Landfill Gas as an Energy Source Rena1, Pratibha Gautam2, Sunil Kumar1 Solid and Hazardous Waste Management Division, CSIR e National Environmental Engineering Research Institute, Nagpur, India; 2Department of Environmental Science & Technology, Shroff S.R. Rotary Institute of Chemical Technology, Ankleshwar, India 1
1. Introduction to Landfill An important constituent of integrated solid waste management is safe and dependable long-term dumping of solid waste . Since the primitive era people have been disposing of waste on the land. With the passing of time, the population increased and society began to expand, and along with the increasing population the amount of solid waste also increased, thus creating glitches in the management and disposal of waste . The piling of waste on land has always brought questions of land encroachment, health issues, and esthetic value. This condition leads to design innovations for landfills. Landfilling is a simple and cost-efficient technique, so it is the most-used practice for municipal solid waste (MSW) disposal [3,4]. In Asian countries, for example, China, 77% of the total MSW is treated in 476 large- and medium-scale waste-to-energy plants. MSW is highly heterogeneous in nature, containing different types of waste. These wastes get disposed of, and thus the landfill becomes a hub of availability of the different types of materials. Even after the long term of deposition, due to the availability of massive materials near about 60e80% of total fresh waste gets accumulated in landfills [5e11]. Although many options are available for waste-to-energy conversion of MSW, waste is still diverted to the land.
1.1 Essential Components of Landfill The basic concept of a secure landfill is a properly designed landfill that can restrict the contact of trash and its subsequent seepage into the ground with the underlying aquifers. In general, leakage through the base of the landfill is unavoidable but can be reduced to practically zero. The essential components of the secure landfill are a bottom liner/liner system, waste disposal cells, leachate collection system, cover system, and gas recovery system (Fig. 6.1).
Current Developments in Biotechnology and Bioengineering. https://doi.org/10.1016/B978-0-444-64083-3.00006-3 Copyright © 2019 Elsevier B.V. All rights reserved.
94 Chapter 6
Figure 6.1 The essential components of landfill.
1.1.1 Liner Systems The liner system prevents the trash and seepage from coming into contact with the soil and aquifer. There are different types of liners. Normally clay, synthetic materials, liner plastic, or a composite type of liner is used to contour the landfill. Linings are also of different types, like single or double or multiple layers. To have a properly lined landfill, the thickness of the liner should also be taken into consideration along with the selection of liner material. The materials that are used as liners in a landfill are: 1. 2. 3. 4. 5.
clay; synthetic membranes; geosynthetic clay; amended soil and other admixtures; composites.
Clay is used to line the landfills that contain nonhazardous waste. The grain size of the clay should meet the 2-mm criterion (Massachusetts Institute of Technology classification).
Landfill Gas as an Energy Source 95 This size of granule provides low permeability and prevents the waste from coming into contact with the soil and the adjoining aquifers. The surface of the clay usually has a negative charge due to the space between the clay particles. The swelling behavior of clay depends partially on the electrolyte concentration within the double layer. The decrement in the double layer thickness causes shrinkage of the clay. This is an important issue for waste disposal sites as the dielectric constant of the leachate is different from that of water. Several factors such as mineral composition, amorphous material percentage, cation absorption, pore fluid chemistry, and the degree of saturation determine the mechanical properties of clay. A field study by Benson et al.  showed that a permeability of 1 107 cm/s or less was achieved when clay liners were compacted at or above the line of optimum moisture. There are distinctive of effects of natural elements on clay permeability and they are divided into the following two categories. 220.127.116.11.1 Desiccation Cracks When the compacted clayey soil liner undergoes a dry period, usually after the construction, it develops a crack. The cracks can be as deep as 1 ft. It happens usually because of shrinkage of the soil. With an increase in soil shrinkage, desiccation cracks also increase . Therefore, the spraying of water is essential. Cracks can also be reduced by compaction of the relatively dry soil with the help of high-compaction equipment . 18.104.22.168.2 FreezeeThaw Degradation This occurs because of alternate freezing and thawing of the clay liner. It also happens when a liner is exposed to subzero temperature. Cracks in the liner due to this phenomenon can occur at several places [14e16]. Table 6.1 shows the specifications for soil usually suitable for liner construction. 22.214.171.124 Synthetic Membranes
Synthetic membranes are considered more durable and puncture proof. Several polymers associate in different proportion with altered additives known as geo-synthetic membrane. There are many forms available normally; however, usually only a few are used, such as butyl rubber; chlorinated polyethylene; chlorosulfonated ethylene; ethyleneepropylene rubber; high-density polyethylene; medium-, low-, and very-lowdensity polyethylene; linear low-density polyethylene; and polyvinyl chloride. The peculiarity of synthetic membranes is that they are different from one another and do not have the same composition [17,18] (Table 6.2). Table 6.1: Specifications for Liner Suitability  Suitable Specification Liquid limit Plastic index Clay fraction
Limit (%) 30 15 25
Most Suitable Specification Liquid limit Plastic index Clay content
Limit (%) 10e15 10e25 Between 18 and 25
96 Chapter 6 Table 6.2: Advantages and Disadvantages of Synthetic Membranes Serial No.
Low-density and high-density polyethylene
Medium-, very-low-, and linear low-density polyethylene Polyvinyl chloride
Advantages Resistant to UV rays, ozone, weathering; performs well at a high and low temperature Resistant to UV, ozone, and weathering; performs well at low temperature
Resistant to UV, ozone, and weathering; performs well at low temperature; resistant toward chemicals, acid, and oils; also resistant toward bacteria Resistant to UV, ozone, and weathering; strength is quite good; performs well at low temperature Chemical resistant; performs well at low temperature; reliable seam quality Chemical resistant; seam quality good Performance is good, seaming characteristics are good
Disadvantages Strength is low, less resistant to hydrocarbons, quite problematic to seam Has good strength; not problematic in seaming, i.e., good toward seaming but seaming reliability is not good Less problematic in seaming
Oil absorption is low; does not perform well after exposure to hydrocarbons and solvent; seam quality not very reliable Poor puncture resistance e
Poorly resistant towards UV, ozone, sulfide; performance not up to the mark at high and low temperature
Modified from A. Bagchi, Design of Landfill and Integrated Solid Waste Management, John Wiley & Sons, Inc., Hoboken, New Jersey, 2004, ISBN 0-471-25499-1.
126.96.36.199.1 Commonly Available Synthetic Membranes Generally four types of membranes are available. These synthetic membranes are manufactured in plants. Table 6.3 shows commonly available synthetic membranes. 188.8.131.52.2 Workability of the Synthetic Membrane The membrane having a thickness of 1.5 mm and above is more tolerant during installation. It is least affected during the seaming process, but, because of its thickness, it becomes quite heavy and requires other specific equipment to lift it. One of the other major concerns in using a synthetic membrane as a liner is seaming, as the membranes that are more chemical resistant are hard to seam .
Landfill Gas as an Energy Source 97 Table 6.3: Manufacturing Processes for Synthetic Membranes Type of Membrane
Process of Manufacturing
Synthetic membranes like HDPE and LLDPE with a textured surface are available on the market. Owing to the presence of a textured surface on both, the interaction between the underlying and the overlying surfaces increases the interface friction. The width of both edges is 6 in. to aid the welding process. The synthetic membrane is attached to nonwoven geofabrics. The thickness is determined according to the width and thickness of the geofabric and the synthetic membrane. This synthetic membrane is manufactured via a process like either extrusion or calendaring. Width should be 4.85e10 m and thickness should be 0.25e4 mm during the extrusion process, and during the calendaring process the width should be 1.5e2.4 m and thickness 0.25e2 mm. Geofabrics, either woven or nonwoven, are coated with polymeric compounds. The thickness of the membrane and geomembrane ranges between 0.75 and 1.5 mm.
HDPE, high-density polyethylene; LLDPE, linear low-density polyethylene.
184.108.40.206 Geosynthetic Clay Liner
Geosynthetic clay liners (GCLs) are a combination of bentonite and geotextiles. They are becoming more prevalent than synthetic and clay liners. Dry bentonite, which is usually a uniform layer, is placed between two geotextiles. A water-soluble adhesive is used to keep the bentonite in the proper place. Geosynthetic liners swell when they come into contact with water. Swelling leads to the formation of a uniform layer of bentonite having a thickness of 12e25 mm. There are four different types of GCLs: • • • •
geotextile-enclosed adhesive-bonded GCL; geosynthetic-encased stitch-bonded GCL; geotextile-encased needle punched; geomembrane-supported adhesive-bonded GCL.
The hydraulic conductivity of GCLs is subtle; the sensitivity of the hydraulic conductivity of GCLs is toward hydration and stress. When the GCL comes into contact with real and synthetic MSW leachate its hydraulic conductivity is increased. Sometimes, due to continuous stress, the hydraulic conductivity of GCLs is found to be decreased significantly [19e21]. 220.127.116.11 Amended Soil and Admixtures
Amended soil and admixture liners are basically manufactured from amenders like bentonite, asphalt, and cement and admixtures, namely, asphaltic concrete.
98 Chapter 6 18.104.22.168.1 Nonbentonite Mixes MSW was tested on four admixtures, i.e., paving asphalt concrete, hydraulic asphalt concrete, soil asphalt, and soil cement, for about 56 months. A subsequent loss in strength was observed but the other physical properties were not seen to be much deteriorated. Owing to the absorption of water the soil cement gained more strength and became less permeable, but cracking of the cement was seen in different highways projects . This admixture is not used in making liners or clay design. Instead it is used as a layer below synthetic membranes for the study of chemical compatibility and retention of operational integrity. 22.214.171.124.2 Bentonite-Amended Soil Bentonite-amended soil basically belongs to the smectite group of clay minerals. The liners are made up of a pile of layers. Each layer consists of an octahedral sheet intercalated between two layers of silica sheet. Owing to its structure, water gets easily absorbed. It is mostly preferred for constructing the final cover. 126.96.36.199 Composite Liners
The composite liner is a compacted liner or GCL having a geomembrane as an underlying cover. Studies suggest that the leakage rate is significantly low compared with clay liners . 188.8.131.52.1 Selection of Liner Material To choose a liner material the following criteria should be taken into account: waste type, type of landfill operation, thickness of the drainage blanket, compatibility of the primary liner with the leachate, and also, in the case of hazardous waste, compatibility of the secondary liner with the leachate. 184.108.40.206.2 Types of Liner System There are two types of liner system: single liner and double liner. The most preferred liners to line a landfill are clay and synthetic membranes. Fig. 6.2 shows the liner system types. 220.127.116.11.3 Liner Thickness The thickness of clay should be 15e30 cm, or 6 in. to 1 ft. This can provide acceptable permeability. The thickness of the clay liner mostly depends on construction-related issues and also on the hydraulic conductivity of soil. As mentioned by Rogowski et al. , the hydraulic conductivity of a soil liner can vary remarkably within a few meters. It is indicated that an area of 1500 cm2 of soil liner is sufficient to incorporate the variability in the hydraulic properties of soil . To have low stratification, fissuring, and large-scale heterogeneity, proper construction and control measures should be taken into consideration for the thickness. 1.1.2 Waste Disposal Cell The waste disposal cell is one of the important components of the landfill. The air space decides the life span of the landfill. The cell or air space is directly proportional to the usable life of the landfill. To create a waste disposal cell, the waste is compacted either on the ground or below the ground in “cells” that contain the waste of only 1 day.
Landfill Gas as an Energy Source 99
Figure 6.2 Types of liner systems.
Figure 6.3 The different types of waste disposal cells .
The principle of the cell is based on three methods: trench, area, and progressive slope . Fig. 6.3 shows the different types of waste disposal cells. 18.104.22.168 Trench Method
The trench method is most suitable in areas that have: • • •
less waste; low groundwater; soil cover of 6 ft.
22.214.171.124.1 Methodology The trench method involves digging up the selected area. An area 4.6 m deep is unearthed. The soil that is dug out is kept for later use as a cover over the waste. To drain off the rainwater, grading is done. The type of subsoil is taken into consideration in this method.
100 Chapter 6 126.96.36.199 Area Method
The area method is most suitable when the: • •
quantity of waste is quite high; possibility of constructing the landfill below ground is negligible.
188.8.131.52.1 Methodology In this method there is compaction of waste and the soil cover over the ground, this method is applicable in any plain area. This method has applicability in many places such as abandoned ground, gullies, strip mines, ravines, valleys, etc. 184.108.40.206 Progressive Slope or Ramp Method
This method is most suitable for slopes or when plain terrain is not available. 220.127.116.11.1 Methodology This method is a combination of the trench and area methods. The waste is compacted, deposited, and covered on the slope. To cover the waste, soil is dug from the front of the daily cell or, if cover soil is not available, it is fetched from another place. 1.1.3 Leachate Collection System To keep water out of the landfill is impossible. The moisture in the waste infiltrates through the garbage. During the percolation of water, harmful chemicals, whether organic, inorganic, or heavy metals, seep along with it, thus contaminating the soil and the subsequent aquifers. To keep the adjoining area and the groundwater safe from the leachate, a collection system is installed for the landfill. Fig. 6.4 shows the leachate collection system. It consists of a drainage layer, leachate trench and pipe, leachate collection pump, lift station pump, and leachate storage tank .
Figure 6.4 The different layers of a leachate collection system .
Landfill Gas as an Energy Source 101 18.104.22.168 Drainage Layer
In the construction of the drainage layer, gravel, sand, or geocomposites are used. Gravel is mostly preferred over a sand drainage blanket, as construction of a sand blanket sometimes creates the problem of clogging. To maintain the leachate flow, the hydraulic conductivity of the drainage layer should be within the limit of 1 102 cm/s . 22.214.171.124 Leachate Trench
A leachate trench is constructed according to the type of liner used. A geotextile is used to line the trench to reduce the entry of fines from the liner. Deeper excavation may be required so that even below the trench the liner has an identical thickness. 126.96.36.199 Leachate Pipe
The leachate pipe carries away the leachate to the storage tank or pond. Throughout the landfill perforated pipes are run to collect and drain the leachate into the leachate pipe. Clogging of the leachate pipe is one of the major issues. Cleaning on a regular basis is required to minimize the clogging. There are methods such as mechanical or hydraulic to clean the pipe. 188.8.131.52 Leachate Collection Pumps and Lift Station
Leachate collection pump capacity is one of the major criteria to consider when installing one. When choosing a pump, delivery and suction capacity need to be checked first. In the lift station usually a submersible pump is used. Since the rate of generation of leachate varies widely, on average a pump having 12 minimum cycles is preferred . 184.108.40.206 Leachate Storage Tank
The tank that stores the leachate is called the leachate storage tank. Normally a leachate tank can hold the leachate for a maximum of 3 days. Leachate can be stored in either a double-walled or a single-walled leachate holding tank. The inside of the encasement needs to be monitored periodically to check for cracks. 1.1.4 Landfill Cover Landfill cover is used to restrict the percolation of water into the landfill. There are four layers of landfill cover: a grading layer, a barrier layer, a gas collection layer, and a foundation layer . Fig. 6.5 shows landfill cover. 220.127.116.11 Grading Layer
The grading layer consists of a (15- to 60-in.) thick coarse-grained material . If the surface is unstable, then a layer of bark or geotextile is added below the surface. To provide a stable surface, a layer of grading is done. It provides a concrete structure to facilitate the
102 Chapter 6
Figure 6.5 The different layers of landfill cover .
venting of landfill gases (LFGs). It increases the water storage capacity as it is helpful to fix the overlying protection layer. 18.104.22.168 Barrier Layer
A barrier layer is constructed mainly to provide a blockage for water to percolate into the lower layer. This layer is very much important and therefore attention is needed during its construction. Clay, a synthetic clay liner, or a synthetic membrane layer is used for construction. It is beneficial for moving the gas in a downward motion and also restricts the movement of gas upward. For hazardous waste, the barrier layer consists of a complex geomembrane layer fused over a compacted clay liner having a hydraulic conductivity of 1 107 . 22.214.171.124 Gas Collection Layer
This layer is present just below the barrier layer. This layer collects the gas from the degradation of organic wastes along with volatile organic compounds. It is constructed of sand, gravel, geotextile, or other materials that are helpful to transfer the gas. Gas collection pipes collect the captured gas from the gas collection layer. The flow of gas can be passive in the case of a closed landfill, where natural pressure is available, or active with the help of a vacuum system. The gas collection layer is necessary for landfills having MSW, abandoned landfills, and miscellaneous landfills. The gas collection layer should have high-in-plan permeability and should be devoid of clogging caused by fine-grained materials. 126.96.36.199 Foundation Layer
This is the interim cover, with some additional compaction before the final cover is constructed. The foundation layer is heavily rolled. If the interim cover is site specific or if it is constructed on an irregular surface, the foundation layer is constructed by adding extra soil.
Landfill Gas as an Energy Source 103 1.1.5 Gas Recovery System There are two types of venting systems to withdraw gas from the landfill: • •
active landfill; passive landfill.
To choose one of the systems, the following should be taken into consideration: landfill design, type of soil, size of usable space, regulation mandates, and type of waste being dumped. 188.8.131.52 Active Landfill
A series of deep excavated wells connected to a header pipe and blower is called an active venting system. The gas can be reused for energy purposes or on-site burning. The gas can also be released into the atmosphere but this requires checking the chemical constituents of the gas along with the location of the landfill. The distance of the landfill from the nearest community should also be considered. 184.108.40.206 Passive Venting System
When the generation of gas is quite low, a passive venting system is installed. It is most suitable for municipal landfills that are smaller than 40,000 m3 in size . A series of isolated gas vents is connected by a perforated pipe to collect the gas released from the waste. This system is mostly seen in nonputrescible containment type landfills.
2. Landfill Gas All biodegradable fractions of waste dumped in landfills undergo the process of anaerobic digestion. As the operating conditions inside the landfill are not controlled and the characteristics of waste are not uniform, many other biochemical reactions occur inside the landfill that are not completely understood. As an output of these reactions, a gaseous mixture is produced, which is termed “landfill gas.” The quantity and quality of LFG depend on several parameters, such as the characteristics of the parent material (waste dumped), the age of the landfill, and various environmental and operational conditions. Waste decomposition in a landfill does not start with anaerobic conditions directly; initially the process starts in aerobic conditions and continues until all the available oxygen in the landfill gets exhausted. Major products of this phase are carbon dioxide, water, and heat. The next phase of the process is the anoxic phase, when higher molecules are broken down into smaller units. Acidic compounds, hydrogen gas, and carbon dioxide gas are the major products of this phase. Water, ammonia, and heat are also simultaneously produced, and it is known as the hydrolysis and acetogenic phase.
104 Chapter 6 The third major phase is the methanogenic phase, which is strictly anaerobic. Methanogenic bacteria take the lead in this phase, the concentration of carbon dioxide gas declines, and methane gas generation starts. Generally it takes around 2 years for a landfill to reach the methanogenic phase and it continues for decades.
2.1 Characteristics of Landfill Gas The characteristics of typical LFG depend upon its composition, which is again a function of many parameters, such as stage of the landfill, composition or type of waste dumped, moisture content, construction of the landfill, and many more . The following parameters can be considered to characterize LFG and assess its potential to be used as an energy source. 2.1.1 Composition of Landfill Gas It is described in the previous section that the composition of LFG depends upon a lot of factors and it cannot be same for any two landfills. Even for a single landfill, the composition of LFG varies, as it is an output of several chemical and biological reactions occurring in a reactor (landfill) without any direct control, and with the passage of time or with the aging of the landfill, the kinetics of this reaction varies a lot. Many researchers have carried out several studies to characterize the composition of LFG and it is concluded that LFG is a mixture of gas having methane (CH4) and carbon dioxide (CO2) as major components. Other gaseous components such as hydrogen disulfide (H2S), nitrogen, and water vapor, with some traces of benzene, toluene, vinyl chloride, organic acids, organosulfur compounds, etc., are also present in small quantities. The minor gaseous components are not limited to these components only, but these are categorized as the most commonly reported trace components. The ratio of CH4 and CO2 varies a lot, but average values can be considered as CH4 50%e60% and CO2 40%e50% [28,29]. Table 6.4 can be referred to for a rough composition of LFG. 2.1.2 Density and Viscosity As the landfill ages from the initial phase to the methanogenic phase, the composition of LFG changes drastically, i.e., from an approximate mixture of 10% H2 and 90% CO2 in the initial phase to 60% CH4 and 40% CO2 in the methanogenic phase; therefore the density and viscosity of this mixture also change. Initially, LFG is slightly heavier than air but in the later stage it becomes lighter than air . 2.1.3 Calorific Value/Heating Value The calorific value of LFG can be defined as the amount of heat produced on combusting a unit volume of gas and can be expressed in kcal/m3, kJ/m3, or BTU/ft3. Calorific value depends directly on the methane content of LFG, i.e., the higher the methane content,
Landfill Gas as an Energy Source 105 Table 6.4: Composition of Landfill Gas  Component
Percentage (Dry Volume Basis) Major Components 47.5 47.0
CH4 CO2 Minor Components N2 O2 Paraffin hydrocarbons Aromatic and cyclic hydrocarbons Hydrogen
3.7 0.8 0.1 0.2 0.1 Trace compounds
H2S CO Other trace compounds
0.01 0.1 0.5
the greater the calorific value. As defined in previous sections, the composition of LFG varies with the age of the landfill, therefore calorific value also varies along with its composition. It has been reported that under good conditions, the calorific value of LFG can be expected to be approximately 7124 kcal/m3 during the methanogenic stage .
2.2 Need for Landfill Gas Collection The release of LFG from unmanaged landfills is a major environmental concern, contributing significantly to higher atmospheric greenhouse gas (GHG) concentrations. Methane is a major component of LFG, and it becomes a major threat because of the fact that it has multifold higher global warming potential than CO2 . Apart from this, methane, being a flammable gas, poses a safety risk if not handled and released properly. Looking at these facts it becomes very much necessary to safely extract the methane content from landfills and, subsequently, energy recovery from this gas can recover the capital expenditure and operating cost involved in gas collection.
3. Recovery of Landfill Gases In the hierarchy of solid waste management, a landfill with a gas recovery system is placed in the category of efficient management options, as this option provides flexibility to recover energy from waste dumped in the landfill. Recovered gas from the landfill in the raw condition may not be suitable for its direct utilization as such, but after a certain degree of treatment, this recovered LFG can be considered a potential replacement of traditional energy sources. Any gas recovery system may include a gas collection and
106 Chapter 6 cleaning/upgrading system. The installation of a gas recovery system at any landfill involves considerable cost investment; therefore economic feasibility has to be checked before installing such recovery systems.
3.1 Gas Collection Systems Migration of LFG into the nearby area can cause fire hazards and other environmental problems. This movement can take place by diffusion or convection. The consequences can be avoided by systematically extracting the gas generated in a landfill through a properly designed gas collection system . For the appropriate design and planning of LFG collection systems, many approaches based on the biodegradability of waste and stoichiometric computation have been proposed. To quantify the LFG generation and gas flow characteristics, many mathematical models have been developed, which help in designing the collection system more precisely. The design of a gas collection or abstraction system may include wells, blankets, and trenches. Well extraction systems consist of a series of wells (usually perforated) having depth to near the bottom of the landfill, whereas a blanket system is constructed below the impermeable layer with sand and gravel .
3.2 Gas Cleaning/Upgrading As the collected raw LFG contains many impurities (such as H2S, NH3, and water vapor) apart from CH4 and CO2 as the major components, it needs to be purified to remove the impurities and upgraded in terms of CH4 percentage. The presence of other gases and impurities makes raw LFG unfit for its direct utilization for any purpose. Table 6.5 describes the effects of different impurities on the utilization of raw LFG. After the necessary cleaning and upgrading, LFG can be used as potential fuel for many purposes. Many technologies are available today to clean and upgrade LFG to its desired level, and some of these technologies are summarized in Fig. 6.6.
3.3 Mechanism for Landfill Gas Combustion LFG has been reported to have around 557 components in trace quantity in addition to the main components, which include CO2 and CH4 . Simple flaring of LFG requires a Table 6.5: Influence of Impurities on Landfill Gas Utilization  Impurity
Influence on Biogas Utilization
CO2 H2O N2 NH3 H2 S
Corrosion, reducing calorific value, antiknock properties of engines Corrosion, damage because of condensation Reducing calorific value, antiknock properties of engines Emissions, antiknock properties of engines, decaying Catalytic converter poison, emissions, health
Landfill Gas as an Energy Source 107
Figure 6.6 Technological options for upgrading biogas [31,33]. Table 6.6: Emissions From Landfill Gas Flaring Gaseous Emission From LFG Flaring
Source of Emission
Methane Nitrogen oxide Hydrogen Carbon monoxide Water vapor
Indicates incomplete combustion of LFG A combustion product Also indicates incomplete combustion of LFG Indicates incomplete combustion A combustion product of carbonaceous compounds, including CH4 A combustion product of carbonaceous compounds, including CH4
Carbon dioxide LFG, landfill gas.
supply of oxygen or air and an ignition source. The mechanism for combustion of methane is described as follows: CH4 þ 2O2 / CO2 þ H2 As per this equation, stoichiometrically the volume of air required for complete combustion of methane is 9.6 times the volume of methane. Table 6.6 summarizes the gaseous emissions from the flaring of LFG. The combustion of LFG in the presence of sufficient air gives a bluish flame, which is short in nature. The flame becomes sooty when air availability tends to reduce. The flaring system for LFG should be designed in such a way that it ensures complete mixing of air so that the products of incomplete combustion can be avoided in the final emitted gas.
4. Landfill Gas Utilization as an Energy Source Upgraded LFG is considered a greener fuel, as its combustion does not lead to the production of harmful gases in such alarming concentrations as regular fuels such as coal and petroleum do. Upgraded LFG is a versatile fuel, the same as biogas, and can be utilized for several thermal applications as well as electricity generation. Other uses of
108 Chapter 6
Figure 6.7 Major alternatives for the utilization of landfill gas (LFG) as an energy source. CCHP, combined cooling heat and power; CHP, combined heat and power; MGT, micro gas turbine; ORCG, organic Rankine cycle generator.
LFG include cooking purposes and as a vehicular fuel. Major technical substitutes for the utilization of LFG as an energy source are summarized in Fig. 6.7.
4.1 Thermal Applications Purified and upgraded LFG contains a high percentage of CH4 and is capable of delivering sufficient heat energy upon burning. This heat can be recovered and utilized through several means as summarized in the following sections. 4.1.1 Heat Utilization by Heating Networks In combined heat and power (CHP) systems, after electricity generation, the rest of the heat is recovered and utilized for various purposes such as water heating systems. District heating networks are the centralized systems that distribute heat to residential and commercial sections. 4.1.2 Organic Rankine Cycle Generators These generators use an organic fluid as the working fluid in place of steam and operate on the principle of the thermodynamic Rankine cycle. The main advantage of such generators is that they can utilize even a low-temperature heat source because the organic fluid selected for these generators is vaporized even at very low temperatures. Commonly used organic fluids are alkyl benzene and siloxanes . One limitation of organic Rankine cycle-based plants is that they are economically feasible only in the size range of 15e20 MW.
Landfill Gas as an Energy Source 109 4.1.3 Utilization of Heat in Absorption Chillers Another system for utilization of the heat produced from LFG is a trigeneration process in which, along with a CHP system, heat is utilized for cooling purposes with the help of absorption chillers; such systems are also known as combined cooling heat and power systems.
4.2 Electricity Generation Electricity generation is another important pathway to utilize LFG in a sustainable manner. Major technological options are discussed in the following. 4.2.1 Combined Heat and Power Generation In a CHP system upgraded LFG can be utilized for electricity generation as well as for heating purposes (process steam recovery, space heating, water heating, or process heat requirement). For power generation, usually upgraded LFG is combusted in an internal combustion engine, which drives a power generator . 4.2.2 Fuel Cell Technology Fuel cells are used to convert chemical energy into electrical energy and can be operated with multiple fuels (fluids containing hydrogen). Hydrogen and oxygen are combined by an electrochemical reaction and electricity is produced through it. This is the main reason that makes this technology different from other technologies, as the combustion reaction does not take place in fuel cells. A power plant based on fuel cell technology can be made by connecting individual units of the fuel cell; this arrangement also has the flexibility of further expansion as per requirement. Different types of fuel cells (based on different electrolytes) are available and further research is also going on to develop new such processes. Three such electrolytes are: • • •
molten carbonate; phosphoric acid; polymer membrane.
Of these, phosphoric acidebased fuel cells are the common ones that utilize reformed methanol as fuel. In general, a fuel cellebased plant is composed of the following three sections: • • •
section for reforming reaction; a stack of fuel cells; system for power conditioning.
In general, fuel cells can be classified into two groups: high-temperature fuel cells, which are operated at a very high temperature (600 C or more), and low-temperature fuel cells, which are operated at considerably lower temperature.
110 Chapter 6 Fuel cell technology is fast gaining popularity because of following advantages: • • • • •
relatively highly efficient; produce low emission and low noise; comparatively low requirement of water; modular arrangement; very few moving parts.
4.2.3 Stirling Engines Stirling engines have a major advantage over other comparative processes in that the combustion here occurs in an external combustion engine. This minimizes the level of purification required for LFG compared with other utilization processes. Stirling engines can be successfully combined for CHP systems to provide a high thermal efficiency. Seventy percent and even higher thermal efficiencies have been reported in the literature for CHP systems based on Stirling engines . The Stirling cycle (ideal condition) consists of the following four cycles: • • • •
working gas compression (isothermally); heat addition at constant volume; working gas expansion (isothermally); heat rejection at constant volume.
Stirling engines provide many advantages over other techniques, for example, different types of fuel can be used in this technology with negligible vibration and very low emissions are produced. 4.2.4 Micro Gas Turbines Micro gas turbines (MGTs) are usually part of a CHP system in which electricity is produced with the help of an MGT. LFG is mixed with compressed air and the mixture is combusted under isobaric conditions, and the gas resulting from combustion is utilized to run turbines. The exhaust gases coming from turbines are at a very high temperature (500e600 C), and can be utilized for heat recovery. It has been reported that a CHP system based on MGT can provide a thermal efficiency greater than 70% . Microturbines are very well suited to small-scale applications and can produce up to 1000 kW of electrical power. Application of MGT-based power can be for electricity supply in a landfill and nearby residencies. MGTs offer the following advantages as an LFG utilization technology: • • •
low emission; parallel moduleetype arrangement can be done to provide stable power; flexible operation;
Landfill Gas as an Energy Source 111 • • • •
small area requirement; low LFG rates with CH4 content around 30% are acceptable; fewer moving parts so low maintenance cost; high overall efficiency.
Although the electrical efficiency of an MGT-based plant is not very significant (15%e25%) because of its relatively small size, owing to the recovery of thermal energy, its overall efficiency can be as high as 80%. A typical microturbine system is based on the Brayton cycle and consists of the following components: • • • • •
combustor; turbine; electricity generator; compressor; recuperator.
Even after high overall efficiency and many advantages, the application of MGT is restricted because of the following disadvantages/limitations: • • •
low electrical efficiency; high capital cost; sensitive to variation in ambient air temperature.
4.3 Other Uses There are several other uses of LFG, for example, it can be used for cooking purposes or can also be used as a transport fuel. The other uses of LFG are shown in the next sections. 4.3.1 Upgrading and Utilization as a Transport Fuel The utilization of LFG as a transportation fuel is very difficult but can be done after extensive cleaning and enhancement of the methane content to more than 97%. This cleaned gas is then compressed at 20e25 MPa and termed as BioCNG. It has been reported that emission levels (CO, NOx, and HC) are slightly higher when BioCNG is used as vehicular fuel compared with compressed natural gas (CNG) used in its place. As far as GHG emissions are concerned, BioCNG as a vehicular fuel can reduce GHG emissions by 63% compared with CNG as a vehicular fuel in heavy vehicles. A study (Table 6.7) shows a comparative assessment of gaseous emissions from heavy vehicles when measuring for CNG and BioCNG. Another advantage of exploring the possibilities of using LFG as a transportation fuel is that it produces a lesser amount of dangerous chemicals. The concept of BioCNG is becoming very popular in many countries such as Sweden, Germany, Italy, Switzerland, and the Netherlands .
112 Chapter 6 Table 6.7: Gaseous Emission From Compressed Natural Gas and BioCNG (for Heavy Vehicles)  Fuel Biogas BioCNG
Particulates (g/km) 0.5 0.1
Calorific Value (kJ/kg) 35,000 52,000
4.3.2 Utilization of Landfill Gas in Gas Grid Systems After the necessary cleaning, LFG can also be upgraded for its injection into a natural gas grid system. As LFG contains lots of impurities and it may be highly corrosive in nature, there are possibilities for it to corrode the equipment; therefore, many countries, such as Germany, Sweden, and Switzerland, have made their own standards for its injection into the natural gas grid . 4.3.3 Utilization for Cooking Purposes Similar to biogas, properly collected, treated, and upgraded LFG can also be used for cooking purposes. Cooking is a better option for developing countries, where the volume of gas is not very high and CHP cannot be a feasible option. Utilization of LFG in place of solid fuel is an environmentally sustainable alternative, as the air pollution in the case of LFG is negligible in comparison to the air pollution caused by solid fuelebased cooking . 4.3.4 Landfill Gas to Liquefied Natural Gas Conversion of LFG into liquefied natural gas (LNG) is also possible and various processes have been developed for the same. One of the common processes is a CO2 wash process, which produces clean methane gas and CO2 of food grade. The raw LFG undergoes several processes, including compression, dehydration, etc., and the output gas after removal of H2S and other impurities contains approximately 70% CH4 and 30% CO2. Comparatively clean output gas is then heated up to 70 F and enters into a membrane chamber where the pressure is reduced to 200 psig and membranes separate O2 and CO2 from the methane. The clean gas produced contains a high content of CH4 and can be liquefied for its use as LNG. Fig. 6.8 presents the outline of LNG production. 4.3.5 Landfill Gas to Methanol Conversion of LFG to methanol can also be achieved after a specific series of treatments. Initially the LFG is made contaminant free and all the impurities except CO2 are removed from it. Then the rest of gaseous mixture (primarily containing CH4 and CO2) is brought to a prefixed ratio of 2.3:1. This ratio is the feedstock of the reformer, which reforms this
Landfill Gas as an Energy Source 113
Figure 6.8 Schematic diagram for the production of liquefied natural gas (WASH process).
Figure 6.9 Conversion of LFG into Methanol.
mixture into a mixture of CO and H2 after the necessary supply of steam. This mixture of CO and H2 then enters into methanol synthesis in the reactor for synthesis of methanol. The schematic diagram of this process is shown in Fig. 6.9.
5. Challenges in Utilization of Landfill Gas Despite the fact that energy utilization from LFG is an environmentally sound and green pathway of solid waste management in any country, its implementation in many places, especially in developing countries, is very limited.
5.1 Major Challenges There are many restrictions on different horizons that limit the application of energy utilization technologies as discussed in the following: •
Technology development The selection of appropriate technology and its successful implementation depend on many parameters and are based on many assumptions, such as expected methane concentration in LFG and its production rate. But in fact there is no consistency in the quality of waste material and therefore the clear estimation of LFG generation and its composition is very difficult. Hence basic research is very much necessary before technology development for a specific landfill site.
114 Chapter 6 •
Economic restrictions Usually LFG utilization techniques demand a huge capital cost, which is the main restriction in its implementation. Landfill operators look for low investment alternatives and prefer to opt for flaring of gas. This restriction can be overcome if some incentives are provided by the government to encourage implementation of such utilization techniques. Involvement of policy makers Sustainable initiatives like the utilization of LFG for energy recovery should be encouraged by regulatory authorities and policy makers, and incentives should be given for such projects. Especially developing countries are lacking in the availability of such policies in the national policy framework that promote the systematic recovery of LFG and its utilization. Active involvement of policy makers and awareness about the importance of such projects can be a positive step toward the removal of this barrier.
5.2 Development of an Action Plan An action plan can be developed for a specific country after identification of the major barriers at the local and national levels. Major points to be covered while developing an action plan are as follows: •
Basic research and site-specific technology selection should be the most important element of an action plan for LFG recovery and its utilization, because they ensure the successful implementation of the project and its payback. As discussed in previous sections, the economic incentive for developing an LFG utilization technique has to be a major part of an action plan because it can encourage any landfill operator to establish a system for LFG recovery and its utilization. Development and strengthening of the legislative and institutional framework is another aspect of developing an action plan for LFG utilization, which can promote research and implementation of various technologies in this field. • Awareness among policy makers and the public about the side effects of LFG (if not recovered and utilized) and the advantages of its utilization for energy recovery is very much necessary to maximize participation in such projects.
All of these points of concern are presented in Fig. 6.10.
6. Conclusions and Perspectives The collected waste of an entire city is dumped at a landfill. Dumped waste undergoes degradation and produces a toxic liquid extract (leachate) and a flammable gaseous mixture (LFG). Because of its toxicity, leachate has huge potential to pollute the soil and its underlying aquifers. Similarly, LFG can also cause accidents if not managed properly. Thus, landfills need to be secured so that they can restrict the contact of trash and its consequent
Landfill Gas as an Energy Source 115
Figure 6.10 Major components of an action plan for landfill gas utilization.
seepage. Along with this, all the LFG generated from the landfill should also be captured, treated, and used for constructive purposes. Different liner systems and leachate collection systems are used to prevent the percolation of leachate into the nearby ground surface, and a gas collection system is used to capture all the LFG produced by a landfill. This LFG, after appropriate treatment and upgrading, can be used for various applications. Upgraded LFG can be utilized as an energy source in different thermal and heat applications. CHP and micro gas turbines are commonly employed for LFG utilization. After the appropriate treatment, LFG can also be utilized for cooking purposes and can also be considered for its application as a vehicular fuel. Despite being a versatile fuel, LFG utilization as a potential energy source is limited because of the higher initial capital investment required and the lack of encouraging policies in this field. Relevant research, awareness programs, and provision of subsidies for such waste-to-energy projects can enhance the acceptance of landfill gas as an energy source at global platform.
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