chapter 2 literature review of waste to energy 2.1. …
TRANSCRIPT
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)