msw cogen term paper report

7/29/2019 MSW Cogen Term Paper Report

1/27

Bio-energy, MEng 6222

Term Paper on CHP Plant that utilizes biodegradable MSW of Mekelle City Page 1

MEKELLEUNIVERSTY

ETIOPIANINSTITUTEOFTECHNOLOGYMEKELLE

DEPARTMENT OF MECHANICAL ENGINEERING

M.ScPrograminEnergyTechnologyBio-Energy, MEng6222

Term Paper Assignment on:

A Cogeneration Plant that utilizes biodegradable waste of Mekelle City

Submitted to: Ftwi Yohaness (Ass. Prof.)

Mr. Mussie Tesfay (Lecturer)

Submitted by: Akatew Haile

Habtewold Ababu

Tariku Firdisa


2/27



Table of Contents Page

Abstract

1. Waste -to-energy Technologies ....1

1.1. Incineration ...1

1.2. Land Fill Gas ...31.3. Anaerobic Digestion.....4

1.4. Gasification..4

2. Case Study: Mekelle Municipal Solid Waste (MSW) ....5

2.1 Solid Waste Generation and characteristics..5

2.2Energy Content of MSW .7

2.3 MSW Collection ..8

2.4 Handling and Disposal of MSW ....8

3. Land Fill Gas utilization ..9

3.1 Why Land Fill Gas for Mekelle MSW? 9

3.2 LFG Generation..103.3 LFG Collection, Pumping and transmission ....13

3.4 LFG Conversion Technologies ..15

3.5 Cogeneration Plant (CHP) utilizing LFG ..18

3.6 Energy Production 19

4. Benefit of the Project .23

5. Conclusion and recommendation .....24

6. References ...25


3/27



Abstract

Land fill gas utilization is one method that is implemented currently in conversion of wastetoenergy

This term paper discusses with Land Fill Gas (LFG) utilization of Mekelle municipal solid waste fo

combined and heat power generation. It also addresses the waste generation rate and types, waste

collection, MSW handling and processing, disposal and energy value (if any) of the municipal solid

waste. Sanitary land Fill of the city which is functional since 2008 (2000 E.C) and planned to serve for

thirty years is regarded to serve as both MSW disposal place and source of LFG. LFG utilization in

fueling the CHP plant that is going to be designed is selected in connection with Sanitary Land Fil

project and the other benefit it gives when compared with other Waste to energy technologies. Land

Fill gas (LFG) generation and conversion technologies are discussed. With the LFG as a fuel

combined heat and power plant is going to be designed. The power produced and the achievable

efficiency is going to be determined. Benefit of the project is explained based on its performance.


4/27



1. Waste-to-energy Technologies

Technology

Waste-to-energy technology involves converting various elements of municipal solid waste such as

paper, plastics, and woods to generate energy by either thermo-chemical or biochemical processes

The thermo-chemical techniques consist of combustion, gasification, and pyrolysis that produce high

heat in fast reaction times. The biochemical processes consist of anaerobic digestion, hydrolysis, and

fermentation using enzymes that produce low heat in slow reaction times. Figure 1 illustrates the

potential output energy technologies and the products that result from these processes. [1]

After determining the composition of the waste, the appropriate waste-to-energy

System can be selected based on the available resources. The most common waste-to-energy

technologies are briefly discussed below.

1.1 Incineration

Incineration, also referred to as mass combustion, is a specialized process that involves the burning

of organic materials in any state to form gaseous and residue. The basic elements of an incinerator

include a feed system, combustion chamber, exhaust gas system and a residue disposal system;

whereas modern incinerators use continuous feed systems and moving grates within a primary

combustion chamber lined with heat resistant materials. The waste must be mixed, dried, and then

heated, all for specific amounts of time and at controlled temperatures.

The advantage of incineration is to combust solid waste, reducing its volume and producing non-

offensive gases and non-combustive ash residues. Volume can be reduced by 80-95% and weight by

70-80% and thus incineration significantly reduces the land required for disposal of municipal wastes.

But incineration has high capital and operating costs. A major consideration is operating problems

which can occur as a result of variability of the waste over time. [2]


5/27



Figure 1: Waste-to-energy technologies and their respective out puts

1.2 Land Fill Gas (LFG) from Sanitary Land fill

Generation of methane from a sanitary landfill is similar to anaerobic digestion, but without

operational control of the process. The waste is simply left as is with no efforts made to increase gas

production; gas is simply captured as it is generated. Typical landfill gas has an energy equivalent to

about half that of natural gas. The methane concentration of the gas is 40 60%. The decomposition

process within a landfill consists of an aerobic stage, anaerobic non-methanogenic stage, anaerobic

methane production build-up stage and finally an anaerobic steady state stage.


6/27



When methane collected off of the landfill is used for energy, the amount released to the atmosphere

is reduced. Landfills are the largest anthropogenic source of methane, accounting for 40% of these

emissions. Gas collection also reduces odors, vegetation damage, and fires, and can be a source of

revenue. [2]

Landfill gas utilization typically requires less maintenance and operation costs compared to anaerobic

digestion. Gas extraction is environmentally beneficial, and considerable economic potential exists for

methane recovery. Landfill gas utilization can be quite simple and economical if a sufficient land and

factory or large building is located near the landfill where the gas can be piped directly into a boiler.

1.3 Anaerobic Digestion

Anaerobic digestion is the decay of organic matter without oxygen producing primarily carbon dioxide

and methane, but also small amounts of hydrogen sulphide, ammonia, and other compounds. The

putrescible and combustible (paper) fraction of the waste is removed and placed in a contained

digester to decay. Three main steps are involved in anaerobic digestion. The first involves the

preparation of the organic fraction of the waste including sorting, separating and size reduction. The

second step involves adding moisture and nutrients, blending, adjusting the pH to about 6.7 and

heating the slurry to about 55-60C. The contents are well mixed for 5-10 days. The third step

involves capture, separation and storage of the gas components.

The purpose of anaerobic digesters is to utilize the gas produced by decomposing waste as a source

of fuel. Waste can be aerobically composted after anaerobic digestion to obtain the benefits of both

biogas as well as compost for soil improvement.

Anaerobic digestion will be more feasible if it is combined with sewage or agricultural waste digestion

Anaerobic digestion is commonly used for treatment of sewage and manure because this material is

uniform and easily degradable. The addition of such materials to MSW would enhance the digestion

process [2].

1.4 Gasification

Gasification is the reaction of organic matter with steam, producing carbon monoxide and hydrogen

Gasification is a modification of pyrolysis in that a limited quantity of oxygen is introduced, and the

resulting oxidation produces enough heat to make the process self-sustaining. Gasification occurs at


7/27



very high temperatures (greater than 700C) and involves the partial combustion of a carbonaceous

fuel, which produces combustible fuel gas rich in carbon monoxide, hydrogen and some saturated

hydrocarbons (mostly methane). The combustible fuel the process produces can be combusted in an

internal combustion engine.

The products of gasification are very useful for making products including methanol, ammonia, and

diesel. The process is quite energy efficient 60% to 90%. Waste volume is reduced by about 90% and

only 8-12% ash is produced compared to 15-20% for incineration. Application of gasification to

municipal waste is still a relatively new development [2].

2. Case Study: Mekelle Municipal Solid Waste (MSW)

The Mekelle city administration municipality bureau has compiled a study report entitled Fina

Feasibility and Preliminary Design report for Mekelle City Integrated Solid Waste Management for

the proper management of the municipality solid wastes by a consulting organization called Promise

Consulting Architects and Engineers.

The objective of the report is to describe the overall assessment, findings and recommendations

made in the study and design of solid waste management system of Mekelle by the consulting

company. The report assessed the existing sold waste management and tried to give responses

accordingly to alleviate threats that are facing the environment and inhabitants of Mekelle city.

This report was prepared based on the initiative of the Mekelle City Administration for improved SWM

system. The system addresses each components of ISWM, i.e., reduction, storage, transportation

recovery and disposal in a cost effective, environmental friendly and sustainable manner. The service

is expected to develop a solid waste master plan that gives a solution to problems related to Solid

Waste services. Previously, waste disposal of the city was in an uncontrolled and non-engineered

way of open dumping and burning at a distance of 5Km from the centre to the old airport road.

2.1 Solid Waste Generation and characteristics

Prior to the report, it was impossible to get a comprehensive study made concerning Mekelle city that

enables to know the rates of generation and characteristics of solid waste from different urban

activities and sources. Even the sources and solid waste category are not clearly identified in the

existing municipal solid waste (MSW) management service.


8/27



The consultant has underlined that basic technical information do not exist in relation to existing SW

rate of generation, composition and source, which is required to undertake comprehensive study on

Integrated Solid Waste Management (ISWM) and good planning and design of solid waste

management infrastructure for improved SW service.

Table 1:City-wise Types of composition of Solid Wastes from Municipal Solid Waste (MSW) (Source

Mekelle Municipal)

Type Solid Waste from MSW

Yearly volume (m )

2005 2010 2015

Organic recyclable(including paper + others) 46347.37 75201.63 90761.14

Organic recyclable(excluding paper) 37368.43 61155.88 73788.50

Organic recyclable(excluding paper +others) 26126.11 42721.20 51547.28

Plastic(all) 10002.49 16570.63 19985.63

Special wastes(excluding Yard wastes) 288.84 458.16 553.38

Hazardous waste rejects 2484.15 4022.18 4838.16

Hazardous plus special rejects(excluding yard waste) 3017.56 4836.26 5814.40

Non-hazardous rejects 10299.71 15770.73 18869.29

Recyclables other than organics & plastic 18335.05 30674.03 36983.93

TOTAL 154269.71 251410.70 303141.71

Note: Rejects are to be transported to Land fill sites;

The consultant had tried to classify the waste categories projecting their yearly capacity (volume)

from the survey made. But the report does not include technical information concerning the MSW. For

instance, it is impossible to get specific types of waste and their energy density. Nevertheless,

according to oral information obtained from the Municipal, the MSW generation rate of Mekelle city is


9/27



estimated to be 0.268kg/capita/day [8]. Currently (2011) the population of the city, as projected by the

consultant, is 302,538. [3]

Based on this population size the daily bulk of MSW will be 81.08 tons and on annual basis it

becomes 29594.267 tons/year. Table 1 above summarizes the general MSW types with their

projected volume.

2.2 Energy content of MSW

Even though the report does not include the energy content, from literatures, the energy content of

the most common MSW is as shown in table below. The energy content of the waste constituents can

vary from one Land fill to another.

Table 2. Average composition and heating value of common MSW.

(Source: Nicholas P. Cheremisinoff,Ph.D, Hand Book of Solid Waste Management and Waste

Minimization Technologies)


10/27



2.3 Collection of the MSW

The collection of MSWs is done by two main responsible groups. They are the private service groups

(companies) and the city administration municipality administration bureau. Private services

(companies) collect the MSW using primary collection methods while the municipal makes collection

and disposal of the solid wastes to the landfill site. With the existing facilities the present collection

efficiency of the City Administration is estimated to be 52% of the solid waste.

The collection of MSW is carried out in to two category stages: Primary collection and Secondary

collection.

Primary collection: The purpose of Primary collection operation is to transfer and store solid waste

materials from the generating bodies to the communal storage and then to the secondary collection

facilities. The following observations have been drawn on the activities of each mode of primary

collection:

Municipal Tractor-trailer

Door to door collection by Privately Service providing companies

Street sweepers with Hand Carts &Wheel Barrows

Secondary collection and Transportation: The secondary collection is used to collect the bulk

quantity of the municipal solid waste from the primary communal storage to the landfill using skip

loader. Presently, the city municipality is the only institution that provides communal storages and

performs secondary collection of garbage to disposal sites.

2.4 Handling and Disposal Of MSW

At the sanitary land Fill site material recovery (recycling) from the waste will be considered. Also

separation of biodegradable waste from hazardous and special waste types will be done. Then the

biodegradable ones will be processed, if any. At last, the biodegradable ones and the hazardous and

special ones are dumped within their own Landfills areas.

As the landfill of the city has no permanent equipments, the MSWs covering and compaction

processes are not regularly executed in the sites. The preliminary design work of promise consult has

considered an average waste-to-cover ratio of 7.5:1.


11/27



3. Land Fill Gas utilization

3.1 Why LFG for Mekelle MSW?

The MSW of Mekelle city has been dumped in two open dumps, around Messobo and Qiuha, in an

uncontrolled manner. But for the farmers and the city community opposed this way of waste disposal,

the Municipal has planned to construct a new Sanitary Land Fill that will serve for thirty years of time.

According to information from the Municipal, this land fill located around Adi-kolomay, has an area of

21 hectare and has begun service three years ago.

The Land Fill studied and designed by Promise Consulting and Architects will have two parts, one for

biodegradable wastes and the other for hazardous wastes. The biodegradable waste (e.g. Papers

and food waste) will decompose over the lifecycle of the landfill as a result of microbial action. This

decomposition yields leachate and gas, consisting primarily methane and carbon dioxide. This land

fill, instead of being source of anthropogenic emitting gases it can be a source of LFG which can be

used for heating and renewable power generation. Energy generation, using LFG, requires less

operating and maintenance costs. Land fill gas is generated as the waste is decomposed and

continues after the closure of the land fill. So LFG has proven to be a reliable fuel source. Utilization

of the LFG avoids problems like odor, possible risk of explosion and fires, pollution and creates

revenue as LFG utilization falls under renewable energy schemes.

In fact Anaerobic Digestion is also another option in converting the MSW to energy. But when

compared to LFG utilization it requires higher operation and maintenance costs. Again for Ad to be

feasible the waste has to mix with sewage or agricultural waste to accelerate digestion. For no

information is available regarding these two wastes, LFG utilization has been chosen.

Although the Municipal has not planned any energy recovery system at the land Fill, this technology

seems the right option in connection with the proposed disposal, Sanitary Land Fill. In this project

(term paper), it is assumed that there is thermal energy demanding processes nearby the land fill.

3.2 LFG Generation

Generation of LFG is a complicated biological process, with essential microbial activity. LFG is

generated as a result of the biodegradation of organic carbon in waste. Approximately 1.87 m3 of LFG


12/27



is produced per kg of degraded organic carbon (with a content of 50 percent CH4). Organic material

in the waste is decomposed in four main phases figure below [4].

Figure 2. LFG Generation after Waste Disposal

The LFG is generated by anaerobic (without oxygen) decomposition of the degradable organic waste

The four main phases are listed below.

Phase I Aerobic: A few days to a few weeks

Phase II Anaerobic, non-methanogenic: One month to 1 year

Phase III - Anaerobic, methanogenic, unsteady: A few months to 2 years

Phase IV - Anaerobic, methanogenic, steady: 10 to 50 years

After the anaerobic phase, the waste will finally stabilize after 30 to 50 years.

The composition of the main components in LFG is shown in Table below.


13/27



Table 3. Composition of the Main Gases and Trace Components in LFG [4]

LFG production varies considerably from one land fill to another, depending on the situation in the

individual country and landfill. The production rate depends on [4]:

Temperature in the landfill: increased temperature accelerates microbiological activity up to that

optimum temperature level.

Moisture content of the waste:Moisture can accelerate bacterial activity or smother it completely if the

waste is completely saturated.

Waste composition:Middle- The composition of the waste affects the decomposition rate: the faster

the organic material decomposes, the higher the rate of LFG production

Waste age:LFG production reaches its maximum capacity after 38 years and normally decreases

after 1530 years, when it is no longer profitable to extract the gas for energy purposes.

Waste structure:Because degrading microorganisms are active in the water film around the waste

particles, smaller particles of organic materials produce more LFG.

Landfill cover:Landfills must be covered to keep out atmospheric air, which will disturb the anaerobic

conditions. The cover material should allow penetration of rainwater to maintain adequate humidity in

the waste.


14/27



Gas Generation Models: There are several models to estimate LFG production and extraction.

These models include the simple zero order model, the first order model, and the most recent, the

multi-phase model.

Based on the available data, we have chosen the first order model in which, LFG generation in a

given amount of waste is assumed to decay exponentially over time using the following equation:

QCH4i = k * Lo * mi * e-kt

Where, QCH4i = annual methane (CH4) generation in the year i of the calculation (m3/year), and k =

methane generation constant (the k value is related the half-life of waste degradation t2 according to

the formula t2 = l(n)/k), Lo = methane generation potential/kg, and mi = waste mass disposed of in

year i.

A first order type model called LANDGEM (PLEXSCAPE 2011) which is an on line model is used in

our case [10]. The following data were entered for this software.

- k = 0.05 (medium degradable)

- population growth rate = 4.4%

- start year = 2000 (E.C), end year = 2030 (E.C)

- waste generation per capita = 0.286 kg/cap. day

- Lo = 160 m3 CH4/tonne

- NMOC = 595 ppmv as hexane

- Methane percentage = 50%

The model resulted in the total LFG, average and maximum methane generated per year in both

charts (below) and tabular form.


15/27



Figure 3: Generated LFG as a function of time

Upgrading of LFG which includes tasks as removing moisture content, carbon dioxide removal

removal of halogenated compounds and hydrogen sulphides requires to be done for LFG to be

utilized in power generation. This task upgrades LFG to high grade fuel improving its quality. With this

assumption the model yielded the following:

Sum: 93,495,928.88 m3 Avg: 3,116,530.96 m3/y Max: 6,900,215.40 m3/y methane

For design purposes considering the maximum value, methane of 18,904.7 m3/day will be obtained.

This figure indicates the amount of methane that is generated from the total Land fill. For all of this

amount cannot be collected, let us assume a collection efficiency of 75% so that the collected amount

will be 14178.5 m3/day or 9296.84 kg/day. This means 387.4 kg/hr and 3393.3475 tons/year o

methane would be collected.

3.3 LFG Collection, Pumping and Transmission

The extraction system in an LFG recovery plan can consist of vertical perforated pipes, horizonta

perforated pipes, ditches, or, in some cases, a membrane covering the landfill under which the

produced gas is collected. The most common method of active gas collection is to extract gas

through vertical perforated pipes, possibly because this is the simplest method where a landfill is

already established. The well is typically drilled with an auger with a diameter of 50100 centimeters


16/27



After drilling, a perforated polyethylene pipe with a diameter of 1015 cm is placed in the middle of

the hole, and gravel is filled in around the pipe. Vertical extraction wells are typically placed 4080

meters apart, depending on the landfill depth [4].

Figure 4: Typical LFG Extraction Well [9]

LFG is extracted by a gas pump or compressor, which provides sufficient vacuum to pull gas from the

landfill. A normal vacuum measures 20100 millibars at the wellhead. The decision whether to use a

pump or a compressor depends on site-specific requirements, particularly the pressure required for

gas transport and the inlet pressure for the gas combustion device [4].

The most widely used gas pump is a radial blower, which is relatively simple and economical. Hence,

in this case vertical pipes for better quality methane and radial blower pump are assumed.

The individual wells can be connected to the pump and utilization system in several ways. The most

common design is to connect the wells to a main collection pipe, which is placed in the optimal way in

the landfill. Figure below illustrates this method.


17/27



Figure 5: Extraction System with Each Well Connected to a Main Collection Pipe [5]

3.4 LFG Conversion Technologies

LFG is a wet gas with different concentrations of a number of trace gases and high moisture content,

which may cause problems like corrosion of the equipment. Depending upon the application, the raw

LFG may require some level of gas processing prior to being utilized in order to reduce these

problems. LFG can be classified into three categories, based on the level of pretreatment/processing

prior to utilization. These are: Low-grade LFG fuel, medium-grade fuel and high-grade fuel [5].

Low- and medium-grade fuel produced from LFG has a heating value of approximately 16.8 MJ/m3 is

roughly one-half the heating value of natural gas. LFG that has been further processed and treated to

produce high-grade fuel has a higher heating value (37.3 MJ/m3) than low and medium grade fuel,

and can be substituted directly for natural gas in pipeline applications [5].

Medium-grade fuel has a broader range of fuel applications than low-grade fuel because of the

reduction in corrosive constituents. Although high grade fuel requires higher processing cost than the

medium-grade, it yields in a higher calorific value almost two times to the medium one. So we have

assumed the LFG will be processed to higher-grade fuel within this project. The cost of processing

can be offset by the reduced maintenance costs resulted from removal of moisture and other gases.


18/27



The two most common technologies for utilizing LFG for power generation purposes are

Reciprocating gas Engines and Gas Turbines.

Reciprocating Gas Engines

Reciprocating engines that use medium grade LFG as a fuel are available in various sizes with

electrical outputs ranging from less than 0.5 MW to more than 3.0 MW per unit. They have a

comparatively low capital cost per kW and a higher efficiency than most gas turbines. The

disadvantages of this technology include higher maintenance costs than for gas turbines and a

requirement for skilled maintenance personnel. Exhaust gases may contain some products of

incomplete combustion and there is a high lubricating-oil consumption, which includes need for

provision of disposal of the waste oil.

Gas Turbines

Gas turbines are available as modular and packaged systems. Gas turbines may have some

application for sites with higher, more stable LFG production rates. Gas turbines are generally larger

than reciprocating engines with electrical outputs ranging from 1 MW to 8 MW for each unit. Gas

turbines also offer the flexibility of modular expansion to suit changes in LFG production however; the

incremental stages are larger than for reciprocating engines. Gas turbines usually have a higher

capital cost associated with initial set up with somewhat lower energy conversion efficienciescompared to reciprocating engines. However, they generally offer superior exhaust emission

characteristics, reduced operating and maintenance costs and greater operational flexibility (in the

ability to maintain reasonable efficiency despite fluctuations in LFG flow and characteristics) than

reciprocating engines. In addition, gas turbines also offer the flexibility to proceed directly to the

combined cycle. These positive features of gas turbines have been found to offset the lower capital

cost and higher energy conversion efficiency of reciprocating engines [5].

The main characteristics of gas turbine are; Best suited for base-load applications; can also handle peaking and load following applications

as well

Combustion turbines are much more compact and lighter than similar capacity reciprocating

engines

NOx emissions from combustion turbines are lower than those from IC engines


19/27



The hot products of combustion expand through specially designed blades mounted on a

shaft, producing a high-speed rotary motion that is generally used for driving an electric

generator that produces electric power

Exhaust gases leaving a turbine are at a high temperature (900oF to 1,100oF). This high quality

heat is excellent for producing high-grade steam

3.5 Cogeneration Plant utilizing LFG

The most common energy application for LFG is on-site generation of electricity using raw or partially

processed LFG as a fuel. Typically, the LFG is used in a reciprocating internal combustion gas engine

or gas turbine driving an electrical power generator. Micro turbines similarly use the LFG as a boiler

fuel for a steam turbine generating facility as well.

Several factors must be evaluated when considering generating electricity with LFG, whether the

technology involves microturbines, reciprocating engines, gas turbines, combined cycle, or steam

turbines. Electrical conversion efficiency, which is an indication of what portion of the energy value of

the LFG can be converted into electrical power, varies with each technology. Other important factors

that must be considered when deciding on whether or not to utilize the LFG for electrical generation

include availability, installation cost, operation and maintenance costs, and emissions, all of which are

site specific.

Table below presents the typical flow ranges required to make the implementation of the following

electrical power generation technologies viable. It also shows the typical power ranges associated

with the various LFG technologies and flow rates [4]. (1 cfm = 9.75 tons LFG/year)


20/27



Table 4: flow range, plant size and electrical efficiency of LFG technologies

Technology Typical flow range Preferred plant Size

Electrical Conversion

Efficiency

Microturbines

4,000 to 20,000 cfm 3 to 18 MW 26-32%

Steam turbines >6,000 to >25,000 cfm 10 to 50 MW 24-29%

Combined Cycle

Systems

5,000 to >25,000 cfm>10 MW 38-45%

Gas Turbine Cogeneration System

Gas turbine systems operate on the thermodynamic cycle known as the Brayton cycle. In a Brayton

cycle, atmospheric air is compressed, heated, and then expanded, with the excess of power

produced by the turbine or expander over that consumed by the compressor used for power

generation.

Gas turbine cogeneration systems can produce all or a part of the energy requirement of the site, and

the energy released at high temperature in the exhaust stack can be recovered for various heating

and cooling applications. Because of the above positive features of gas turbine, we have selected a

Centaur 40 gas Turbine which is a product of Solar Turbines Company with 3.515 MW e capacity and

applicable for CHP plant [11].

3.6 Energy Production

In gas Turbines the compressed air from compressor reacts with the fuel in combustion chamber. The

turbines rotational motion is as a result of some part of thermal energy of the hot flue gases. Here the

fuel is methane produced from processed LFG to a high-grade fuel. According to the following

equation, two moles of air is required for complete (stoichiometric) combustion of methane.

CH4 + 2(O2 + 3.76N2) CO2 + 2H2O + 7.52N2

Methane flow rate is 0.1076 kg/s, using the molar mass of air (28.97 kg/kmol), the air required to burn

the fuel of 0.1076 kg would be 0.3896 kg. But in gas turbines excess air up to 300% can be applied.


21/27



Hence, if we consider an excess air of 200%, the balanced combustion equation of the fuel with the

air would become;

CH4 + 3O2 + 11.28N2 CO2 + 2H2O + 11.28N2 + O2

From this equation the air-fuel ratio (A/F) becomes;

A/F = 3 * 28.97 kg of air/16 kg of CH4 = 5.432

So, the mass of air required for 200% excess air combustion of fuel will be;

ma = 5.432 * 0.1076 = 0.5845 kg

Mass of flue gas that would result from o.1076 kg combustion of CH4 would be,

mg = mCO2 + mO2 + mN2 + mH2O

= 0.2959 + 0.2152 + 1.062 +0.2421 = 1.815 kg (on wet basis)

= 1.5731 kg (on dry basis)

The cycle diagram of the CHP plant employing Gas Turbine is as shown in figure below. Let us

assume ambient conditions for air entering compressor. i.e. Temperature T 1 = 27oC = 300 K, specific

heat capacity cp = 1.005 kJ/kg.K, Pressure P1 = 1 bar, and k = 1.4. The temperature of air after

compression is found from isentropic compression of an ideal gas,

T2 = T1*(r)(k-1)/k

Where r = 15.6, is compression ratio assumed for Centaur 40 Gas Turbine.

T2 = 300*(15.6)0.286

= 657.68 K


22/27



Figure 7: cycle diagram of Gas turbine Cogeneration (open cycle)

Ignoring pressure drop in the combustor, for adiabatic processes P2 = P3 and P4 = P1 .

The compressor work required for this compression is,

Wc = ma*cpa*(T2 T1) = 0.5845 kg/s*1.005 kJ/kg.K*(657.68 - 657.68)K = 0.21115 MW

The lower heating value (LHV) of the fuel is used in cogeneration applications. With assumption of

high-grade fuel of LFG, LHV of methane is 37.5 MJ/kg. For the flue gas expanding in the turbine let

us assume non-ideal gas conditions: specific heat capacity of the flue gas cpg = 1150 J/kg.K and k=

1.33. The temperature of exhaust gas for the selected turbine is 710k. Using these values the

temperature of flue gas at entrance of the gas turbine will be;

T3 = T4*(r)0.248

= 710*(15.6)0.248

= 1403.77K


23/27



Efficiency of the combustion chamber will be,

cc = mg*cpg*(T3 T4)/(mf*LHV)

= 0.5845kg/s * 1.150 KJ/kg.K (1403.77 710)K /(0.1076 kg/s * 37500 KJ/kg)

= 0.2338

The gross electrical output of the gas turbine is

WT = mg*cpg*(T3 T4)

= 1.815kg/s*1.15KJ/kg.K*(1403.77 710)K

= 1.4481 MW

The net electrical output of the turbine is

WTnet = WT Wc = 1.4481 - 0.21115 = 1.2369 MW

The overall gas turbine efficiency now becomes,

T = WTnet/(mf*LHV) = 1.2369*106/(0.1076*37.5*106) = 0.3066 = 30.66%

Gas turbines with heat-recovery steam generators (HRSGs) are commonly used in chemica

process industries (CPI) plant. They can be operated in either the cogeneration mode or the

combined-cycle mode. In the cogeneration mode, steam produced from the HRSG is mainly used for

process applications, whereas in the combined-cycle mode, power is generated via a steam turbine

generator. The HRSG generates steam utilizing the energy in the exhaust from the gas turbine.

As a result in this project it is supposed that there are processing industries like Textile Industry or

other chemical Industries which use the heated water for processing purposes.

The Pinch and approach method of determining the parameters of HRSG is based on the difference

between the gas temperature leaving the evaporator and temperature of saturated steam and the


24/27



difference between saturated steam and temperature of entering water respectively. These methods

suggest, for un-fired HRSG, a temperature difference range of 15oF 30oF i.e. 59oC 86oC [6].

But the demand of hot water of industries for washing, cleaning and heating services is about 80 oC

So let us fix the temperature of heated water at 80oC and assume the cold water of 0.5 kg/s enters at

27oC. Now the parameters at HRSG become;

Water: mw = 0.5 kg/s, T1 = 27oC = 300K, h1 = hf@T1 = 113.25kJ/kg

T2 = 80oC = 353 K, let quality (x) = 0.2 for more warm water is required.

h2 = (hf+ x*hfg)@T2 = 796.67kJ/kg

Exhaust gas:Tin = 710K, mg = 1.815kg/s

The energy required to heat up water to the required temperature is

Q = mw*(h2 h1)

= 0.5(kg/s)*(796.67 - 113.25) kJ/kg = 341.71kW

With assumption of 1% loss of heat, temperature of flue gas at exit of HRSG is found from the energy

balance of HRSG;

mg*cpg(Tin Tout)*(hl) = mw*(h2 h1)

Where hl = 0.99 for 1% loss of heat.

(Tin Tout) = 341.71kW/(1.815(kg/s)*1.15(kJ/kg.K)*0.99 = 331.22K

Tout = 710 - 331.22 = 378.78K = 105.78oC

Efficiency of the HRSG now becomes

HRSG = mw*(h2 h1)/( mg*cpg(Tin Tout))

= 341.71kW/(1.815(kg/s)*1.15(kJ/kg.K)* 331.22K)

= 0.4943 = 49.43%

The overall cogeneration plant efficiency is the sum of the two efficiencies.

overall = 0.3066 + 0.4943 = 0.8009 = 80.09%


25/27



For smaller gas Turbines, the cogeneration parameters are electrical conversion 24 % -31%, thermal

conversion 50% and overall conversion efficiency is 74-81 [7].

4. Benefit of the Project

This project benefits all the stake holders involved in the waste collection, transmission and disposal

system. It also gives substantial advantage to the Municipal and nearby hot water demanding

processing industries.

This project creates job opportunities for collection through disposal of the waste. Waste collection

creates job for poor women road sweepers and collectors as is being done by tractor trailer now

Waste processing before dumping and material recovery also needs labors. Men should be hired for

construction and then operation of the project.

The main advantage is the pollutant gas that is used to emit to the environment becomes a reliable

fuel for power generation. Problems such as odor and risk of fire and explosion from the land fill will

be inhibited. Energy generated by this technology also falls under renewable energy category.

The power generated by this plant can be used by neighboring society or can be added to the grid. If

this project place if far from grid line it is better to be used locally for cost of transmission will be high

compared to the power produced. The project owner will get more revenue from this project for the

project requires small operating and maintenance costs.

The thermal energy of exhaust gas, that used be rejected to the environment, is extracted by water

that is to be used for industrial processing purposes. The money used to expend on purchase of fue

for evaporating water can be used for other expenses. As the energy of the exhaust gas decreases

their polluting potential decreases.

In general, waste and then a pollutant mater is used for energy generation. It is smart technology and

environmental friendly. The city, the society, the municipal and processing industries benefit a lot.


26/27



5. Conclusion and Recommendation

Conclusion

This initial design of CHP system from the LFG using the Mekelle City Adi Kolomay landfill site gives

a clue as wastes that generate from the community has economical and environmental values if they

are managed in a controlled and studied manner. The economical use of the design is the generation

of electricity and heat power. The electricity generated from this landfill enables to cover a significant

amount of energy demand of the city. And the heat that exhausts from the power producing turbine is

used for process heat activities (with the use of HRSG) needed by the nearby industries or other

bodies that needs it for their various activities.

It keeps the environment by creating a clean and odorless sanitation conditions. As a result, it

improves the health condition of those farmers who do their activities in the nearby areas.

Besides the energy recovery system LFG utilization keeps the surrounding clean, inhibits problems

like emission of methane, possible risk of fire and explosion, odor from the land Fill, creates job

opportunities. Moreover it brings, if the technology is applied, technology transfer to the country.

Recommendation

Even if the study made by Promise consult is to make a standard landfill site, due to financiashortage that faces the City Administration Municipality Bureau, there is no enough material to make

standard compaction operation. It would be better for the municipality bureau to make financial raising

agreements with NGOs and other governmental bodies that work on environmental issues.

Waste types have to be determined in a detail way that can give better technical information

regarding to their name and energy content. It would be informative if they are not put in group.

There should be integration between the municipality bureau and the issue concerned

departments of Mekelle University. This could help to work on experimental analysis of the

energy content of the landfill site and on the real use of its energy using CHP.

Cost analysis of this project is not done for lack detailed information on equipments needed

and their prices in LFG utilization for power production. So it is difficult to say this project if

profitable without cost analysis as it uses highly processed LFG and the power obtained is

small.

The hot water demand of nearby industries should be identified rather than assumption


27/27


6. References

1. Energy technology Bulletin: WTE Technologies

2. Catherine Tatarniuk, the feasibility of waste-to-energy in saskatchewan based on waste

composition and quantity

3. Promise Consult: Consulting architects and Engineers, Final Feasibility and Preliminary Design

report for Mekelle City Integrated Solid Waste Management

4. Horacio Terraza and Hans Willumsen, Guidance Note on Landfill Gas Capture and Utilization

5. Land Fill Gas Utilization technologies, (unknown author, web material)

6. V. Ganapathy, ABCO Industries, Heat recovery Steam generators: Understanding the Basics.

7. UNEP, Thermal Energy Equipment: Cogeneration,www.energyefficiencyasia.org

8. Mr. Mengisteab, Expert at Mekelle Municipal City, Waste Disposal and Management Section

9. SCS Engineers, Design of Land Fill Gas Systems, Part 1.

10. LANDGEM Gas Generation model,www.plexscape.com/services

11. Caterpillar Company, Solar Turbines, Gas turbine Generator systems
http://www.energyefficiencyasia.org/http://www.energyefficiencyasia.org/http://www.energyefficiencyasia.org/http://www.plexscape.com/serviceshttp://www.plexscape.com/serviceshttp://www.plexscape.com/serviceshttp://www.plexscape.com/serviceshttp://www.energyefficiencyasia.org/

msw cogen term paper report

Documents