related to specific parameters is described by the objective function. The closer the parameters of the mixture are to the reference values, the lower the value of this function will be, indicating better performance. The ant algorithm, which is one of the swarm algorithms in the field of artificial intelligence, was employed to minimize the objective function.

2.4. Functionality of the Application

The application provides flexibility of use by providing a number of functions available to the user. These are primarily:

  • selecting waste included in the mixture and its mass,

  • generating the composition of subsequent portions of input material,

  • updating of the mass of available waste,

  • assigning weights to individual waste parameters,

  • generating a report on the simulations performed.

Hazardous waste incineration plants usually have their own laboratory where they test the properties of the waste they receive. After loading the database with this information into the application, the user has at his disposal a list of all available waste along with their more or less precise characteristics. For the purposes of the program, waste should be defined by at least five parameters, which are the parameters taken into account in the algorithm. These are properties tested by laboratories because they are crucial for the combustion process. Based on the list of available waste and their parameters, the operator can select any waste to create a batch mixture, and also set the total mass of the batch material. This is a convenient solution that provides flexibility. It happens that some hazardous waste requires priority in directing to thermal processing, and the mass assignment facilitates adapting the portion of the feed material to the characteristics of the rotary kiln and the thermal process.

Given the high frequency of loading in a waste incineration plant throughout the day, an important feature of the application is its capability to generate multiple waste mixtures. The tool does not have any predetermined upper limits in this regard. Nevertheless, the total waste consumption can potentially serve as a constraining element. In addition, tests conducted on the mixture by simulating multiple waste selection showed stability and compliance with set parameters. In all of the obtained results, the simulated portion of the feed material is in compliance with requirements.

Updating the mass of waste used to create the batch is a key aspect that has been programmed to ensure smooth operation of the incinerator. This function allows the operator to create additional mixtures and prevents mass loss. The application automatically deducts the corresponding mass from the waste incorporated in the mixture based on the indicated results.

Giving the selected parameter or parameters greater weights, and thus greater importance when generating the composition of batches of feed material, may be an alternative in case of temporary problems that may occur in the installation. You can imagine a situation in which there was a temporary but significant drop in process efficiency. In such a case, the operator may temporarily increase the weight of the calorific value. The algorithm will then select ingredients in a way that favors this parameter.

The tool has ability to improve the handling of loading procedures by preserving data generated for each waste portion and creating detailed reports based on this data. Application users can obtain a report that provides guidance on the proportion of input materials within a specific time frame. By knowing the loading frequency at a given facility, the application can generate loading schedules for batches of feedstock per day, per week, etc. While wider ranges may not be particularly relevant due to the dynamic nature of waste incineration, they can still be demonstrated when necessary.

3. Results and Discussion

The results of the simulations are presented in the context of the stability and quality, and checking the functionality enabling the use of weights for the selected parameter.

3.1. Results of the Simulations—Stability of the Solution

It was assumed that subsequent portions of the feed material might be generated, each weighing 100 kg. The view of the application window during one of the tests is shown in Figure 2.
Table 2 lists the results of ten simulations involving determining the mass of a portion of the input material (100 kg) and selecting waste to be used to create a portion of the input material. A weight of 1 was used for each of the parameters considered. This is an initial assumption, which aims to check the quality of solutions for the case of unified significance of the parameters. This simulation allows for assessing the stability of the solution.

The obtained results indicate compliance with the adopted ranges. Simulation 1 may serve as an example. In this case, the following properties of the mixture are obtained: calorific value 15.52 MJ/kg, pH 7.53, content of selected basic salts 5.7%, content of chlorine 2.1% and content of halogens 0.97%. In all conducted tests, the parameters of the acquired mixture are within the specified parameter ranges, most of them with a significant margin of error. The results obtained are stable and consistent with established guidelines.

3.2. Results of the Conducted Simulations—Introduction of Parameter Weights

Figure 3 shows the results after introducing a higher weight for one of the parameters, here: calorific value, which was given a weight of 5. In this case, the results will show the quality of the solution with the preferred parameter. The given simulations had the same input data, i.e., the mass of a portion of the input material (100 kg) and the types of waste selected for the mixture, such as simulations with unified weights, testing the stability of the results.
Table 3 summarizes the results of ten simulations (Mixture 2) performed for the same conditions, except for the weight of one of the parameters, as in the previous test campaign. For comparison, previously obtained results were also presented.

As can be seen, the calorific value in this set of solutions is closer to the reference value than in solutions without the use of increased weight for one of the parameters. This can be observed on the example of the previously described Simulation 1. The values of the parameters of the mixture, oriented primarily on the calorific value, are as follows: CV = 17.08 MJ/kg, pH = 7.83, salts Na, K, Ca = 7.14%, Cl = 1.73%, halogens = 0.67%. In this case the values of the three parameters improved in regard to the reference value. In addition to the distinguished calorific value, these are the chlorine content and halogens content. In such a case, the algorithm changes the mass shares of individual wastes or changes the composition of the suggested mixture by adding another waste specified by the user or subtracting waste included in the previous simulation. The value of the parameter with increased importance is brought closer to the reference value at the expense of some of the remaining parameters. It should be noted, however, that in each case the values of the remaining parameters are within the given range of values.

4. Conclusions

The developed optimization tool is an application for selecting the portion of feed material for a rotary kiln. It is based on the analysis of selected parameters of waste. The application, enabling the selection of components for a batch of the input material, is intended to compose the mixture in a hazardous waste thermal treatment plant in a way that ensures effective use of the energy contained in the waste, maintains specific operating parameters of the installation and minimizes the occurrence of operational problems in the incineration plant. The proposed tool minimizes the impact of alkaline and acidic components on the furnace lining and limits the possibility of excess emissions. The application test was performed, which included ten simulations determining the mass of a portion of the input material (assumed 100 kg) and selecting waste to be used to create a portion of the input material. An initial weight of 1 was used for each of the parameters considered. The values of all parameters were within the norm. The ranges of values for individual parameters were as follows: calorific value 14.96–20.66 MJ/kg, pH 5.59–8.11, content of alkaline salts 1.42–7.39, content of chlorine 1.1–3.83, content of halogens 0.08–0.97. Application tests showed that the basic condition was met, which was the creation of mixtures with established properties. One of the functions of the tool is the ability to give higher weights to selected parameters, shifting the result towards this waste property. Similarly, simulations were carried out to check the stability of the solution after changing the weight of one of the parameters—the weight of the calorific value increased to 5. The remaining assumptions and types of waste in individual simulations were unchanged. In this case, the ranges of values for individual parameters were as follows: calorific value 17.08–18.69 MJ/kg, pH 5.68–8.23, content of alkaline salts 1.42–8.20, content of chlorine 1.12–4.37, content of halogens 0.08–0.96. The values of the favored parameter definitely approached the reference value. Despite this, the resulting batches of feed material still meet the initial guidelines in terms of all considered parameters. An interesting direction for further development of the application seems to be the expansion of a module for parallel emission calculations for a given batch composition. Currently, the application is limited only to the furnace space and does not take into account the products of the thermal process. It seems reasonable to also take this aspect into account when developing the application in the future. In this approach, it might be possible to couple the application with numerical calculations of the thermal process.


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