CHAPTER 2

LITERATURE REVIEW OF WASTE TO ENERGY

2.1. Waste to Energy (WtE)

Waste to energy (WtE) is the process of generating energy in the form of

electricity and heat from the primary treatment of waste, or the processing of waste into

a fuel source. WtE is a form of recovery. Most WtE processes generate electricity or

heat directly through combustion, or produce a combustible fuel commodity, such as

methane, methanol, ethanol, or synthetic fuels. Modern waste to energy plants are very

different from the trash incinerators that were commonly used until a few decades ago.

Unlike modern ones, those plants usually did not remove hazardous or recyclable

materials before burning. These incinerators endangered the health of the plant workers

and the nearby residents, and most of them did not generate electricity. Waste to energy

generation is being increasingly looked at as a potential energy diversification strategy,

especially by Sweden, which has been a leader in waste to energy production over the

past 20 years. The typical range of net electrical energy that can be produced is about

500 to 600 kWh of electricity per ton of waste incinerated. Thus, the incineration of

about 2,200 tons per day of waste will produce about 1,200 MWh of electrical energy.

2.2. Resources of Waste to Energy (WtE)

Waste to energy (WtE) technologies leads to substantial reduction in the overall

waste quantities requiring final disposal. Feedstock for waste to energy plants can be

obtained from a wide array of resources including municipal solid wastes (MSW),

agricultural residues, animal wastes, wood wastes and industrial wastes.

2.2.1. Municipal Solid Waste (MSW)

Municipal solid wastes (MSW) can be converted into energy by

thermochemical or biological technologies. The major energy resource in municipal

solid waste is made up of food residues, paper, fruits, vegetables, plastics etc. which

make up as much as 75-80 percent of the total MSW collected. At the landfill sites, the

gas produced by the natural decomposition of MSW can be collected, scrubbed and

cleaned before feeding into internal combustion engines or gas turbines to generate heat

and power.

2.2.2. Agricultural Residues

Crop residues include all agricultural wastes such as bagasse, straw, stem, stalk,

leaves, husk, shell, peel, pulp, stubble, etc. Current farming practice is usually to plough

these residues back into the soil, or they are burnt, left to decompose, or grazed by

cattle. Agricultural residues are characterized by seasonal availability and have

characteristics that differ from other solid fuels such as wood, charcoal, char briquette.

Crop wastes can be used to produce biofuels, biogas as well as heat and power through

a wide range of well-proven technologies.

2.2.3. Animal Wastes

The biogas potential of animal manure can be harnessed both at small and

community scale. In the past, this waste was recovered and sold as a fertilizer or simply

spread onto agricultural land, but the introduction of tighter environmental controls on

odour and water pollution means that some form of waste management is now required,

which provides further incentives for waste to energy conversion. The most attractive

method of converting these waste materials to useful form is anaerobic digestion (AD).

2.2.4. Wood Wastes

Wood processing industries primarily include sawmilling, plywood, wood

panel, furniture, building component, flooring, particle board, moulding, jointing and

craft industries. Wood wastes generally are concentrated ate the processing factories.

Wood wastes has high calorific value and can be efficiency converted into energy by

thermal technologies like combustion and gasification.

2.2.5. Industrial Wastes

The food processing industries produce a large number of organic residues and

by-products that can be used as biomass energy sources. These waste materials are

generated from all sector of the food industry with everything form meat production to

confectionery producing waste that can be utilized as and energy source. Since the early

1990s, the increased agricultural output stimulated an increase in fruit and vegetable

canning as well as juice, beverage, and oil processing. Wastewater from food

processing industries contains sugars, starches and other dissolved and solid organic

matter. A huge potential exists for these industrial wastes to be biochemical digested to

produce biogas, or fermented to produce ethanol, and several commercial examples of

waste to energy conversion already exit around the world.

2.3. Technologies of Waste to Energy (WtE)

In recent decades, various WtE technologies have been developed to converse

wastes into usable energy. The promising WtE conversion technologies are

thermochemical conversion methods (incineration, pyrolysis, and gasification) and

biochemical conversion methods (anaerobic digestion and ethanol fermentation).

Electricity, heat, fuel gases, liquids, and solids are the primary recovery products of

those technologies. In practice, combinations of two or more of these methods may be

used, but there are various difficulties with these current technologies.

Figure 2.1. Waste to Energy Technologies

2.4. Thermal Conversion

Thermal conversion is a process that turn biomass into energy with the

concurrent or subsequent release of heat energy. Incineration, pyrolysis and gasification

Waste

Thermochemical conversion

Incineration

Pyrolysis

Gasification

Biochemicalconversion

AnaerobicDigestion

EthanolFermentation

methods are included in this technology. These methods tend to produce fewer

emissions while converting trash into usable energy.

2.4.1. Incineration

Incineration is a waste treatment process that involves the combustion of

organic substances contained in waste materials. Incineration of waste materials

converts the waste into ash, flue gas and heat. Incinerators reduce the solid mass of the

original waste by 80% - 85% and the volume by 95% - 96%, depending on composition

and the degree of recovery of materials such as metals from the ash for recycling.

Incineration is a technique essentially applied by waste devastation in a furnace by

monitoring burning at high temperatures which takes place between 750 and 1,100˚C.

The heat produced by an incinerator can be used to generate steam which may then be

used to drive a turbine in order to produce electricity. The amount and thermal potential

of the collected material, the effectiveness of the processing system, and nature of

energy manufactured are major factors determine WtE recovery. Incineration has a

number of outputs such as the ash and the emission to the atmosphere of flue gas. Thus,

incineration can cause severe environmental pollution, but can also be an

environmentally friendly if it is combined with energy recovery, control of emissions

and an appropriate disposal method for the ultimate waste. In MSW incinerators, the

bottom ash constitutes approximately 25 to 30% by weight of the solid waste input.

Additional treatment can improve bottom ash characteristics and would allow its use in

concrete aggregates. Fly ash quantities are much lower, generally 1 to 5% by weight of

the input and is required in order to make it environmentally safe for landfill disposal.

Figure 2.2. Basic Linear Structure of Incineration Plant

The basic linear structure of an incineration plant is showed in Figure 2.2 and

may include the following operation: incoming waste reception, storage of waste and

raw materials, pretreatment of waste, loading of waste into the process, thermal

treatment of the waste, energy recovery and conversion, flue gas cleaning, residue

management or discharge, emissions monitoring and control, waste water control and

treatment, bottom ash treatment and solid waste discharge.

Incineration has many benefits especially in terms of destroying contaminant

medical wastes and other life-risking garbage. Also, incineration largely utilizes waste

to energy technology. For example, thermal treatment is very popular since they have

a shortage of land in Japan. In addition, the energy produced by incineration plants is

in high demand in nations such as Sweden and Denmark. However, incinerators also

have their downside. Even though incineration can decrease the quantity of waste and

generate heat and electricity, it is quite expensive to construct a reliable incineration

plant. Residues from the incinerator like fly ash and bottom ash can potentially harm

people and the environment if it isn’t disposed properly.

2.4.2. Pyrolysis

Pyrolysis is the thermal decomposition of materials at elevated temperatures in

the absence of oxygen to produce bio-oil, syngas and solid char. Pyrolysis process emits

mainly methane, hydrogen, carbon monoxide and carbon dioxide. It involves a change

of chemical composition. It is most commonly used in the treatment of organic

materials. the organic materials present in the biomass substrate starts to decompose

around 350 to 550˚C and it can proceed until 700 to 800˚C. In general, pyrolysis of

organic substances produces volatile products and leaves a solid residue enriched in

carbon, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called

carbonization. Pyrolysis is considered as the first step in the processes of gasification

and combustion. It differs from other processes like combustion and hydrolysis in that

it usually does not involve the addition of other reagents such as oxygen or water.

Overall, the pyrolysis process can be classified as slow and fast depending on

the heating rate. During the fast pyrolysis process, biomass residues are heated in

absence of oxygen at high temperature using higher heating rate. Based on the initial

weight of the biomass, fast pyrolysis can provide 60 to 75 % of liquid biofuels with 15

to 25% of biochar residues. The process is characterized by small vapour retention time.

However, quick chilling of vapour and aerosol can ensure higher bio-oil yield. It can

provide liquid biofuels for turbine, boiler, engine, power supplies for industrial

applications. Slow pyrolysis can yield good quality charcoal using low temperature and

low heating rates. The vapour residence time can be 5 to 30 min in this process. The

quality of bio-oil produce in this process is very low. The main advantage of pyrolysis

process is the reduction in volume of the waste. Pyrolysis is also instrumental in the

discovery of many important chemical substances, such as phosphorus and oxygen.

Figure 2.3. Process Flow Diagram of Pyrolysis

2.4.3. Gasification

Gasification is a thermo-chemical transformation process that coverts organic-

or fossil fuel-based carbonaceous materials into carbon monoxide, hydrogen and

carbon dioxide. The process of producing energy using the gasification method has

been in use for more than 180 years. This is achieved by reacting the materials at high

temperatures (700˚C), without combustion, with a controlled amount of oxygen or

steam. The resulting gas mixture is called syngas or producer gas and is itself is a fuel.

The power derived from gasification and combustion of the resultant gas is considered

to be a source of renewable energy if the gasified compounds were obtained from

biomass. Syngas may be burned directly in gas engines, used to produce methanol and

hydrogen. For some materials gasification can be an alternative to landfilling and

incineration.

Gasification can generate lower amounts of some pollutants as sulfur oxides

(SOx) and nitrogen oxides (NOx) than combustion. In principle, gasification can

proceed from just about any organic material, including biomass and plastic waste.

Comparable to incineration, gasification similarly generates bottom ash that needs to

be detached and accurately preserved. In many gasification processes most of the

inorganic components of the input materials, such as metals and minerals, are retained

in the ash. In some gasification processes, this ash has the form of a glossy solid with

low leaching properties, but the net power production in slagging gasification is low

and costs are higher. The syngas produced in gasification method contain mainly of

hydrogen and carbon monoxide which combusted in an isolated container to harvest

electricity and chemicals as indicated. Regardless of the final fuel form, gasification

itself and subsequent processing neither directly emits nor traps greenhouse gases such

as carbon dioxide.

The gasification process varies from other practice by various features such as

reactor atmosphere, reactor design, internal and external heating and operating

temperature. While other biofuel technologies such as biogas and biodiesel are carbon

neutral, gasification in principle may run on a wider variety of input materials and can

be used to produce a wider variety of output fuels. Several waste gasification processes

have been proposed, but few have yet been built and tested, and only a handful have

been implemented as plants processing real waste, and most of the time in combination

with fossil fuels.

Figure 2.4. Process Flow Diagram of Gasification

2.5. Biochemical Conversion

Biochemical conversion process makes use of the enzymes of bacteria and other

microorganisms to break down biomass. In most of the case, microorganisms are used

to perform the conversion process: anaerobic digestion, fermentation, and composting.

Biochemical conversion is one among the few which provide environment-friendly

direction for obtaining energy fuel from MSW.

2.5.1. Anaerobic Digestion (AD)

Anaerobic digestion is a sequence of process by which microorganisms break

down biodegradable material in the absence of oxygen and results in the production of

biogas and digestate. The process is used for industrial or domestics purposes to manage

waste or to produce fuels. Anaerobic digestion is used as part of the process to treat

biodegradable waste and sewage sludge. As part of an integrated waste management

system, anaerobic digestion reduces the emissions of landfills gas into atmosphere.

Anaerobic digestion is widely used as a source of renewable energy. The process

produces a biogas, consisting of methane, carbon dioxide, and traces of other

contaminant gases.

The four key stages of anaerobic digestion involve hydrolysis, acidogenesis,

acetogenesis and methanogenesis. The overall process can be described by the chemical

reaction, where organic material such as glucose is biochemically digested into carbon

dioxide (CO2) and methane (CH4) by the anaerobic microorganisms.

In hydrolysis stage, the complex organic materials are broken down into low-

molecular-weight compounds such as amino acids, fatty acids, and simple sugars. The

biological process of acidogenesis results in further breakdown of the remaining

components by acidogenic bacteria. here, volatile fatty acids (VFAs) are created, along

with ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts. In

acetogenesis stage, acetic acid, carbon dioxide, and hydrogen are formed from the

VFAs by acid-forming bacteria or acetogens. The terminal stage of anaerobic digestion

is the biological process of methanogenesis. Here, methanogens use the intermediate

products of the preceding stages and convert them into methane, carbon dioxide, and

water.

Anaerobic digestion is particularly suited to organic material, and is commonly

used for industrial effluent, wastewater and sewage sludge treatment. Anaerobic

digestion, a simple process, can greatly reduce the amount of organic matter which

might otherwise be destined to be dumped at sea, dumped in landfills, or burnt in

incinerators.

C6H12O6 → 3CO2 + 3CH4

2.5.2. Ethanol Fermentation

Ethanol fermentation, also called alcoholic fermentation, is a biological process

which converts sugars such as glucose, fructose and sucrose into cellular energy,

producing ethanol and carbon dioxide as byproducts. Alcoholic fermentation is

considered as an anerobic process because yeasts perform this conversion in the

absence of oxygen. Ethanol fermentation has many uses, including the production of

alcoholic beverages, the production of ethanol fuel, and bread cooking. Ethanol

fermentation produces unharvested byproducts such as heat, carbon dioxide, food for

livestock, water, methanol, fuels, fertilizer and alcohols.

2.6. Waste Collection and Disposal Management of Yangon City

Waste is collected and transported by Pollution Control Cleansing Department

(PCCD) personnel and the process consists of the first collection (from the source of

waste to the collection site/iron container) and the second collection (from the

mentioned relaying facilities to the disposal site) (Figure 2.5)

The first collection is performed by either so-called bell-collection, designed

site collection or on-street collection. The bell-collection is performed in highly

populated residential areas and the PCCD personnel directly visit households and

offices by ringing the bell and collect waste by cart. The waste collected by bell-

collection is temporary deposited at the temporary waste tanks made of concrete or

bamboo according to the population density iron containers. Waste discarded in the

street container is transferred to the temporary waste tanks or iron containers by carts.

The second collection is performed by transferring the waste temporarily stored

at the waste tanks to the disposal site by PCCD personnel by trucks with manual

transshipment. Waste collected by the iron containers does not require transshipment

and delivered as is to the disposal site.

Figure 2.5. Waste Collection Flow

2.6.1. Treatment and Disposal of Waste

Waste produced in the city of Yangon is currently disposed as landfill directly

at the final disposal site managed by PCCD. Table 2.1 shows the Final Disposal Site

(FDS) and Temporal FDS operated as of 2018.

Waste produced in the city is mainly treated by Htein Bin Disposal Site to which

waste from North and West districts are mainly delivered and Htawe Chaung Disposal

Site to which waste from South and East districts are mainly delivered. Waste is

received 24 hours a day at each site. There are two such Temporal FDS in the city and

the land area of each site is around 0.1-1 ha. All disposal sites in the city of Yangon are

open-dump type which does not have seepage control work or leachate treatment

equipment and there is no coverage soil for the dumped waste.

Table 2.1. Final disposal site in Yangon City

Facility

Name

Type Amount of waste

accepted

(tons/day)

Area

(Acre)

In

service

Area

Year stared

in service

Htain Bin Disposal

site

1287.5 150 70 2002

Htawe

Chaung

Disposal

site

1070.50 147 47.4 2001

Dala Temporary 21.76 1.3 N/A 2003

Seikkyi

Khanung

Temporary 7.11 0.25 N/A 2003

Total 2387.12

2.7. Yangon Waste to Energy Plant (YWTEP)

Yangon, the former capital of Myanmar, is driving the country’s rapid

democratization as economic center of the country. The city has a population of around

5.21 million and the amount of daily waste has reached around 3,000 tons in 2017 which

shows significant increase from the daily amount of 1,550 tons in 2011.

Therefore, appropriate waste treatment was become an urgent necessity. The

waste in the city of Yangon is currently managed by the Pollution Control and

Cleansing Department (PCCD) of Yangon City Development Committee (YCDC)

from the administration perspective. The plant was designed and built using state-of-

the-art technology developed by JFE Engineering Corporation (JFE) which is capable

of achieving the high-power generation and waste volume reduction efficiencies that

can effectively aid in the improvement of the electricity shortage situation and the

achievement of appropriate solid waste management in Yangon City. The cost for

building the plant, US$16 million, was equally shared by both governments that is

Myanmar and Japan while the technology was provided by Japan. YCDC as Project

Owner, and JFE as EPC contractor jointly held an opening ceremony for the “Yangon

Waste to Energy Plant” in Shwe Pyi Thar Township, Yangon City on April 7th, 2017.

Figure 2.5. Yangon Waste to Energy Plant

Yangon Waste to Energy Plant (YWTEP) use the incineration method to destroy

wastes up to 60 tons/day and can generate 20MW. JFE Hyper Stoker System is used to

incinerate the waste collected at the waste pit. This plant features an environmentally

friendly form of energy generation. This plant is operated 24 hours a day and 310 days

a year. In the plant, the waste treatment fee (tipping fee) and sale of electricity power is

the main source of income. Sale of electric power is indispensable factor for business

feasibility. Table 2.2. shows the electricity balance of YWTEP.

Table 2.2. Balance of electricity

Item Specs Note

Amount of general waste

treated

1,000

tons/day

500 tons × 2 units

310,000

tons/day

Annual aggregated waste amount

treated

Days of operation per year 310 days 20 MWh × 1 unit

*At rated operation for standard waste

Output of power generator 20 MWh *At rated operation for standard waste

Electricity used in the plant 4 MWh *At rated operation for standard waste

Electricity sold 16 MW/day *At rated operation for standard waste

119,040

MWh/year

*At rated operation for standard waste

This plant produces bottom ash and fly ash daily and handles it in a safe way.

Bottom ash or incinerated ash from complete combustion falls into the ash cooling

system via the bottom ash chute where fire is extinguished, and moisture is added before

the ash is temporality stored in the container. The ash is then brought to the final

disposal site by trucks regularly. In this program, the half-moist ash cooling system is

used. Fly ash collected by the filtering-type dust collector is delivered to the container

by the fly ash conveyor and store temporary there, and then discharged to outside the

site regularly (by external detoxifying treatment or delivery to the final disposal site for

hazardous waste). The specifications of major equipment constituting the plant are

presented in Table 2.3.

Table 2.3. Major equipment specifications

Equipment Item Unit Specification

Receiving Waste pit 7 days

Waste crane Unit 2

Furnace Model JFE hyper stoker

furnace

Capacity Ton/day/unit 500

Number Unit 2

Flue gas

cooling

Model Heat recovery

boiler

Steam pressure for constant use

(Heater outlet)

MPa(G) 4.8

Steam temperature for constant

use

deg.C 420

Flue gas

treatment

Acidic gas removal - Dry treatment

(Injection of

powder slaked

lime)

Dioxins removal - Dry treatment

(Injection of

powder activated

carbon)

Dust removal - Filtering-type dust

collector (bug

filter)

Nitrogen oxide removal - Combustion

management

Heat

recovery

Model Extraction-

condensing

turbine

Number unit 1

Steam pressure for constant use

(turbine inlet)

MPa(G) 4.6

Steam temperature for constant

use (turbine inlet)

deg.C 415

Emission pressure kPa (A) 25

Generator output (for standard

waste)

MW 20.0

Ash

removal

Bottom ash (incineration ash) Semi-dry type ash

cooling system

One of the purposes of this plant is to reduce greenhouse gas (GHG) emissions

through the incineration process instead of disposing at landfills. This plant also

includes advanced air and water pollution control systems in accordance with the law

of Myanmar and flue gas emission level in full compliance with the law and WHO

standard. Reference emissions and project emissions of GHG emission reductions are

presented in Table 2.4.

Table 2.4. GHG emission reductions

4,663 tCO2e; GHG emission reductions at 4th year after project start (2020)

GHG emission reductions 4,663 tCO2e

Reference emissions 12,073 tCO2e

(CH4 emissions from landfill site) (7,496 tCO2e)

(CO2 emissions from electricity) (4,576 tCO2e)

Project emissions 7,409 tCO2e

(CO2 emissions from waste incineration) (4,913 tCO2e)

(NO2 emissions from waste incineration) (369 tCO2e)

(CO2 emissions from electricity) (2,102 tCO2e)

(CO2 emissions from fossils fuel consumption) (26 tCO2e)