3. Energy Production From Plastic Solid Waste (PSW)

3 Energy Production From Plastic Solid Waste (PSW) S.M. Al-Salem Environment & Life Sciences Research Centre, Kuwait Institute for Scientific Research, Kuwait

3.1 Key Concepts Waste is a byproduct of the day-to-day activities that we as consumers conduct. Solid waste (SW) originating from various sectors can be handled in many ways. There have been various established technologies on an industrial scale, where waste is valorized to meet the demand of a certain country or community. In addition, plastics comprise a very significant proportion of this waste. We have detailed in the past two chapters the fact that plastics encompass a high calorific value (CV). This energy content is what makes plastics a material that is highly desired for incineration processes. Incineration is the technology that is typically associated with energy recovery. Although many of us would also relate energy recovery from wasteto-landfill gas generation (LFG) or biogassing (or even thermo-chemical treatment), but it is releasing the embodied energy content of a material is what is considered to be an energy recovery scheme. It is not possible to talk about the technologies and key concepts in energy recovery from plastics without detailing the major key concepts of SW incineration. SW incineration can be found at the most advanced level of treatment, and is certainly considered the highest in preference order when it comes to the PSW management hierarchy (see Chapter 2: Major Technologies Implemented for Chemicals and Fuel Recovery). Incineration can also be found coupled with other technologies. It could be dedicated to treat a certain byproduct (e.g., ash), and it can also be a treatment option for a certain waste stream, such as medical waste or hazardous waste. Incineration feedstock is something that needs careful consideration, and should meet certain requirements. The most important parameter that must be well controlled is the lower CV which was noted earlier in Chapter 2, Major Technologies

Implemented for Chemicals and Fuel Recovery. The LCV is the energy content that makes the feedstock lucrative enough to incinerate and then recover energy. If this parameter is controlled, other parameters will be considered as associated to the LCV. Therefore, quality, account of seasonal variation, source of material, and composition, etc., will follow the careful monitoring of LCV. This is the reason why the incineration of extreme composition of materials (e.g. plastics and sand alone) is not recommended for incineration and co-incineration. This is despite having high energy content for plastic feedstock on its own [1]. Moreover, there must be very good consideration for the economical values of waste that is used for the incineration process. This is in terms of continuous availability of feedstock material and the utilization of the right economy of scale of the unit. The throughput of the material must be constant and somewhat stable in generation throughout the four seasons to ensure generation of energy is fairly constant all year long. Many third world (developing) countries have a continuous and well-documented scavenging activity in waste dumps, landfill sites, and around incineration and other treatment plants (Fig. 3.1). Scavengers of waste usually target plastics. This is in order to resell this material to recyclers and private industries that can operate a cheaper process with a feedstock originating from waste at lower prices than market ones. This also reduces the CV of the incineration unit feedstock due to the recovery of valuable fractions of plastics with high energy value intended for incineration. At the heart of any integrated solid waste management (ISWM) system, lies energy recovery through incineration processes. Incineration also relies heavily on landfill sites where residues are typically landfilled as a final disposal method. Of

Plastics to Energy. DOI: https://doi.org/10.1016/B978-0-12-813140-4.00003-0 © 2019 Elsevier Inc. All rights reserved.

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Figure 3.1 Documented waste scavengers in an active landfill operating in the state of Kuwait, targeting mainly plastic articles.

course, landfilling of a large amount of waste will not be achieved in this technique due to the very simple fact that the majority of the SW has been incinerated. Taking careful consideration of the type of incineration process for energy recovery (or general waste to energy, WtE technologies) is a must. Energy recovery from waste that is rich in plastics is a success story in many societies and countries. However, it is invaluable to control, as mentioned earlier, the energy content and, consequently, other parameters through proper material separation and a highly efficient collection system (Fig. 3.2). Social acceptance and accountability are also key issues that have to be considered for a good energy recovery scheme, where the sustainability of the process itself is controlled by the optimization of the economic, environmental, and social aspects. There should also be a good use of the resources surrounding the material availability, where ultimately the decision to incinerate for energy recovery is higher in preference than biological treatment or TCT on an individual basis. There should be a general acceptance for the increasing operational and maintenance costs associated with the investment of energy recovery plants. It is essential also to establish a good economy for energy and the byproducts produced

(electricity, heat, steam, metals, ash, etc.). The number of jobs created and the green (environmental) image also make for grand incentives for societies to support such ventures. Various societies have had their share of success stories when it comes to energy recovery from plastics and waste. McDougall et al. [2] detailed various case studies on both a country and city basis that shows that WtE, and in particular, incineration, is something that is essential for reclamation and treatment of products from waste. One of which is the case of Prato (Italy) where the control of waste, especially film plastics that account for 140 tonnes, is managed post separation in two material recovery facilities to recycling schemes. The Hampshire (UK) IWMS system on the other hand, has an incineration plant that processes 10% of the collected recyclables in the area, with only 5.5% diverted to composting. Many developing world countries have a large dependency on incineration for energy recovery. The State of Kuwait, an oil-dependent West Asian nation with one of the highest SW generation rates in the world (5.75 kg per capita per day), is currently faced with serious issues regarding the management of resources in terms of SW accumulation and reclamation. It has been announced that in the recent future, recycling of rich plastic and organic municipal solid waste

3: ENERGY PRODUCTION FROM PLASTIC SOLID WASTE (PSW)

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Waste generation (including plastics)

Recyclables

Collection and transportation

Materials recovery and sorting

Recycling

Emission control

Incineration (WtE)

Energy ash

Residue handling

Treatment options

Other treatment (including landfilling)

Recycling products energy soil improvement, etc.

Figure 3.2 Principle components of waste streams treatment options highlighting important stages for a successful energy recovery scheme.

(MSW) that will rid the country and landfills of some 50 % of total waste load, will be treated in a 1 million tonnes per annum capacity incineration unit for energy recovery purposes, and be integrated with the national grid [3]. Kuwait also hosts two incineration plants that processes waste, including plastics in 5 tonne per day units. These energy recovery plants are monitored for their emissions and also produce 5 % ash that is buried in a nearby landfill site. In other Middle East countries, established LFG recovery systems for energy production revolve around recovering methane gas (C1) to power up turbines and other units. These are hosted in Turkey, namely in its capital city, Ankara where 60% of the organic fraction from MSW is utilized in a zero landfill waste scheme to power 22.5 MW for the national grid [4]. The United Arab Emirates (UAE) is also following suit within the municipality of Dubai in an Al-Quais landfill site valorizing 22 years of MSW buried underground [5]. Feedstock for energy recovery via incineration, especially originating from waste, has to have a minimum requirement of categories (blend) and composition with a certain energy content when it

comes to plastics. Table 3.1 shows the minimum requirement of plastic materials that must be present within the feedstock to be able to be considered for incineration. There is a general confusion that exists among sectors and authorities that handle and deal with energy recovery units. The confusion lies among the three main terms which are used in this type of work. These terms are thermal treatment, combustion, and incineration. Thermal treatment (or destruction) is the process whereby the material is mixed with oxygen with or without the aid of an external energy source, and a solid ash is recovered from the bottom of the vessel while carbon dioxide (CO2) and water are exited from the top. Thermal treatment allows for the recovery of heat and energy, which are also typically produced from the process. Combustion on the other hand, is the reaction itself that takes place between a hydrocarbon (HC) with oxygen (O2) to produce carbon dioxide (CO2) and water. On the other hand, incineration is the process where materials are heated in the presence of oxygen to oxidize organic compounds. Incineration will utilize the main combustion

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Table 3.1 Waste Definition and Classification Based on Point of Origin and Source Lower CV (MJ/kg)

Category Organics (food and kitchen waste)

15 20

Plastics: PE

45

PVC

15 25

Polystyrene

40

Polypropylene

45

Fines (,12 mm mesh)

15

reaction where HCs are burned (combusted) with oxygen to release embodied energy thus: HC 1 O2 -CO2 1 H2 O 1 Energy

(3.1)

Hence, incineration is a type of thermal treatment that combusts the HC for energy release. Thermal treatment also hosts other treatment methods such as gasification (use of gasifying agents such as oxygen, steam, air, etc.), hydrolysis, and of course, combustion, where the following stoichimometric reaction takes place [6]: C20 H32 O10 1 x1 O2 1 x2 H2 OΔ -y1 C 1 y2 CO2 1 y3 CO2 1 y4 H2 1 y5 CH4 1 y6 H2 O 1 y 7 Cn Hm (3.2) Coefficients (x and y), denoted in the previous equation, will balance both sides of the chemical reaction. For example, when certain polymerized materials are incinerated, the following reactions will take place while the possibility of producing energy is present at certain combustion conditions [7]: Polyethylene (PE): C2 H4 1 3O2 -2CO2 1 2H2 O

(3.3)

Butyl rubber: C9 H16 1 13O2 -9CO2 1 8H2 O

(3.4)

C2 F4 1 O2 -9CO2 1 4HF

(3.5)

Teflon:

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Various plastic materials produce (postincineration) ash and metals due to the presence of contaminants in the feedstock material. The product gases are likely to host various numbers of inorganic and organic compounds as well. The combustion reaction of organic materials and HCs will ultimately result in complex byproducts. When complete combustion with total oxidation of all carbon, sulfur, etc., is achieved, no oxidation of nitrogen (N2), in theory, is to take place. There are a number of parameters that can control and manipulate a combustion reaction with a certain feedstock. These are the lower and higher CVs of the material used as a feedstock (see Table 2, Chapter 2: Major Technologies Implemented for Chemicals and Fuel Recovery). The unit design also plays a major role in energy recovery and range of combustion reactions. However, it is best to validate the feedstock material by examining three parameters for applicability of combustion and combustibility without auxiliary fuel [1]. These are the ash content (residual), combustible fraction, and the moisture content of the feedstock (i.e., raw waste with plastics, organics, etc.). The Tanner Triangle Plot given below in Fig. 3.3 shows the range of combustibility of materials. The feedstock is applicable for combustion (without auxiliary fuel) if it falls within the shaded area, where moisture ,50%, ash ,60% and combustible fraction is . % [See Ref. [1] for full details on waste assessment for combustibility and properties. The same reference gives the criteria for waste incineration (combustion technologies).].

3.2 Types of Incineration Units Incineration units vary in size, design, and operation. They will all operate a combustion system that works mainly to reduce the volume and size of the feedstock materials (typically commingled SW) and temperature range. Incineration units can be divided into a number of categories based on the ability to destroy contaminants [7]. These categories are rotary kiln, moving grate, multiple heart, liquid injection, fluidized bed, and finally, multiple chamber. However, before going into detail and before examining case studies of plastics used as feedstock to each type, in addition to the application of these


%Moisture (W )

10

90

20

80

30

70

40

60

50

W = 50%

50

60

30

C=

70

25%

40

80

20 A=

90

10

90

80

70

60

50

40

30

20

10

4-%

% Ash (A)

% Combustible (C)

Figure 3.3 Material combustibility assessment tanner triangle plot according to World Bank Report [1].

units to waste materials in general, it should be noted that this classification is based on unit type and process specific categorization. In other words, there is another way of categorizing incineration units based on the feedstock treatment of the combustion system, which are mass burning and homogenized feedstock burning. It should also be noted that mass burning is the most widely used and well-developed technology when it comes to incineration. Mass burning is the combustion of unsorted MSW, with the aim of converting feedstock into useable energy [8] under certain operating conditions. This technology requires little and almost no pretreatment of feedstock. The majority of treatment plants around the world operate this type of incineration using a moving grate operation. This technique is also the most feasible when it comes to the economy of scale, and is considered the height of in rate of return of WtE technologies. However, just like in the case of all incineration units, there are major environmental implications associated with this technology. There has always been a risk of formation of dioxins [dioxin is the term collectively given to the structurally related polychlorinated chemicals to polychlorinated dibenzo para dioxins (PCDDs) and polychlorinated dibenzofurans], nitrogen oxides (NOx), and the deterioration of local air quality that will ultimately

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cause a public health concern from the pollutant exposure of its chimney stack. Mass burning using movable grates is considered to be the only technology that fulfills the criteria of proven feasibility by the World Bank [1]. Unlike mass burning technologies, pretreated incineration plants are very limited due to their complications in preparing and sorting the feedstock material. The reader should also note that energy recovery is done in a different part (or section) of the incineration process after the combustion chamber. Fig. 3.4 outlines the incineration unit plant as a whole to give the reader a better understanding of the process flow diagram of waste incineration. In Fig. 3.4, it is key to understand that all types of combustion chambers (vessels) will operate to facilitate the material feedstock reaction with O2 to produce the heat required for energy recovery from the exothermic reaction. These materials are what we can consider as fuel to this process. In our case, plastic-rich feedstock or a plastic material. The furnace can also operate depending on the mode of the flow of gases and the feedstock material as depicted in Fig. 3.5. The first type of mass burning incinerator unit that is considered quite common is the rotary kiln incinerator. Fig. 3.6 shows a typical rotary kiln incineration plant used in MSW incineration. The unique feature of this unit is the combustion chamber, which is a heated rotating cylinder mounted at an angle with baffles to add the required turbulence for the process. The rotary kiln itself consists of a layered burning unit where the material is transported through the furnace by the rotation of the cylinder. Most organic materials are incinerated in this type of unit. It has been also reported to incinerate the solids and sludge. The temperature of operation in this type of process can vary between 800°C and 1650°C, and the kiln cylinder can be of a wide range in diameter between 1 and 5 m, and 8 and 20 m in length, with a throughput range between 0.1 and 20 tonnes per hour. Moving grate technologies in incineration plants are predominant in the waste management market. The main feature of this type of unit is the transportation facility of the material, which is done through the furnace in a roller grate after an overhead carne feeds waste into a dedicated hopper (chute) to the furnace (Fig. 3.7).

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Material/waste source

Boiler

Treatment sorting, homogenizing, shredding

Combustion process

Ash (slag) recovery

End use

Stack emission

Temperature reduction & heat recovery

Air pollution control

Safe disposal

Figure 3.4 Schematic flow diagram of typical incineration plant.

Figure 3.5 Classification of furnace type with respect to flow of gases and feedstock materials. From van Blijderveen, Ignition and Combustion Phenomena on a Moving Grate With Application to the Thermal Conversion of Biomass and Municipal Solid Waste, PhD thesis, University of Twente, Enschede, The Netherlands, December (2011) [9].


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Atmosphere

Quench

Air Natural gas, liquid feed

Afterburner extension

Transfer duct

Packed column scrubber

Main burner Ash bin

Air Natural gas, liquid feed

Rotary kiln incinerator

ID fan

ID fan

Solids feeder

Afterburner

Rotary kiln

Stack

Demister

Secondary burner

Venturi scrubber

Carbon bed adsorber

Hepa filter

Scrubber liquid recirculation

Primary air pollution control system

Redundant air pollution control system

Figure 3.6 Schematic diagram of a typical rotary kiln incineration unit. From USEPA (J. Lee, D. Fournier, Jr., C. King, S. Venkatesh, C. Goldman, 1997. Evaluation of rotary kiln incinerator operation at low to-moderate temperature conditions EPA/600/SR-96/105 March)[10].

Figure 3.7 Schematic diagram of a moving grate incineration unit equipped with energy (power) generation and emission control [11].

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Figure 3.8 Design of the grates in waste incineration plants [9].

Grates used in this process can vary in design and operation. The majority of which will consist of rows of bars which move with or against the waste flow (Fig. 3.8). The movement of the grate is what results in good mixing of the feedstock. Roller grates are also commonly found in many plants that deal with waste feedstock. Feedstock is first dried in the combustion chamber (100°C) and then heated under pyrolytic (e.g., inert) conditions at about 250°C. Oxygen (O2) gas is then supplied to have a combustion reaction occurring at 450°C. Oxygen is also additionally supplied and large heat is then recovered as energy. The design and movement of the grate is key in transporting and agitating the waste material. As it is a mass burning type of unit, there is no immediate need for pretreatment of the feedstock before incineration. In energy recovery installations (Fig. 3.7), the hot gases resulting from the combustion process are used to create steam for power generation purposes. Energy recovery will be discussed at later stages of this section. Also, it should be noted that moving grates give the highest possible treatment capacity, where throughput can be managed between 10 and 4300 tonnes per day [12]. Fluidized bed incinerators are, as their name implies, based on the principle of fluidization of the feedstock material by suspending the solids using air (fluidizing agent). Fluidization is a welldefined and reported unit operation in chemical engineering that has a very versatile application and use. Fluidized bed incinerators are also reported to incinerate waste very rapidly, and can deal with a number of feedstock from waste, including MSW and PSW. The most common type of this unit in the pressurized fluidized bed combustion process where fuel (in this case MSW or PSW) is injected under pressure into the combustion chamber hosts inert sand particles that will transfer the heat to the fuel. Hence, fluidization has always been a good process

in providing good heat and mass transfer, control of product range, and good control of the combustion reaction. Heat is also supplied throughout the combustion process as necessity persists. Air is supplied into the bottom of the particle bed that leads to their suspension during the fluidization process [6]. Table 3.2 gives a summary of the advantages and disadvantages of the main incineration unit technologies used in energy production from waste and consequently, plastic materials. Another less common process is the multiple hearth combustion furnace used in the incineration of various contaminants (solids) and sewage sludge. The concept was developed back in the 1900s for treating and roasting iron ore (Fe2O3), where air cooled vertical cylinders are used to incinerate solids and sludge. This type of unit is not used for the treatment of MSW and certainly not plastics or polymeric waste on their own, but is considered a main type of incineration unit in waste management (Fig. 3.9). The feedstock is slowly fed from the top through the stacked hearth. The outer shell is typically manufactured of steel, where a hollow cast iron rotating shaft runs through the center. Operating temperature of this unit can significantly vary from 800°C to above 1600°C. A lesser common type of waste incineration unit is the electric infrared unit used in sludge treatment plants (Fig. 3.10). A furnace, coupled with a conveyer belt, will extend the length of the unit. Infrared heating elements will be placed on the top roof of the conveyer belt. The waste containing high moisture (such as sludge) will be dried along the length of the unit’s conveyer. Ash will also be recovered from the process and excess air might vary significantly between 20% to 70% [13]. All the various designs of incineration units will share a number of common aspects. They will all allow the thorough mixing of feedstock material with air in temperatures high enough to achieve a combustion reaction between 750°C and 1000°C.

Table 3.2 Summary of the Advantages and Disadvantages of the Main Incineration Technologies [1] Technology Name Advantages

Moving Grate No need for sorting, pretreatment or shredding of feedstock Technology meets and demand of the markets Accommodates variation in CV of feedstock due to design 85% thermal efficiency of process with throughput of multiple of furnaces

Disadvantages

High capital (CAPEX) and operational (OPEX) cost

Rotary Kiln No need for sorting, pretreatment or shredding of feedstock Overall thermal efficiency of 80 % Accommodates variation in CV of feedstock due to design High capital (CAPEX) and operational (OPEX) cost Not as common as the moving grate

Fluidized Bed Simple design and well established technique Low associated costs 90 percent thermal efficiency. Suitable for various types of waste. Not common technology due to capacity constraints

Limited throughput capacity compared to grate technology

Figure 3.9 Cross section diagram of a multiple hearth incinerator used in sludge treatment [13].

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Figure 3.10 Cross section diagram of sludge/waste incineration unit of electric infrared type [13].

They will all also allow flue gases to be scrubbed for removal of hazardous chemicals, and particulate matter (PM) will be removed by electrostatic precipitation and filters. While incineration occurs, the energy is recovered as heat from the flue gases resulting from the process. All energy recovery schemes will require the cooling down of the flue gases by using a boiler, which then will allow the gases to exit and be part of the air purification and pollution control configuration of the plant. Three main types of boilers are available to cover the wider spectrum of energy and utility production. These are the hot water boiler, and low and high pressure (HP) steam boilers. The flue gases exiting the boiler will ultimately result in having heat, steam, power, or a combination of these utilities produced for the national grid or energy distributor, to be then used by the general public. The types of boilers are summarized in Table 3.3. Various types of incineration units can be used with the available boilers and energy recovery configurations. In addition, the type of the waste and level of contaminants govern the process due to the unit’s operation in handling the various types of hazardous waste. Fig. 3.11 shows how the steam boiler may be configured with the rotary kiln incineration process overall. The amount of energy recovered is governed by the previously stated parameters discussed in this chapter. As noted previously, grate type incineration units are the most common incinerators of waste plants. This technology is noted to produce large amounts of heat due to the constant oxygen supply provided. HP-steam boilers are a lucrative (and possible) route for the recovery of combined heat and power (CHP). Fig. 3.12 shows a schematic flow diagram of this

process, with energy recovery and the flow gases cleanup processes as described previously in [14 15].

3.3 Incineration of Plastics The problem of plastics accumulation as a major part of the waste stream is something that could be managed via energy recovery methods. As noted in the past chapter, these technologies are considered the most beneficial for the end user and consumer due to the fact that utilities (power, steam, and heat) are produced from them. Plastics are a very lucrative source of energy due to their nature as a crude oil derivative product. However, there exists the major problem of economic viability in the industry when it comes to all methods of recovery. The advantage of energy recovery is the possibility of overcoming this problem by producing high-end and easily marketed products (utilities). The nature of plastics gives the feedstock rich characteristics in terms of energy content similar to conventional fuels. Excess heat can be easily produced from the incineration of plastics, and can also serve households and communities as a utility supply. A good example of applying incineration of plastics-rich feedstock is the United Kingdom, where the majority of the units used in incineration are of the moving grate type, and treat waste with high percentages of PSW. Incineration of PSW can also reduce the amount of waste (by volume) in the range of 90% 99% depending on the constituting polymers. This significantly reduces the dependency on landfills. Plastics in incineration process serve two main objectives according to Westerhout et al. [16].


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Table 3.3 Type of Boilers Used in Energy Recovery Systems From Incineration Units Boiler Principle

Hot Water Boiler

LP Steam Boiler Flue gases will produce steam in the temperature range of 120 250°C and a pressure around 20 bars

Most complex in operation and design

The boiler can feed to a cooling tower but no energy recovery is possible with this configuration

35% of efficiency is achieved with power generation, and 75% of combined steam and power

80% thermal recovery is possible

CHP can be in a process of 85% overall efficiency

Heat generation only

Heat generation only

No power or combination of steam and power with heat is possible

No power or combination of steam and power with heat is possible

Steam, power, and heat

Simple in design and has low CAPEX

Simple in design and has low CAPEX

Limited use and application due to low energy content of hot water

Limited use and application due to low energy content of hot water

If cooling tower is used to cool gases, no recovery of energy is possible

If cooling tower is used to cool gases, no recovery of energy is possible

Flue gases will heat the water to be used in heat production The boiler efficiency can reach 80 % (i.e., thermal and overall recovery) Higher temperature of water (above 160°C) can be achieved with manipulating operational pressure of the unit

Energy application

Notes

HP-Steam Boiler

They act as a fuel source to supply energy in combustion phase, and they act as a reducing agent for pyrolysis and gasification in replacing coke. The use of plastic materials as a supplement fuel has been reported in the steel making industry using incineration units as well [17,18]. Plastics can be incinerated in high amounts within MSW. Moreover, there exits a number of issues with plastics being incinerated with MSW that one should look out for. These could be summarized as per the following:

• Heavy metal content in feedstock must be eliminated to be able to produce reusable slags and ash from incineration [19,20].

Possibility of producing power and heat, or a combination of both, in addition to, steam

Combination of steam and power with heat is also achieved and possible after settingup proper turbine units Complex in design Highly profitable and can be used in various communities to cover local needs

• Incineration units nowadays are moving toward technologies that will operate with relatively low temperatures around 850°C, hence applicability of plastics material content within this range of temperature must not be overlooked. As previously noted, emission control is a major technical point when it comes to the incineration process. One of the main technologies that can reduce carbon emissions of the incineration process is the oxygen/recycled flue gas (O2/RFG) combustion system. This technique used by chemical and combustion engineers, is based on separating the N2 gas from the air, while pure O2 gas is mixed

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Figure 3.11 Incineration of hazardous waste plant showing configuration with steam boiler [14] [15].

Figure 3.12 Incineration of SW using moving grate technology with energy recovery as shown by Holder et al. [15].


with CO2 rich recycled flue gas from the combustion chamber. Thus, the easy recovery of CO2 from this rich carbon stream becomes an effort of reducing it and controlling its emissions. A number of energy recovery plants use techniques such as coal fired power generation systems and integrated gasification combined systems (ICGS) [21,22]. Fig. 3.13 shows an experimental setup used by Chen and Huang [22] in their work. PE plastic granules were used to simulate PSW incineration, and were fed at a rate of 24 g/min after reaching steady state temperature conditions. It was reported that the emission of CO decreased with the rising ratio of the recycled flue gas with O2 levels below 40%. This facilitates for the user a window for manipulating the process conditions for a better emission control. Tang et al. [23] studied the behavior of polymeric materials present in MSW. Combustion conditions were achieved utilizing a thermogravimetric analyzer where polyvinyl chloride (PVC), leather, and rubber were subjected to various heating ramps. O2, CO2, and N2 were used as gaseous atmospheres, ensuring combustion and pyrolysis conditions, while creating a means to determine the effect of CO2 addition on the behavior of the material. No change in weight loss of materials studied occurred below 600°C with respect to reaction media. Smol et al. [24] also determined the benefits of incineration with the aim of waste treatment, namely sewage sludge, and linked the utilization of incineration of ash, after processing it in the form of vitrified microspheres, to a reduction of thermoplastic resin weight and an

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improvement of the flow of resin in the composite, in thermoplastics, and in the production of layers resistant to scratching, and finally, in the production of anti-condensation paints and thermal insulation layers. The work stemmed out of the importance of recovering phosphorus (P), which is an emerging topic due to the benefits of recovering this chemical and enforcing stringent environmental regulations [25]. The reader might have noted earlier that plastics that have a high CV can be comparable or superior to conventional fuels in many cases. In fact, 25% of the CV of MSW is estimated to be from the plastics present in it [26]. The average CV of MSW is typically assumed to be 20,000 Kj/kg [1]. Plastic materials will combust in two distinct phases: a pyrolysis phase followed by a combustion phase [26 28]. In the first phase, the plastic material will decompose chemically from heat into gaseous compounds related to the type and nature of the plastic material [27]. Combustion will then occur at the flame when the gases enter it. Due to combustion reaction conditions, the combustion gases will typically be very stable, with a small atomic structure (two or three atoms) such as H2O, CO2, NO, CO, and SO2 [29]. Particular attention must be paid to incomplete combustion conditions where insufficient amount of O2 gas can lead to the formation of CO and the release of more complex, structured chemicals which are considered a major concern in regards to the incineration process (e.g., chlorobenzene).

Figure 3.13 Experimental setup of plastics incineration used by Chen and Huang [22].

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Bujak [30] conducted a major scale study where a rotating kiln incineration unit was used to treat the plastic scrap of a plastic tape manufacturing facility. In addition to the recovered heat, the pilot plant unit eliminated all emissions of SO2, CO, NOx, and dust, which showed great potential for future upgrading of existing industries in order to meet environmental regulations. A major reduction in environmental stressors were also detailed and achieved in Frey’s et al. [31] work after simulating grate furnace incineration of MSW with a fraction of plastic mixed with refuse derived fuel (RDF). Incineration of tyres is also manageable and can be of great benefit for energy recovery producers where incinerating used rubbers are considered as a fuel source [32]. All of which deems plastic and polymeric materials a good source of energy for potential treatment in incineration units.

3.4 Governing Regulations and Key Criteria Incineration of fossil fuels is considered to be the main source of greenhouse gases (GHGs) emissions into the atmosphere. Incineration technologies within the WtE context are regulated in every country and region in the world, with the main concern having always been in how to meet permissible limits of air pollution control criteria. Waste management regulations are strict, on occasion, with incineration facilities in controlling their feedstock materials, and consequently, the release of some chemicals from their exhaust gas systems. In many cases, regulations will require the separation of recoverable materials and recyclables from the feedstock. However, the percentage of recovery varies from one country to another, where, for example, in the United States, 25 % is mandatory [33]. European Union (EU) and the United States incineration plant regulations cover and categorize incineration units into three main types depending on their emission strengths and type of waste feedstock they handle. The first is the mass burn combustor. It was noted earlier that this is quite a common technique in handling throughputs due to ease of operation (in relative terms). Some 55 % of MSW within Europe and the United States is processed in such a manner. Due to the excess air being fed in the system, gas exhaust volumes are

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always a cause for worry with these type of units. The second is RDF combustors. These are a subclassification of the pretreatment type of incineration unit, and RDF is typically co-incinerated with coal. The last classification is the modular combustor, which is the smallest in size and feed throughput, with a variation in the amount of air being fed in for the execution of the combustion reaction. Incineration generates chemicals that are considered major criteria GHGs and are climate relevant. Incineration is associated with the release of CO2, CO, nitrous oxide (N2O), ammonia (NH3), and nitrogen oxides (NOx). Dioxins and other chemicals are also released from this process. Incinerating one tonne of MSW is associated with the (minimal) release of 700 kg of CO2 [34]. Table 3.4 shows the different volumes of exhaust gases released from incineration units. It can be noted that MSW incineration is considered the lowest in release exhaust gas volume in comparison to other feedstock materials. This can be attributed to the commingled nature of MSW, and can be considered as an added advantage to MSW incineration. The incineration unit exhaust gases will contain various pollutants released into the atmosphere depending on the zone of combustion with the design of the unit and the operating temperature. The higher the temperature, the more pollutants will be destroyed in the process. The zones within the incineration unit that will contribute to product formation are the combustion and post combustion zones. Both NOx and dioxins are formed in the combustion zone, but with different temperature ranges. NOx is formed at temperatures higher than 1450°C, while dioxins are formed within the temperature range of 150 400°C [15]. Dioxins are the result of PVC waste. There exists a variety of heavy metals that are present in incineration units’ feedstock materials which will be in the incineration bottom ash or Table 3.4 Incineration Units Exhaust Gas Release Volume by Feedstock [34]

a

Feed Type

Estimated Volume (m3/Mg Waste)a

MSW

5500

Hazardous waste

7000

Sewage sludge

8000

Dry basis. Mg (Mega gram equivalent to 106 grams).


released as PM from the exhaust system [35]. Table 3.5 shows major criteria pollutants that are monitored and regulated in incineration processes. Control of these emissions is achieved using scrubbers among other installations.

59

use of incineration in blast furnaces has always been applied in Germany for the production of iron for steel manufacturing by reducing iron ore (Fe2O3) to iron (Fe) using fossil fuels as reducing agents in the process. Plastics have been applied as a reducing agent in incineration for production of steel. Stahlwerke Bremen (Germany) and British Steel (United Kingdom) are two examples of companies that have tried such strategies in treating waste whilst reducing costs and environmental burdens with the use of PSW. Capacity of operation have reached 7000 tpd of iron using this technique, with the added advantage of using PSW as a feedstock with minimal sulfur content and minimal dioxin emissions as well [41]. Replacing coal with plastics in blast furnace incineration has become more dominant nowadays in industries the world over, and has reached Asian ventures such as the Steel Making Company in Korea [42 44] as well. Germany is also reported to have the highest number of incineration units in Europe, exceeding 53 with a capacity surpassing 10.7 million tpa [45]. The United States hosts over 190 incineration units with a capacity of 110 tpd. The European Trade Association of Diisocyanates & Polyols Producers (ISOPA) declared its support of incineration of MSW with energy recovery schemes back in 2007, since it utilizes the continent’s PSW, which on average makes up 7 % of total SW loads and produces a constant ash content that can be used in various civil engineering applications [46]. The BSL incineration process is another example where chlorine rich plastics (containing PVC) are treated using rotary kiln incineration with the aim of producing HCl from the energy of the process itself [41]. The process cannot accept only PVC waste, as the high CV will cause temperature control problems. However, 45

3.5 Notes and Case History Success Stories The process of energy recovery by incinerating plastic materials has immense environmental benefits. Not only does it reduce the amount of PSW, but it also destroys harmful blowing agents, chemical additives, (mainly chlorofluro carbons present in PSW shreds), granules, and foams [36]. Fire retardants present in plastics also increase the complexity of the incineration process. Energy recovery schemes utilizing WtE technologies can be found at the heart of many governmental initiatives toward waste management. The United Kingdom is estimated to provide 17 % of its electricity from waste by the year 2020 [37]. The United Kingdom’s target was declared back in 2006 to produce 700 MWe from MSW, which rids the country of 25 % of its total waste load. Various counties around the United Kingdom have noticed this implementation of new WtE plants over the past decade or so. Hampshire hosts a number of energy recovery plants from waste, collectively treating about 45 % of its MSW. Kent also hosts a 500 kilo tonnes per annum (ktpa) facility for MSW incineration [37 40]. Within the EU, many states consider WtE technologies as a main source of power using mass burning to cover utility demand for communities around the continent. Denmark, Sweden, and Germany are prime examples of this strategy. The Table 3.5 Major Pollutants and PM Regulations [15]

a

Region

PM (mg/m3)

Cadmium (Cd, mg/m3)

Mercury (Hg, mg/m3)

PCDD/F (Ng/m3)a

United Statesb

0.18 115

0.001 18

0.001 0.55

0.0022 5

0.05

0.1

N/A

0.1 5

EU

10

0.05

Japan

150

N/A

c

Provided in unit of toxic equivalency. Values encompass all nonhazardous incinerator types designated by EPA regulations. c Combined with thallium containing compounds. PCDD/F, polychlorinated dibenzo para dioxins and dibenzofurans. b

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ktpa is the overall throughput capacity of the process with a heat production capacity of 25 MW with 15 ktonnes of PVC content. Fig. 3.14 depicts the BSL process, where a combustion chamber is fitted after the rotary kiln reaches 900 1200°C of operating temperature. HCl is released and recovered during this stage. The energy (steam), slag, and HCl are considered to be the products of this process. The main advantage of this configuration is that dioxins and furanes are diminished. The United Kingdom’s Department of Environment, Food, and Rural Affairs (DEFRA) has declared that the country host 15 incineration plants with a capacity that exceeds 3 million tonnes of MSW [47]. All of the previously stated employ what is commonly known as co-incineration treatment, where plastics are fed with another type of SW stream to the incineration unit. Plastic waste is also mixed, in some cases, with coal, not only waste. Boavida et al. [48] combusted PSW with coal using a fluidized bed combustor (CFB). The objective of the work was to reach a fuel mixture with as little variation in heat as possible. The volatile release was the main cause of variation when PSW was added, hence, essential to control in

Air

Slag

Pretreatment (shredding depending on waste size)

Rotary kiln

Post combustion

HCl absorption

Flue gas purification HCl purification

Figure 3.14 BSL incineration process of plastics.

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terms of shape and type of polymer. In addition, CFB combustors are increasing in popularity due to: (1) Their less complex emissions control system of the configuration. (2) Their high combustion efficiency with a simple operation and fast response. (3) Reduction in boiler size. (4) Low corrosion with easier ash removal. One of the most common technologies around Europe is the Ebara Co. CFB, with more than 100 units installed around the world. The Spanish capital of Madrid has a commissioned unit that handles 10% of the city’s waste with a throughput of 9 % of PSW. The main advantage of this technology is that it has no moving parts, and it is equipped with a slanted bed floor to produce a revolving sand motion [37]. Cement kilns have been commonly used as well in PSW incineration. For the past three decades, cement kilns have been using PSW as an alternative feedstock (Fig. 3.15). The design of the unit suits PSW very well [41] due to the fact that kilns are able to reach desired temperatures with ease, have a substantial length size, a long residence time of the fuel in the kiln, and alkalinity of the kiln’s environment. Gas temperatures within the kiln can reach up to 2000°C in the combustion zone, and the gas

Waste

Water

TO

Gas cooling

Steam

Water


61

Figure 3.15 Schematic flow diagram of a typical cement kiln [2].

residence time is about three seconds, with temperatures around 1200°C. The nature of the kiln allows the solid feedstock to flow in the opposite direction along its kiln (200 m), resulting in a long residence time of the gases of 4 6 s at 1800°C, and 15 20 s at 1200°C [49]. There exists some 250 units in cement plants around Europe with a production rate of 170 million tonnes of cement per annum [17]. Poland is a prime example of this cement industry, with about 11 mtpa cement production [41]. According to past surveys, the following countries have a high percentage of alternative fuel use in the percentile values given: Holland (72 %), Switzerland (34%), and Belgium (30 %). There exists two process that are dominant in this application, a dry and a wet process. The former relies on the introduction of the material in a dry form in the kiln, while the latter introduces it as a slurry. A disadvantage of the wet process is that it needs more energy requirement (5000 MJ/tonne for the wet process in comparison to 3600 MJ/tonne for the dry process), since no water has to be evaporated in the dry process. The emerging Chinese market has a very unique and rapid development rate in comparison to other countries. The energy consumption in China is unfavorable due to the energy endowment. Keeping

in mind that China alone produces 29% of the world’s MSW [50], landfilling using sanitary landfill sites is the predominant method of waste treatment [51]. However, incineration and composting are the other two alternative technologies used for waste management in the country. Moving grate furnaces, rotary kilns, and fluidized beds are the common technologies used. In the year 2014, 188 incineration units were in operation in China treating over 32 % of the total MSW [52]. There seems to be a great investment in the alteration of technologies and management of waste plans around the world toward energy production. Incineration has not only proven itself as an economical investment, but also as a very good method for energy production around the world. This is evident from the examples illustrated from around the world. However, the production of energy can also benefit from the production of various fuel sources which chemical and fuel production technologies from plastics can deliver. These methods, which were touched upon in Chapter 2, Major Technologies Implemented for Chemicals and Fuel Recovery, will be illustrated later on in this book to give the reader the full view of the treatment methods available that can be used to valorize PSW.

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List of Abbreviations C1 CFB CHP CO CO2 CV EU GHGs HC HP ICGS ISWM ktpa LFG LP MSW N2 N2 O NH3 NOx O2/RFG O2 P PCDD/ F PE PSW PVC RDF SO2 SW TCT UAE WtE

methane gas fluidized bed combustor combined heat and power carbon monoxide carbon dioxide calorific value European Union greenhouse gases hydrocarbon high pressure integrated gasification combined systems integrated solid waste management kilo tonnes per annum landfill gas generation low pressure municipal solid waste nitrogen nitrous oxide ammonia nitrogen oxides oxygen/recycled flue gas oxygen phosphorus polychlorinated dibenzo para dioxins and dibenzofurans polyethylene plastic solid waste polyvinyl chloride refuse derived fuel sulfur dioxide solid waste thermo-chemical treatment United Arab Emirates waste to energy

References [1] World Bank, 1999. Technical Guidance Report, municipal solid waste incineration. [2] F.R. McDougall, P.R. White, M. Franke, P. Hindle, Integrated Solid Waste Management: A Life Cycle Inventory, second ed., Blackwell Publishing, Cornwall, UK, 2008. ISBN 0-63205889-7. [3] KN, 2016. Kuwait News Bulletin. Available http://www.kuwaitnews.com/kuwait/ from: baladi/121389-2016-12-03-09-28-14 [4] SP, 2013. South pole news. Mamak landfill gas recovery.

TO

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[5] CW, 2013. Construction Weekly Online, Dubai launches landfill gas recovery project. Available from: http://www.constructionweekonline.com/article-20404-dubai-launches-firstlandfill-gas-recovery-project/ [6] D.A. Vallero, Fundamentals of air pollution, U.S. Environmental Protection Agency/ Elsevier Academic Press, MA, US, 2008. [7] T.M. Letcher, D.A. Valero, Waste: A Handbook for Management, Academic Press Elsevier, US, 2010. [8] McLanaghan, S.R.B., 2002. Delivering the landfill directive: the role of new and emerging technologies. [9] M. van Blijderveen, Ignition and Combustion Phenomena on a Moving Grate With Application to the Thermal Conversion of Biomass and Municipal Solid Waste, PhD thesis, University of Twente, Enschede, The Netherlands, 2011. December 2011. [10] Lee, J., Fournier, D., Jr., King, C., Venkatesh, S., Goldman, C., 1997. Evaluation of rotary kiln incinerator operation at low to-moderate temperature conditions EPA/600/SR-96/105 March. [11] Database of Waste Management Technologies, 2017. Waste to Energy 1-2: grate incineration technology. Available from: http://www.epem. gr/waste-c-control/database/html/WtE-01.htm [12] Lew, R., 2016. Moving grate incineration: preferred WTE technology. Available from: http://www.bioenergyconsult.com/moving-grateincineration/ [13] USEPA, 1996. Sewage sludge incineration— US EPA, AP-42, Vol. I. Available from: https://www3.epa.gov/ttnchie1/ap42/ch02/final/ c02s02.pdf [14] European Union for Responsible Incineration and Treatment of Special Waste (EURITS), 2017. Available from: http://www.eurits.org/ [15] A.L. Holder, E.P. Vejerano, X. Zhou, L.C. Marr, Nanomaterial disposal by incineration, Environ. Sci. Process. Impacts 15 (2013) 1652 1664. [16] R.W.J. Westerhout, J.A.M. Kuipers, W.P.M. van Swaaij, Experimental determination of the yield of pyrolysis products of polyethylene and polypropylene. Influence of reaction conditions, Indus. Eng. Chem. Res. 37 (3) (1998) 841 847. [17] Janz, J., Weiss, W., 1996. Injection of waste plastics into the blast furnace of steelwork, in:


[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

Proceedings of the 3rd International Iron Making Congress, Gent, Belgium, September 16 18, pp. 114 119. Ariyama, T., 1996. Development of a new scrap melting process based on massive coal and plastics injection, in: Proceedings of the 3rd International Iron Making Congress, Gent, Belgium, September 16 18, pp. 314 321. J. March, Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed., John Wiley & Sons, US, 1992. E. Kowalska, Z. Wielgosz, J. Pelka, Use of post life waste and production waste in thermoplastic polymer compositions, Polym. Polym. Composites 10 (1) (2002) 83 91. N. Kimura, K. Omata, T. Kiga, S. Takano, S. Shikisima, The characteristics of pulverized coal combustion in O2/CO2 mixtures for CO2 recovery, Energy Conserv. Manage. 36 (1995) 805 808. J. Chen, J. Huang, Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O2/ RFG combustion technology, Fuel 86 (2007) 2824 2832. Y. Tang, X. Ma, Z. Lai, Y. Fan, Thermogravimetric analyses of cocombustion of plastic, rubber, leather in N2/ O2 and CO2/O2 atmospheres, Energy 90 (2015) 1066 1074. M. Smol, J. Kulczycka, Z. Kowalski, Sewage sludge ash (SSA) from large and small incineration plants as a potential source of phosphorus e Polish case study, J. Environ. Manage. 184 (2016) 617 628. Y. Kalmykova, K.K. Fedje, Phosphorus recovery from municipal solid waste incineration fly ash, Waste Manage. 33 (2013) 1403 1410. Magee, R.S., 1989. Plastics in municipal solid waste incineration: a literature study, prepared by the Hazardous Substances Management Research Centre of the New Jersey Institute of Technology for the Society of the Plastics Industry. Washington (DC). ASIN: B00071NT32, Society of the Plastics Industry. E.A. Boettner, G.L. Ball, B. Weiss, Combustion Products From the Incineration of Plastics, National Technical Information Centre, Springfield, VA, 1973. USEPA# 670/ 2-73-049.

63

[28] K. Wark, C.F. Warner, W.T. Davis, Air Pollution: Its Origin and Control, 3rd ed., Addison-Wesley, US, 1998. [29] Dynamac, A., 1989. An overview of synthetic materials producing toxic fumes during fires, EPA contract 68-01-6239, Rockville (MD). [30] J.W. Bujak, Thermal utilization (treatment) of plastic waste, Energy 90 (2015) 1468 1477. [31] H. Frey, B. Peters, H. Hunsinger, J. Vehlow, Characterization of municipal solid waste combustion in a grate furnace, Waste Manage. 23 (2003) 689 701. [32] J.A. Conesa, R. Font, A. FuIlana, J.A. Caballero, Kinetic model for the combustion of tyre wastes, Fuel 77 (13) (1998) 1469 1475. [33] USEPA, 1991. Plastics wastes: management, control, recycling and disposal. United States Environmental Protection Agency, pollution technology review no. 201 (1991), prepared by: Randall Curlee T, Das S, ISBN# 0-81551265-1. [34] Johnke, B., 2000. Emission from waste incineration, good practice guidance and uncertainty management in national greenhouse gas inventories, reviewed by: Hoppaus, R., Lee, E., Irving, B., Martinsen, T., and Mareckova, K. [35] S. Abanades, G. Flamant, B. Gagnepain, D. Gauthier, Fate of heavy metals during municipal solid waste incineration, Waste Manage. Res. 20 (2002) 55 68. [36] APC, 1999. American Plastics Council, Development of hydrocyclones for use in plastic recycling, Technical paper. [37] L. Yassin, P. Lettieri, S.J.R. Simons, A. Germana, Study of the process design and flue gas treatment of an industrial-scale energy-from-waste combustion plant, Industr. Eng. Chem. Res. 46 (2007) 2648 2656. [38] Ares, E., Bolton, P., 2002. Waste incineration. Research paper 02/34, House of Commons Library, London. [39] L. Yassin, P. Lettieri, S.J.R. Simons, A. Germana, Energy recovery from thermal processing of waste: a review, Eng. Sustainab. (Proc ICE) 158 (2005) 97 103. [40] Quant, A., 2005. Annual monitoring report 2004/05; Hampshire Minerals and Waste Development Framework, Winchester, UK. [41] A. Tukker, H. de Groot, L. Simons, S. Wiegersma, Chemical Recycling of Plastic

64

[42]

[43]

[44]

[45] [46]

[47]

PLASTICS

Waste: PVC and Other Resins, European Commission, Delft, the Netherlands, 1999. DG III, Final Report, STB-99-55 Final. PISMC, Pohang Iron and Steel Making Company, Technology of blast furnace with waste plastics in Europe, POSCO Internal Report (1996) 1 9. PISMC, Pohang Iron and Steel Making Company, Technology of blast furnace with waste plastics in Europe, POSCO Internal Report (1996) 11 27. M. Asanuma, T. Ariyama, M. Sato, R. Murai, T. Sumigama, Combustion and gasification reaction behaviour of plastics in blast furnace, J. Iron Steel Inst. Japan 83 (1997) 545 550. Pollution Issue, 2007. Incineration. Available from: pollutionissues.com API, 2007. Alliance for the Polyurethanes Industry. Available from: http://www.polyurethane.org/recycling DEFRA, 2006. Municipal Waste Management Statistics 2004/05, March.

TO

ENERGY

[48] D. Boavida, P. Abelha, I. Gulyurtlu, I. Cabrita, Co-combustion of coal and nonrecyclable paper and plastic waste in a fluidised bed reactor, Fuel 82 (15 17) (2003) 1931 1938. [49] N. Horvat, F.T.T. Ng, Tertiary polymer recycling: study of polyethylene thermolysis as a first step to synthetic diesel fuel, Fuel 78 (4) (1999) 459 470. [50] S. Dong, T.W. Kurt, Y. Wu, Municipal solid waste management in China: using commercial management to solve a growing problem, Utilities Policy 10 (2001) 7 11. [51] Y. Zhang, N. Deng, J. Ling, C. Xu, A new pyrolysis technology and equipment for treatment of municipal household garbage and hospital waste, Renew. Energy 28 (2003) 2383 2393. [52] X. Li, C. Zhang, Y. Li, Q. Zhi, The status of municipal solid waste incineration (MSWI) in China and its clean development, Energy Procedia 104 (2016) 498 503.