Determination of the optimal area of waste incineration in a rotary kiln using a simulation model

Waste Management 42 (2015) 148–158

Contents lists available at ScienceDirect

Waste Management journal homepage: www.elsevier.com/locate/wasman

Determination of the optimal area of waste incineration in a rotary kiln using a simulation model J. Bujak ⇑ ´ skiego 6, 85-950 Bydgoszcz, Poland Polish Association of Sanitary Engineers, Bydgoszcz Division, Rumin

a r t i c l e

i n f o

Article history: Received 4 February 2015 Accepted 28 April 2015 Available online 16 May 2015 Keywords: Mathematical model Rotary kiln Thermal treatment of waste (incineration) Performance testing

a b s t r a c t The article presents a mathematical model to determine the flux of incinerated waste in terms of its calorific values. The model is applicable in waste incineration systems equipped with rotary kilns. It is based on the known and proven energy flux balances and equations that describe the specific losses of energy flux while considering the specificity of waste incineration systems. The model is universal as it can be used both for the analysis and testing of systems burning different types of waste (municipal, medical, animal, etc.) and for allowing the use of any kind of additional fuel. Types of waste incinerated and additional fuel are identified by a determination of their elemental composition. The computational model has been verified in three existing industrial-scale plants. Each system incinerated a different type of waste. Each waste type was selected in terms of a different calorific value. This allowed the full verification of the model. Therefore the model can be used to optimize the operation of waste incineration system both at the design stage and during its lifetime. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Waste incineration systems have their own well-defined specificity during both the design and execution stages, especially during operation (Yang et al., 2007). Their primary purpose is to convert the waste into a form considered as safe, while maintaining/satisfying a number of conditions required by the regulations (Chaerul et al., 2008; DIRECTIVE 2010/75/EU; Duan et al., 2008; Insa et al., 2010; Jang et al., 2006; Lee et al., 1991; Woolridge et al., 2008; Zimmer and McKinley, 2008). Some of the most advanced elements of waste management are thermal conversion processes of waste. They involve the oxidation of waste through incineration (Ampaitepin and Tetsuo, 2010), gasification (Lee and Kim, 1996) and degradation, for example through pyrolysis (Chiarioni et al., 2006; Conesa et al., 1996; Kovács et al., 2007; Leung and Wang, 1999; Li et al., 1999; Miranda et al., 2013; Wallman et al., 1998; Xianwen et al., 2001). The main advantages of this type of process are as follows:  the possibility of converting waste into safe form;  significant reduction in its weight and volume;  the possibility of recovery of substantial amounts of heat.

⇑ Tel.: +48 501541185; fax: +48 523220853. E-mail address: [email protected] http://dx.doi.org/10.1016/j.wasman.2015.04.034 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

Waste is highly heterogeneous in terms of size, elemental composition and physicochemical properties. This is particularly true of animal waste and bones (Poskrobko, 2013), medical and hazardous waste (Dean, 1996; Lombardi et al., 2013) and meat and bone meal which are usually burned in rotary kilns. Burning such types of waste is difficult and may be unstable. Moreover, the incineration process must be carried out specifically to follow a number of additional conditions and restrictions. Such basic conditions and restrictions are:  the appropriate level of emissions to the atmosphere,  adequate level of transformation of ash and slag,  providing the minimum temperature outside the afterburner chamber of 850 °C or 1100 °C,  ensuring the minimum residence time of flue gases in the afterburner chamber:  adequate volume and design parameters of the rotary kiln and afterburner chamber. Analysis of various waste combustion process, taking into account the above conditions and limitations, requires the use of advanced computing and simulation tools (Lin and Ma, 2012; Lombardi et al., 2013; Marias, 2003; Mujumdar and Ranada, 2006; Ndiaye et al., 2010; Nguyen et al., 2009; Niessen, 2010; Xia et al., 2014; Yang et al., 2007; Zanoelo and Meleiro, 2007).

J. Bujak / Waste Management 42 (2015) 148–158

This paper presents a mathematical model based on a rotary kiln technology, of thermal treatment of waste. It simulates the process flow and exchange of heat and enthalpy flow in the combustion and afterburner chambers as a result of a variety of waste incineration. A similar model was presented by Lombardi et al., 2013. Its objective was to predict the temperature inside the rotary kiln and on its outer surface when taking into account the different boundary conditions. The simulation model presented in this article provides a stream of incineration of waste as a function of its calorific value. Based on the derived parameters and taking into account the boundary conditions, the field of optimum plant operation can be determined. The analytical tool presented herein can be used as follows:  at the planning and preliminary design phase of the rotary kiln and afterburner chamber,  before the planned modernization in order to determine the possibility of increasing the capacity and efficiency of the system. The computational model has been verified in three actual industrial-scale facilities.

2. Materials and methods 2.1. Computational model The model is based on the known and proven energy flux balances and equations that describe the specific losses of energy flux while considering the specificity of waste incineration systems which were described in detail by (Bujak, 2009; Lombardi et al., 2013). The model is universal. It can be used both for analysis and testing of systems burning different types of waste (animal and bones, meat and bone meal, hazardous and medical). It includes the option to use of any kind of additional fuel. Types of waste incinerated and additional fuel are identified by a determination of their elemental composition. Calorific values for solid waste are calculated using the Dulong equation (Lombardi et al., 2013). The computational model consists of 2 blocks: the rotary kiln also known as combustion changer and afterburner chamber. The basic input parameters are:  the flux of incinerated waste: mi-w, (kg/h)  elemental composition of waste: C, H, O, N, S, Cl, Ash, H2O, (kg) or (%)  elemental composition of additional fuel used for the incineration of waste and maintaining the desired temperature outside the afterburner chamber: C, H, O, N, S, Cl, Ash, H2O, (kg) or (%)  minimum temperature of flue gases outside the afterburner chamber: ti-fg(min), (°C)  maximum temperature of flue gases outside the afterburner chamber: ti-fg(achmax), (°C)  minimum residence time of flue gases in the afterburner chamber: si-r(achmin), (°C)  the setpoint concentration of oxygen outside the combustion chamber: O2i-ach, (%)  temperature of the air used for combustion of waste and additional fuel: ti-a(rk), (°C)  heat transfer coefficient of the outer surfaces of the rotary kiln: Ui-rk, (W/m2 K)  heat transfer coefficient of the outer surfaces of the afterburner chamber: Ui-ach, (W/m2 K)

149

Basic parameters calculated by the simulation model:  calorific value of waste incinerated: Q(LCV)i-w, (MJ/kg)  calorific value of additional fuel: Q(LCV)i-af, (MJ/kg)  fuel flux delivered to the rotary kiln and the afterburner chamber: mi-af, (m3/h) or (kg/h)  the flux of the mass of flue gas leaving the rotary kiln: mo-fg(rk), (kg/h)  the temperature of flue gas leaving the rotary kiln: to-fg(rk), (°C)  chemical composition of the flue gas leaving the rotary kiln: CO2, H2O, N2, CO, SO2, O2, HCl, Dust, (kg) or (%)  the flux of the mass of bottom ash leaving the rotary kiln: mo-bash(rk), (kg/h)  the volume of the rotary kiln: Vo-rk, (m3)  the flux of the mass of flue gas leaving the afterburner chamber: mo-fg(ach), (kg/h)  the temperature of flue gas leaving the afterburner chamber: to-fg(ach), (°C)  chemical composition of the flue gas leaving the afterburner chamber: CO2, H2O, N2, CO, SO2, O2, HCl, Dust, (kg) or (%)  the flux of the mass of bottom ash leaving the afterburner chamber: mo-bash(ach), (kg/h)  the volume of the afterburner chamber: Vo-ach, (m3)  all heat and enthalpy fluxes described in Item 2.2. 2.1.1. Combustion chamber Fig. 1 shows the calculation algorithm for mass flux and parameters of the flue gas leaving the combustion chamber, based on the elemental composition of medical waste, additional fuel and air used for combustion. The calculation determines calorific value and chemical enthalpy flux contained in the waste and additional fuel and physical enthalpy flux in the air used for combustion. For purposes of the calculation algorithm, additional fuel may be gas, liquid, solid or even a waste of a suitable calorific value (which ensures reaching the appropriate combustion temperature). However, extra fuel is considered only if the calorific value of the waste is so small that it makes it impossible to achieve a predetermined minimum temperature of the gases leaving the combustion chamber. The model takes into account the loss of energy flux in the form of: – chemical enthalpy of heated ash removed from the combustion chamber – physical enthalpy of heated ash removed from the combustion chamber – the heat flow to the environment by the outer wall of the combustion chamber. Other assumptions used in the model: – the system operates under set conditions; – the rotary kiln operates at a constant speed of rotation. 2.1.2. Afterburner chamber The diagram of calculations of the mass flux and flue gas parameters discharged from the thermoreactor chamber, made on the basis of the chemical composition of flue gas leaving the combustion chamber, additional fuel elemental composition and the air used for combustion is illustrated in Fig. 2. The balance of the flux of energy and mass in the thermoreactor chamber includes the following assumptions: – afterburning process occurs under set conditions – afterburning process occurs with the excess air ratio of k > 1 – additional fuel can be any gas or liquid fuel.

150

J. Bujak / Waste Management 42 (2015) 148–158

WASTE x1

kg/s

C

x1-C

H

x1-H

O

x1-O

N

x1-N

S

x1-S

Cl

x1-Cl

F ASH

x1-F x1-ash

H2O

x

Q(LCV)

1-H 2 O

ADDITIONAL FUEL x2

kg/s

C

x2-C

i-w

m i-w t i-w

OUTLET GASES - RK

E i-w

y1

kg/s

CO 2 Q(LCV) m i-af(rk)

i-af(rk)

ti-af(rk)

H2O

ROTARY KILN m o-fg(rk)

( Combustion

t o-fg(rk)

y1-CO y

N2

y1-N

CO

y1-CO

2

H

x2-H

O

x2-O

SO 2

y1-SO

N

x2-N

HCl

y1-HCl

S

x2-S

ASH

x2-ash

HF O2

y1-HF y1-O

Dust

y1-dust

H2O

x 2-H

E i-af(rk)

E o-fg(rk)

chamber )

m i-a(rk) t i-a(rk)

2O

E o-dust(rk)

2

1-H 2 O

2

2

E i-a(rk)

AIR USED FOR WASTE AND FUEL COMBUSTION x3 kg/s O2

x3-O

N2

x3-N

H2O

x3-H

2 2

E o-bash(rk)

2O

E o-es(rk)

m o-bash(rk) Fig. 1. Algorithm for calculations – combustion chamber.

The model takes into account the energy loss in the form of heat flux transmitted to the environment by the outer wall of the afterburner chamber.

While the following are lost in or discharged (output) from the recovery system: E˙o-fg(ach)

–

E˙o-es(rk)

–

E˙o-es(ach)

–

E˙o-bash(rk)

–

E˙o-bash(ach)

–

E˙o-dust(ach)

–

2.2. Energy balance and thermal efficiency of the heat recovery system The system of thermal treatment of waste including its equipment components, when combined with other systems, may be treated as an open thermodynamic system (Fig. 3) exchanging mass, energy and heat with the environment. The following are input to the tested system: E˙i-w E˙i-af(rk)

– –

E˙i-af(ach)

–

E˙i-a(rk)

–

E˙i-a(ach)

–

chemical enthalpy flux of medical waste (kW) chemical enthalpy flux of additional fuel delivered to the combustion chamber (kW) chemical enthalpy flux of additional fuel delivered to the afterburner chamber (kW) physical enthalpy flux of air used for waste incineration and additional fuel combustion in the rotary kiln (kW) physical enthalpy flux of air used for after-burning gases and additional fuel combustion in the afterburner chamber (kW)

physical enthalpy flux of flue gases – potential usable energy (kW) heat flux lost to the atmosphere through the external surface of the rotary kiln (kW) heat flux lost to the atmosphere through the external surface of the afterburner chamber (kW) physical and chemical enthalpy flux of hot bottom ash (rotary kiln) (kW) physical and chemical enthalpy flux of hot bottom ash (afterburner chamber) (kW) physical and chemical enthalpy flux of dust (kW)

Energy balance equation can be written as:

E_ iw þ E_ iaf ðrkÞ þ E_ iaf ðachÞ þ E_ iaðrkÞ þ E_ iaðachÞ ¼ E_ ofgðachÞ þ E_ oesðrkÞ þ E_ oesðachÞ E_ obashðrkÞ þ E_ obashðachÞ þ E_ odustðachÞ

ð1Þ

Introducing the following equations:

X ðE_ tl Þ ¼ E_ oesðrkÞ þ E_ oesðachÞ þ E_ obashðrkÞ þ E_ obashðrkÞ

ð2Þ

151

J. Bujak / Waste Management 42 (2015) 148–158

OUTLET GASES - RK y1 kg/s CO 2 H2 O N2 CO SO 2 HCl HF O2 Dust

y1-CO 2 y1-H O 2 y1-N 2 y1-CO y1-SO 2 y1-HCl y1-HF y1-O 2 y1-dust

mo-fg(rk) t o-fg(rk)

ADDITIONAL FUEL y2 kg/s

E o-fg(rk)

C H O N S ASH H2O

Q(LCV)i-af(ach) m i-af(ach) ti-af(ach)

y2-C y2-H y2-O y2-N y2-S y2-ash y2-H O 2

OUTLET GASES z1 kg/s

E o-dust(rk)

AFTERBURNER CHAMBER

mo-fg(ach) t o-fg(ach)

E i-af(ach)

E o-fg(ach) Eo-dust(ach)

m i-a(ach) t i-a(ach) E i-a(ach)

CO 2 H2 O N2 CO SO 2 HCl HF O2 Dust

z1-CO 2 z1-H O 2 z1-N 2 z1-CO z1-SO 2 z1-HCl z1-HF z1-O 2 z1-dust

AIR USED FOR FUEL AND AFTER-BURNING GASES

y3

kg/s

O2 N2 H 2O

y3-O 2 y3-N 2 y3-H O

E o-bash(ach)

2

E o-es(ach)

m o-bash(ach) Fig. 2. Algorithm for calculations – after-burning chamber.

E i-af(rk)

E i-af(ach) E i-a(ach) AFTERBURNER CHAMBER

E i-w

ROTARY KILN

E o-es(ach) E i-a(rk) E o-fg(ach) E o-dust(ach)

E o-es(rk)

E o-bash(rk)

E o-bash(ach)

Fig. 3. Incinerator unit – energy flows.

E_ iw þ E_ iaf ðrkÞ þ E_ iaf ðachÞ þ E_ iaðrkÞ þ E_ iaðachÞ ¼ E_ fg þ

and

E_ ofg ¼ E_ ofgðachÞ þ E_ odustðachÞ Eq. (1) can be rewritten as follows:

ð3Þ

X ðE_ tl Þ

ð4Þ

where R (E˙tl) is the total loss of energy fluxes (kW). Defining thermal efficiency as the ratio of usable heat flux carried away from the system to the total flux of heat and enthalpy

152

J. Bujak / Waste Management 42 (2015) 148–158

supplied to the system and taking into account formula (4), the thermal efficiency has the following form:

E_

fg g¼ _ P Efg þ ðE_ tl Þ

– hazardous waste 12–13 MJ/kg (Meraz et al., 2003) – medical waste 19–22 MJ/kg (Bujak 2010; Meraz et al., 2003). The simulation tests were carried out for three different values of excess oxygen in the flue gas leaving the afterburner chamber: 8%, 9% and 10%. Measurements of the concentration of oxygen in the flue gas were carried directly outside the afterburner chamber. The decrease in excess oxygen in the flue gas below the set point forces opening of the valves supplying the air flux to the afterburner chamber. The air is fed in a smooth manner. The set point of the concentration of oxygen in such systems is most often maintained in the range of 7–10% (Bujak and Sitarz 2016; Lombardi et al., 2013; Wielgosin´ski 2012). Concentration values below 7% cause high carbon emissions – above 100 mg/m3 (at reference conditions). On the other hand, the large excess of oxygen in the flue gas (above 11%) causes its cooling which in turn increases additional fuel consumption. In this area, there are also high emissions of carbon oxides. In compliance with the applicable regulations in the European Union, the minimum temperature in the afterburner chamber cannot be lower than 1100 °C for waste containing more than 1% of halogenated organic compounds and 850 °C for waste with up to 1% of these compounds. This means that the afterburner chamber must be equipped with a special burner switching on if achieving said temperatures proves impossible. The rotary kiln (afterburner chamber) is also equipped with an independent burner. The burner performs two basic functions:

ð5Þ

or

E_

fg g¼ _ Ew þ E_ af cch þ E_ af ach þ E_ acch þ E_ aach

ð6Þ

The thermal efficiency defined by formula (6) can be determined only empirically because it requires measurements of usable energy flux and physical enthalpy flux of flue gases. For this reason, this method is called a direct method. Formula (5) shows an example of the indirect method for calculating the thermal efficiency. 3. Results and discussions Fig. 4a shows test results of the effect of changes in calorific value and the excess oxygen in the flue gas on the flux of recycled waste, taking into account the following conditions: – system operation: steady state (excluding the start-up and extinguishing periods) – afterburner chamber volume: Vo-ach = 16.5 m3 – the minimum residence time in the afterburner chamber: si-r(achmin) = 2.5 s – the minimum flue gas temperature outside the afterburner chamber: ti-fg(achmin) = 1100 °C – the maximum flue gas temperature outside the afterburner chamber: ti-fg(achmax) = 1200 °C – air temperature used for the waste incineration: ti-a(rk) = 25 °C – loss of heat flux through the outer surface of the rotary kiln and the afterburner chamber to the environment: E˙o-es(rk) + E˙o-es(ach) = 130 kW – additional fuel: high-methane natural gas.

– it initiates the process of incineration, – it supports the burning of waste if its calorific value is too low – when no possibility of autothermal combustion exists. During incineration of waste with very low calorific value Q (LCV) < 5 MJ/kg, both burners must work to ensure a stable process of incineration in the rotary kiln and to maintain a minimum temperature of the flue gas outside the afterburner chamber. When excess oxygen in the flue gas increases, its temperature in the combustion and afterburner chambers is reduced. Therefore, maintaining a minimum temperature in the afterburner chamber requires the increased flux of additional fuel. Flue gas stream leaving the afterburner chamber is the sum of the flue gas fluxes from the combustion of waste and supplemental fuel. Higher fuel consumption of additional fuel generates a higher total flux of the flue gas leaving the afterburner chamber. With the increase in the flue gas flux, the residence time in the afterburner chamber is decreased (below the minimum). To keep it at a given level (e.g.,

The simulation studies were carried out for waste with the calorific value of 5–30 MJ/kg. In this respect, is the vast majority of the waste is burned in the rotary kilns such as: – animal remains 6.9–28.3 MJ/kg (Poskrobko 2013), – animal bones 7.5–17 MJ/kg (Staron´ et al., 2010), – meat and bone meal 13–30 MJ/kg (Cascarosa et al., 2012; Senneca 2008), 895

900 750

800

655 584

670 485

572

600

278

500

395 456

415

345

5,0

700

400 339

261

9,4

245

164

158

11,1

300

14,6

118

109

20,0

200

Waste flux - m i - w (kg/h)

770

100 0

10,0 82

25,0

Calorific value - Q(LCV) i -w (MJ/kg)

8,0 30,0

9,0

Oxygen in flue gas -O 2i -ach (%) 8,0

9,0

10,0

Fig. 4a. The dependence of incinerated waste flux as a function of the calorific value and the excess oxygen in the flue gas after the afterburner chamber.

153

J. Bujak / Waste Management 42 (2015) 148–158

2.5 s), with the increasing excess of oxygen in the flue gas, less waste should be burned. This can be seen in Fig. 4a, where the largest flow of incinerated waste mi-w = 670 kg/h with the calorific value of 5 MJ/kg occurs with an excess of oxygen O2i-ach = 8%. With such parameters of combustion, the flux of additional fuel (Fig. 4b) is the smallest and equals mi-af = 103.9 kg/h. With the increase of the oxygen concentration to a level of O2i-ach = 10%, fuel consumption increases further to reach the value of mi-af = 143.2 kg/h. As a result, the flux of incinerated waste (5 MJ/kg) decreases, reaching the value of mi-w = 278 kg/h. With the increase of the calorific value of the waste (more than 5 MJ/kg) during its combustion, the temperature and flux of the flue gas increase, independent of the concentration of oxygen. This causes additional reduction in fuel consumption (Fig. 4b) and the resulting flue gas flux. As a result, increasing amounts of waste are incinerated (Fig. 4a, Table 1). In a characteristic point, hereinafter referred to as the break point, system capacity (the flux of waste incinerated) reaches its maximum value (e.g. mi-w = 895 kg/h). After reaching the maximum value, the flux of waste incinerated decreases together with a further increase in calorific value. In this area of operation, incineration process no longer requires additional fuel supply. Within the maximum calorific value of the waste Q(LCV)i-w = 25–30 MJ/kg, the maximum flue gas temperature (Table 1) at the outlet of the afterburner chamber ti-fg(achmax) = 1200 °C becomes a fundamental limitation in the amount of waste incinerated. In such cases, the flux of flue gases leaving the afterburner chamber is much smaller than the calculated amount which means that its minimum residence time in the afterburner chamber is considerably longer than the setpoint si-r(achmin) = 2.5 s. For this reason, we can increase the flux of waste incinerated by increasing the air flow which reduces the temperature of the flue gas outside the afterburner chamber below the maximum limit. Thus, for example, at the oxygen concentration of O2i-ach = 8% in the calorific value of Q(LCV)i-w = 30 MJ/kg, the flux of incinerated waste is mi-w = 82 kg/h. For the same calorific value and the excess of oxygen in the flue gas of O2i-ach = 10%, the waste flux increased to mi-w = 245 kg/h. These tests were carried out on the assumption that the walls of the rotary kiln and the afterburn chamber are well insulated. Heat flux loss through these surfaces to the environment is relatively low, ranging from 3% to 5% of the total energy flux supplied to the system as a waste and additional fuel. These types of systems are built mostly when they are assumed to be located inside the respective halls or rooms. In many cases, the thermal conversion systems are built on the outside of buildings – in the open air. In such cases, most of the systems are not sufficiently insulated. Their loss of heat flux to the environment can be even as much as 15% of the total energy flux supplied. Fig. 5 143,2

160 140

illustrates such system in operation. Break points occur at much higher calorific values than in identical systems with low heat flux flow to the environment. Furthermore, for high concentrations of oxygen in the flue gas (10% or more) the point is outside the field of operation. Here, in the entire range of calorific values, its increase occurs parallel to the increase in the flux of waste incinerated. At the highest tested value of 30 MJ/kg, the incinerated waste flux was 306 kg/h. On the other hand, in terms of the low calorific value of waste, the incinerated waste flux is very small: from 0 to 20 kg/h. And finally, large heat flux losses to the environment are the cause of substantial increases in consumption of additional fuel (Fig. 6). With flue gas emissions at the oxygen concentration of 8% and 9%, waste burns without additional fuel when the calorific value of waste is higher than 15.0 MJ/kg or 20.0 MJ/kg, respectively. The largest consumption of natural gas occurs during incineration of waste with low calorific value and high amount of excess air (10% oxygen concentration in the flue gas). Within the value of 5–10 MJ/kg the system operates as a conventional gas boiler. Incinerated waste flux is zero (Fig. 6) and the consumption of natural gas is maximized – 196.5 kg/h. Such high oxygen concentration in the flue gas together with high heat flux exchange with the environment makes the process of waste incineration almost unprofitable. Comparison of the efficiency of the installation as a function of heat flux loss to the environment and the concentration of oxygen in the flue gas outside the afterburner chamber is presented in Table 2. The thermal efficiency is determined by Formula 5. Analyzing the results obtained, the greatest impact on the thermal performance of the tested systems is by the heat flux exchange with the environment through their outer surfaces. This means that there is a direct connection between poorly insulated outer surfaces of the rotary kiln and corresponding reduced efficiency of the system. In terms of the tested case, the difference is as high as several percent – 10.9%. The oxygen concentration in flue gases has only a negligible impact on the thermal efficiency. The above analysis shows that it is very difficult to choose the optimal operating point of this type of system without proper tools e.g. a mathematical model. Under changing operating conditions, such as the calorific value of the waste flux and the temperature of air used for combustion, both the waste flux and the additional fuel undergo changes. In addition, there are a number of boundary conditions, which significantly limit the actual work field. Knowledge of the performance characteristics of the technology and conducting of economic analysis based on this knowledge allow finding the best operating range. Fig. 7 shows an example of the performance characteristics of the waste incineration system with the following boundary conditions and assumptions:

122,8

120 103,9 100

8,0 9,0

94,2

80

10,0

65,9

60

44,8

40

10,0 9,0 8,0

20 0

5,0

9,4

11,1

14,6

20,0

25,0

30,0

Fig. 4b. The flux of additional fuel supplied as a function of the calorific value and the excess oxygen in the flue gas after the afterburner chamber.

154

J. Bujak / Waste Management 42 (2015) 148–158

Table 1 The mass flux balance and the flue gas temperature leaving the afterburner chamber as a function of the calorific value. Calorific value of the waste Q(LCV)i-w (MJ kg1)

Oxygen – outlet flue gases O2i-ach (%)

Inlet waste flux mi-w (kg/h)

Inlet fuel flux mi-af (kg/h)

Inlet air flux mi-a (kg/h)

Outlet flue gas flux mo-fg (kg/h)

Outlet bottom ash and dust flux mo-bash+dust (kg/h)

Physical enthalpy flux of outlet flue gases Eo-fg (kW)

Outlet flue gas temper to-fg (°C)

5.0 9.4 11.1 14.6 20.0 25.0 30.0

8.0

670.0 895.0 770.0 345.0 158.0 109.0 82.0

103.9 0.0 0.0 0.0 0.0 0.0 0.0

4867.8 4904.3 4870.8 2772.7 1707.7 1450.5 1314.3

5580.0 5717.0 5570.0 3086.0 1848.0 1553.0 1393.0

61.6 82.3 70.8 31.7 17.7 6.5 3.3

2157.2 2170.6 2217.8 1248.0 744.0 617.5 548.6

1100 1100 1166 1200 1200 1200 1200

5.0 9.4 11.1 14.6 20.0 25.0 30.0

9.0

485.0 655.0 750.0 584.0 261.0 164.0 118.0

122.8 44.8 0.0 0.0 0.0 0.0 0.0

5142.8 5135.4 5138.0 5077.7 3046.2 2355.8 2042.7

5706.0 5775.0 5819.0 5608.0 3278.0 2510.0 2156.0

44.6 60.3 69.0 53.7 29.2 9.8 4.7

2154.2 2162.3 2154.5 2155.9 2220.2 993.1 845.1

1100 1100 1100 1175 1200 1200 1200

5.0 9.4 11.1 14.6 20.0 25.0 30.0

10.0

278.0 395.0 456.0 572.0 415.0 339.0 245.0

143.2 94.2 65.9 0.0 0.0 0.0 0.0

5409.4 5409.1 5403.1 5420.6 5270.5 5295.3 4611.8

5805.0 5862.0 5883.0 5940.0 5639.0 5614.0 4847.0

25.6 36.3 42.0 52.6 46.5 20.3 9.8

2137.7 2145.6 2143.2 2151.2 2174.0 2200.6 1891.0

1100 1100 1100 1100 1167 1200 1200

800 740

700 600

595

538 549

500 472

461

400 400

334

400 376

327

12,6

200

246 0

10,0

300

313

201

5,0

306

17,2

160

11

20

39

100

61

0 20,0

25,0

30,0

10,0

9,0

8,0

8,0

9,0

10,0

Fig. 5. The dependence of incinerated waste flux as a function of the calorific value and the excess oxygen in the flue gas after the afterburner chamber – loss of heat flux through the outer surfaces of the rotary kiln and the secondary combustion chamber is: 390 kW.

196,5

191,1

171,0

185,8

200

8,0 170,7

151,2 150

9,0 105,7

119,8

10,0

86,4

100

0,0

67,5

50

9,0

0,0

8,0

0,0

0 5,0

10,0

10,0

12,6

17,2

20,0

25,0

30,0

Fig. 6. The additional fuel flux supply as a function of the calorific value and the excess oxygen in the afterburner chamber – loss of heat flux through the outer surfaces of the rotary kiln and the secondary combustion chamber is: 390 kW.

155

J. Bujak / Waste Management 42 (2015) 148–158 Table 2 Comparison of thermal efficiency of systems with different heat exchange with the environment depending on calorific value and oxygen concentration. Calorific value (MJ kg1)

Oxygen concentration (%)

Thermal efficiency – loss of heat flux to the environment, 130 kW (well insulated walls) (%)

Thermal efficiency – loss of heat flux to the environment, 390 kW (walls without insulation) (%)

The difference in thermal efficiency (%)

5.0 10.0 15.0 20.0 25.0 30.0

8.0

92.6 94.4 87.4 84.8 81.6 80.3

83.2 84.4 83.5 84.3 81.4 80.1

9.4 10.0 3.9 0.5 0.2 0.2

5.0 10.0 15.0 20.0 25.0 30.0

9.0

92.5 94.4 93.5 90.5 87.2 85.9

83.0 84.5 83.6 84.2 83.6 84.1

9.5 9.9 9.9 6.3 3.6 1.8

5.0

10.0

92.3

Outside the field of operation 83.5 83.5 83.9 84.1 84.1

–

10.0 15.0 20.0 25.0 30.0

94.4 93.4 94.3 93.5 92.6

10.9 9.9 10.4 9.4 8.5

– combustion technology: rotary kiln – afterburner chamber volume: Vo-ach = 16.5 m3 – minimum temperature outside the afterburner chamber: ti-fg(achmin) = 1100 °C – minimum residence time of flue gases in the afterburner chamber: si-r(achmin) = 2.5 s

– oxygen concentration in the flue gas leaving the afterburner chamber: O2i-ach = 9.0% – maximum temperature outside the afterburner chamber: ti-fg(achmax) = 1200 °C – air temperature used for the waste incineration: ti-a(rk) = 25 °C – minimum system capacity: mi-w(min) = 300 kg/h – loss of heat flux to the environment through the outer surface of the rotary kiln and the afterburner chamber: E˙o-es(rk) + E˙o-es(ach) = 130 kW (well-insulated system) – additional fuel: high-methane natural gas Q(LCV)i-af = 48.5 MJ/kg. Analyzing Fig. 7, we see that Curve 1 determines the dependency of the incinerated waste flux as a function of its calorific value. It illustrates the maximum flux of incinerated waste including the two boundary conditions: afterburner chamber volume of Vo-ach = 16.5 m3 and flue gas residence time of si-r(achmin) = 2.5 s. Curve 2, introduces further limitations in the form of minimal flux of incinerated waste: mi-w(min) = 300 kg/h. Thus, between Curve 1 and Curve 2, a field is created which identifies the area of waste incineration involving three limitations. The higher the temperature of the flue gas inside the rotary kiln and afterburner chamber, the higher the likelihood of a rapid wear and tear of the refractory lining. To minimize this process, another limitation is introduced, namely, the maximum temperature of ti-fg(achmax) = 1200 °C in the form of Curve 3. The right part of incineration area is cut off, and thus a new field is created between lines 1, 2 and 3. This is a secure combustion area, called the work area. Curve 4 divides it into two parts: – the left side of the work area requiring additional fuel supply to maintain a minimum temperature of the fuel gas outside afterburner chamber – the 1100 °C line. The more we move away from Curve 4 towards the left side, the more additional fuel fluxes are required by the combustion process.

Line 4 - minimum temperature: 1100 oC The breaking point after the afterburner chamber

750 kg/h @ 11.1 MJ/kg

Line 3 - maximum temperature: 1200 oC after the afterburner chamber

Flux of waste mass - m

i-w

(kg/h)

900 Work area - with additional fuel 800

Optimal work area - without additional fuel Line 1 - incinerated waste flux

700

Volume of afterburning chamber: 16.5 m The residence time: 2.5 s Oxygen: 9.0%

600 500

3

Line 2 - minimum load: 300 kg/h

400 300

2 1

200 4

100

5

10

15

20

3

25

Calorific value of the waste - Q(LCV)

30 i-w

35

(MJ/kg)

Fig. 7. Performance characteristics of heat recovery system equipped with rotary kiln, taking into account the boundary conditions.

156

J. Bujak / Waste Management 42 (2015) 148–158

Fig. 8. Thermal waste treatment system - facility A. Table 3 Characteristics of tested systems and waste disposed in them. Facility name

Waste type

Calorific value – working (MJ kg1)

System performance (kg h1)

Technology type

A B C

Pig bones Medical Plastic + paper

5.7 21.9 30.0

700 400 250

Rotary kiln Rotary kiln Rotary kiln

– the field on the right illustrates the process of incineration without the use of additional fuel. It was called the optimal work area. Optimal work area is also characterized by relatively high fluxes of incinerated waste. Sometimes in practice it turns out that the most cost-effective waste incineration will occur near the left side of 1100 °C line, that is, with a negligible additional fuel consumption. This situation can occur because there is a maximum flux of incinerated waste within this area. Increased amount of incinerated waste and the associated higher income can compensate for the additional fuel costs. As we can see, determination of the optimum work field is not easy and requires appropriate computational tools. The above operation analysis was related to a particular case with a number of limitations. In practice, each system is different and characterized by different factors: – – – – –

heat flux loss to the environment the type of additional fuel the type and design of concrete lining air temperature used for the waste incineration the minimum flue gas temperature outside the combustion chamber, such as 850 °C.

The mathematical model presented in this article can greatly facilitate and accelerate tests for optimization of heat recovery systems. 4. Model verification 4.1. Introduction The model to determine the performance characteristics of thermal waste treatment system was verified in three actual

industrial-scale facilities. Fig. 8 shows one of the test facilities burning meat waste in the form of pig bones – facility A. Each system (Table 3) incinerated different types of waste. Each waste type was selected in terms of a different calorific value. This allowed the full verification of the model. The flux of the waste mass to be incinerated was weighed prior to loading. Calorific values and elemental composition of waste were determined by laboratory testing. Chemical enthalpy stream in additional fuel (high-methane natural gas) was measured by a GM gas meter. The gas volume measurement was performed at medium pressure of approx. 3 bar. At the same time, pressure and temperature of gas were continuously measured allowing the determination of volume of gas under normal conditions. With the volume of gas under normal conditions (p = 1 bar, t = 0 °C) and calorific value (obtained on the basis of physical and chemical analyzes by the Gas Company), chemical enthalpy flux contained in natural gas was calculated. 4.2. Measurement system The measurement system consisted of three main components: (a) flux measurement: – flux of chemical enthalpy of additional fuel fed into the rotary kiln and the afterburner chamber – using a GM flow meter manufactured by Vortex, with a measuring accuracy of 1.25% – mass of medical waste, measured by electronic scales with 0.5 % accuracy (SC1), – mass ash, measured by electronic scales with 0.5 % accuracy (SC2) – the volume of flue gas (part of the emission computer), measured by flowmeter D-FL 100, with 1.25% accuracy (b) temperature and oxygen measurement using sensors with a measuring accuracy of 2.0 % for: – flue gas temperature at the outlet of the afterburner chamber (T1), – oxygen concentration in the flue gas leaving the afterburner chamber (O2) (c) continuous flue gas monitoring (emission computer) consisting of:  measuring component:

157

J. Bujak / Waste Management 42 (2015) 148–158 Table 4 Validation of the calculation model with experimental data. Input and output data O2i-ach (%)

Calculation model Q(LCV)i-w (MJ kg1)

Experimental testing

5.7

21.9

30.0

5.7

21.9

30.0

698.0 88.3 4952.1 5671.0 67.4 2160.1

142.0 0.0 1613.2 1741.0 14.2 687.3

82.0 0.0 1314.3 1393.0 3.3 548.6

719.0 93.5 4654.2 5403.0 63.7 2099.8

148.0 0.0 1678.9 1813.0 13.9 697.9

79.0 0.0 1399.1 1474.0 4.1 566.2

492.0 118.5 5173.2 5735.0 48.7 2154.3

250.0 0.0 2805.6 3032.0 23.6 1241.7

118.0 0.0 2042.7 2156.0 4.7 845.1

491.0 134.4 4892.1 5474.0 43.5 2041.9

241.0 0.0 3012.4 3226.0 27.4 1325.7

114.0 0.0 2231.9 2340.0 5.9 922.5

297.0 131.2 5421.7 5822.0 27.9 2139.3

390.0 0.0 5272.3 5621.0 41.3 2190.6

245.0 0.0 4611.8 4847.0 9.8 1891.0

281.0 140.5 5621.8 6013.0 30.3 2287.9

405.0 0.0 4987.6 5356.0 36.6 2052.7

239.0 0.0 4825.6 5053.0 11.6 1933.4

8.0 Inlet waste flux, mi-w, (kg/h) Inlet fuel flux, mi-af, (kg/h) Inlet air flux, mi-a, (kg/h) Outlet flue gas flux, mo-fg, (kg/h) Outlet bottom ash and dust flux, mo-bash+dust, (kg/h) Physical enthalpy flux of outlet flue gases Eo-fg, (kW) 9.0 Inlet waste flux, mi-w, (kg/h) Inlet fuel flux, mi-af, (kg/h) Inlet air flux, mi-a, (kg/h) Outlet flue gas flux, mo-fg, (kg/h) Outlet bottom ash and dust flux, mo-bash+dust, (kg/h) Physical enthalpy flux of outlet flue gases Eo-fg, (kW) 10.0 Inlet waste flux, mi-w, (kg/h) Inlet fuel flux, mi-af, (kg/h) Inlet air flux, mi-a, (kg/h) Outlet flue gas flux, mo-fg, (kg/h) Outlet bottom ash and dust flux, mo-bash+dust, (kg/h) Physical enthalpy flux of outlet flue gases Eo-fg, (kW)

– system of collection and transport of flue gas sample – system of dust measurement and the reference parameters (static pressure, temperature and flue gas velocity) necessary to perform calculations – measuring panel with analyzers  conversion and computing component: – measurement data concentrator processing data from the analyzers and sensors while converting it from analogue to digital format – emission computer implementing acquisition, archiving, verification and presentation of measurement data and creation of graphs and generation of reports  support component: – panel with technical gases necessary for continuous system calibration.

4.3. Comparison of the results obtained on the basis of the calculation and test models In order to evaluate the fit between the simulation model and empirical data, the results (Tables 4) obtained experimentally were compared with those calculated by a mathematical model. The input data for both models is identical. For the verification of the calculation model against empirical data, standard deviation was used as a measure of the location and dispersion, standard deviation being the average of deviations of the characteristics from the arithmetic mean. The actual (obtained during the measurements) flux of waste incinerated was different by an average of 11.8 kg/h from the calculated using a mathematical model representing 4.1% of the average value of the flux. In the case of the gas volume flux, the difference was 3.75 Nm3/h, or 6.8% of its average value. 5. Conclusions Computational analysis showed that testing of the thermal performance of waste disposal requires the right tools and mathematical models. The complexity of the combustion processes with regard to the current legal status and limitations of the design

and material requires thorough investigation in order to select the best option. Determining the best work area should be complemented by economic analysis. As shown by the tests, regardless of the level of the oxygen concentration in the flue gas, the flux of waste disposal increases with the decrease of calorific value. At a specific point, hereinafter referred to as the breaking point, it reached its maximum value. After reaching the maximum value, the flux of waste incinerated decreased together with a further decrease in calorific value. For high concentrations of oxygen in the flue gas (10% or more) and high heat loss the point could not be observed under the given boundary conditions. Here, in the entire range of calorific values, its increase occurs parallel to the increase in the flux of waste incinerated. At the highest tested value of 30 MJ/kg, the incinerated waste flux was 306 kg/h. The additional fuel flux in the form of natural gas was significantly dependent on the calorific value of the waste and the oxygen concentration in the flue gas. With the increasing calorific value of the incinerated waste, the gas flux fed to the system was lower. On the other hand, higher oxygen concentration in the flue gas caused increased additional fuel consumption. In the case of very high concentrations of oxygen (10% or more) and large heat loss to the environment from the outer surface of the rotary kiln and the afterburner chamber, the additional fuel must be supplied even at high calorific values of waste. The thermal efficiency of the system was largely dependent on the intensity of heat loss to the environment. It depended on the design and materials used for the construction of partitions rotary kiln and the afterburner chamber. The rotary kiln worked at a constant speed. The effect of the constant speed on the intensity of heat exchange was not analyzed. It undoubtedly occurs, although it should be noted that the incineration of waste in this type of system is carried out at speeds of from 1 to 3 revolutions per hour. The computational model was verified in three actual industrial-scale facilities. Each system incinerated a different type of waste. Each waste type was selected in terms of a different calorific value. This allowed the full verification of the model. The differences between the results obtained on the basis of a

158

J. Bujak / Waste Management 42 (2015) 148–158

mathematical model and the real facility were only a few percent 4.1% for flux of waste incinerated and 6.8 % for gas volume flux. Therefore the model can be used to optimize the operation of waste incineration system both at the design stage and during its lifetime. References Ampaitepin, S., Tetsuo, T., 2010. The waste-to-energy framework for integrated multi-waste utilization: waste cooking oil, waste lubricating oil, and waste plastics. Energy 35, 2544–2551. Bujak, J., 2009. Experimental study of the energy efficiency of an incinerator for medical waste. Appl. Energy 86, 2386–2393. Bujak, J., 2010. Experimental study of the lower heating value of medical waste. Polish J. Environ. Study 19, 1151–1158. Bujak, J., Sitarz, P., 2016. Reduction of NOx and CO emissions through the optimization of incineration parameters in a rotary kiln. Environ. Prot. Eng. 42 (4) (in press). Cascarosa, E., Gea, G., Arauzo, J., 2012. Thermochemical processing of meat and bone meal: a review. Renew. Sustain. Energy Rev. 16, 942–957. Chaerul, M., Tanaka, M., Ashok, V., Shekdar, A.V., 2008. A system dynamics approach for hospital waste management. Waste Manage. 28, 442–449. Chiarioni, A., Reverberi, A.P., Fabiano, B., Dovı, V.G., 2006. An improved model of an ASR pyrolysis reactor for energy recovery. Energy 31, 2460–2468. Conesa, J.A., Marcilla, A., Font, R., Caballero, J.A., 1996. Thermogravimetric studies on the thermal decomposition of polyethylene. J. Anal. Appl. Pyrol. 36, 1–15. Dean, K., 1996. Heat-recovery incinerator for a community hospital. ASHRAE J. 38, 49–53. DIRECTIVE 2010/75/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 24 November 2010 on industrial emissions (integrated pollution prevention and control). Duan, H., Huang, O., Wang, Q., Zhou, B., Li, J., 2008. Hazardous waste generation and management in China: a review. J. Hazard. Mater. 158, 221–227. Insa, E., Zamorano, M., Lopez, R., 2010. Critical review of medical waste legislation in Spain. Resour., Conserv. Recyc. 54, 1048–1059. Jang, Y., Lee, C., Yoon, O., Kim, H., 2006. Medical waste management in Korea. J. Environ. Manage. 80, 107–115. Kovács, T., Zsély, I., Kramarics, Á., Turányi, T., 2007. Kinetic analysis of mechanisms of complex pyrolytic reactions. J. Anal. Appl. Pyrol. 79, 252–258. Lee, C.C., Huffman, G.L., Nalesnik, R., 1991. Medical waste management. Environ. Sci. Technol. 25, 360–363. Lee, J.S., Kim, S.D., 1996. Gasification kinetics of waste tire-char with CO2 in a thermobalance reactor. Energy 21, 343–352. Leung, D.Y.C., Wang, C.L., 1999. Kinetic modeling of scrap tire pyrolysis. Energy Fuels 13, 421–427. Li, A.M., Li, X.D., Li, S.Q., Ren, Y., Shang, N., Chi, Y., et al., 1999. Experimental studies on municipal solid waste pyrolysis in a laboratory-scale rotary kiln. Energy 24, 209–218. Lin, H., Ma, X., 2012. Simulation of co-incineration of sewage sludge with municipal solid waste in a grate furnace incinerator. Waste Manage. 32, 561–567.

Lombardi, F., Lategano, E., Cordiner, S., Torretta, V., 2013. Waste incineration in rotary kilns: a new simulation combustion tool to support design and technical change. Waste Manage. Res. 31, 739–750. Marias, F., 2003. A model of a rotary kiln incinerator including processes occurring within the solid and gaseous phase. Comput. Chem. Eng. 27, 813–825. Meraz, L., Dominguez, A., Kornhauser, I., Rojas, F., 2003. A thermochemical conceptbased equation to estimate waste combustion enthalpy from elemental composition. Fuel 82, 1499–1507. Miranda, M., Cabrita, I., Pinto, F., Gulyurtlu, I., 2013. Mixtures of rubber tyre and plastic wastes pyrolysis: a kinetic study. Energy 58, 343–352. Mujumdar, R., Ranada, P., 2006. Simulation of rotary cement kilns using a onedimensional model. Chem. Eng. Res. Design 84, 165–177. Ndiaye, L.G., Caillat, S., Chinnayya, A., Gambier, D., Baudoin, B., 2010. Application of the dynamic model of Saeman to an industrial rotary kiln incinerator: numerical and experimental results. Waste Manage. 30, 1188–1195. Nguyen, T.D.B., Kang, T.H., Lim, Y.I., Eom, W.H., Yoo, K.S., 2009. Application of urea based SNCR to a municipal incinerator: on-site test and CFD simulation. Chem. Eng. J. 152, 36–43. Niessen, W.R., 2010. Combustion and Incineration Processes: Applications in Environmental Engineering, fourth ed. CRC Press, Andover, Massachusetts, USA. Poskrobko, S., 2013. Identification and stabilization of combusting animal waste with active participation of bone material – emission of SO2 and HCl. Fuel Process. Technol. 113, 20–27. Senneca, O., 2008. Characterisation of meat and bone mill for coal co-firing. Fuel 87, 62–70. Staron´, P., Kowalski, Z., Krupa-Zuczek, K., Wzorek, Z., 2010. Thermal utilization of mixtures of bone waste. Polish J. Chem. Technol. 12, 26–30. Wallman, P.H., Thorsness, C.B., Winter, J.D., 1998. Hydrogen production from wastes. Energy 23, 271–278. Wielgosin´ski, G., 2012. Pollutant formation in combustion processes, advances in chemical engineering, Dr. Zeeshan Nawaz (Ed.), ISBN: 978-953-51-0392-9, InTech, . Woolridge, A.C., Phillips, P.S., Denman, A.R., 2008. Developing a methodology for the systematic analysis of radioactive healthcare waste generation in an acute hospital in the UK. Resour., Conserv. Recyc. 52, 1198–1208. Xia, Z., Li, J., Wua, T., Chen, C., Zhang, X., 2014. CFD simulation of MSW combustion and SNCR in a commercial incinerator. Waste Manage. 34, 1609–1618. Xianwen, D., Xiuli, Y., Chuangzhi, W., Wennan, Z., Yong, Ch., 2001. Pyrolysis of waste tires in a circulating fluidized-bed reactor. Energy 26, 385–399. Yang, W., Nam, H., Choi, S., 2007a. Improvement of operating conditions in waste incinerators using engineering tools. Waste Manage. 27, 604–613. Yang, Y.B., Sharifi, V.N., Swithenbank, J., 2007b. Converting moving-grate incineration from combustion to gasification – numerical simulation of the burning characteristics. Waste Manage. 27, 645–655. Zanoelo, E.F., Meleiro, L.A.C., 2007. A dynamic optimization procedure for noncatalytic nitric oxide reduction in waste incineration plants. Chem. Eng. Sci. 62, 6851–6864. Zimmer, C., McKinley, D., 2008. New approaches to pollution prevention in the healthcare industry. J. Clean. Prod. 16, 734–742.