Technical Analysis of Landfill Gas Recovery Systems for the Production of High BTU Gas

By Gary J. Nagl
Gas Technology Products

Abstract

In the past, landfill gas recovery projects consisted of the installation of gas gathering systems followed possibly by desulfurization and siloxane removal then by combustion in gas engines or turbine/generator sets for the production of electrical power or in boilers for the production of steam. These types of projects were very attractive since very little processing was required even though the gas had low to medium heating value (300–500 BTU/CF). Unfortunately, the power companies would not and still will not pay a very high price of the power.

Times have changed. With the today’s high energy prices, landfill gas recovery projects to produce high BTU gas, which can be injected into natural gas pipelines, are becoming very attractive even without tax breaks; however, a good deal more gas processing is required to achieve the 900+ BTU/CF heating value. No one technology is capable of performing the required task; however, by combining commercially proven technologies, high BTU gas can be achieved with very little technical risk.

The real technical risks associated with landfill gas recovery systems for the production of high BTU gas are mostly related to the operational and maintenance aspects of the equipment. Compared to normal landfill operations, gas recovery systems are relatively complex; however, with the substantial revenues that can be obtained from the recovered energy, these projects can easily support a higher grade of operating and maintenance personnel, thus eliminating the risks associated with the technical and operational aspects of the producing high BTU gas.

Introduction

Upgrading landfill gas to high BTU gas requires the almost complete removal of all compounds other than methane. This requirement necessitates removal of the following components from a landfill gas stream:

Unfortunately, there is no one process that can remove all of these components in one step. Consequently, a series of processes must be combined to achieve the desired results, which is a gas consisting of the following compositions;

A discussion of the processes available to remove the various contaminants follows.

Hydrogen Sulfide Removal

There are a variety of commercially proven processes available to remove H2S from gas streams. These processes can be divided into wet and dry processes.  The dry processes are generally non-regenerable and are characterized by having low capital costs but high relative operating costs ($/lb. of H2S removed). Wet processes are just the opposite in that they are usually regenerable but have high capital costs and low relative operating costs. The point at which the economics switch from the dry systems to the wet systems is usually around 300 pounds of H2S/day.

The oldest dry system still in use today is the iron sponge process, which consists of wood chips impregnated with hydrated ferric oxide. The H2S reacts with the ferric oxide to form iron sulfide. The spent media can be regenerated by passing air through the bed, which converts the iron sulfide back to ferric oxide and elemental sulfur. This process has been successfully employed for many years on landfill gas; however, it has lost some appeal due to the environmental and safety problems associated with handling and disposing of the spent media.

Another dry system that has been successfully employed on landfill gas consists of an iron oxide impregnated on a ceramic base. As the H2S passes over the media it reacts with the iron oxide to form innocuous iron pyrite. The process is not regenerable, but it is attractive because the safety problems associated with iron sponge are eliminated. The two commercial processes employing this technology are Sulfur-Rite® and SulfaTreat®.

Activated carbon can also be employed for selectively removing H2S from landfill gas; however the high cost of replacing and disposing of the carbon generally limits its use to polishing applications (very small amounts of H2S to be removed).

The only wet system in commercial use today for selectively removing H2S from landfill gas in one step is the LO-CAT® process. The process is based on liquid redox chemistry and employs an aqueous solution of chelated iron to directly convert the H2S to elemental sulfur. This technology has been in use at a Landfill in Florida since 1995, and several other units are currently being installed.

All of the above processes are capable of removing 99.9+% of the H2S and have been proven in landfill gas service for several years.

Hydrogen Sulfide/Carbon Dioxide Removal

Several commercially proven, liquid absorption processes are available for the simultaneous removal of H2S and CO2. These technologies are further characterized by employing chemical or physical absorbents. Chemical absorbents selectively remove acidic compounds by reacting with them to form weakly bonded compounds, which disassociate upon the application of heat.  Physical absorbents rely on the solubility of the acidic components in the absorbent, which are then released by merely reducing the pressure. When acidic gases constitute a major portion of the gas stream to be treated such as landfill gas, physical absorbents become more attractive due to the lower energy requirement.

Chemical solvents, which are widely used in the natural gas processing industry consists of alkanolamines such as mono-, di-, and methylethanolamine. A gas at elevated pressure is contacted with the amine in an absorber. The acidic components are absorbed and react with the amine. The amine solution is then depressured to release any absorbed hydrocarbons before entering the stripper column in which the solution is heated to its boiling point. The acidic components disassociate and are released while the stripped solution is cooled and returned to the absorber column. Due to the large quantity of acidic components and the corresponding large heat requirements, chemical absorption systems are rarely employed in treating landfill gas.

The simplest physical absorption system consists of contacting the landfill gas with water at elevated pressures, which will preferentially absorb the acidic components. The effluent water from the absorber is then stripped with air to remove the absorbed components. This processing technique has been successfully employed in the treatment of landfill gas in Europe; however, its drawback is that a great deal of water must be circulated through the absorber resulting in high parasitic power consumptions.

The Selexol™ process is a proprietary technology, which employs a physical solvent that is derived from a dimethyl ether of polyethylene glycol. The solvent has a strong affinity for CO2, H2S, water and heavy hydrocarbons; however, it has a very low affinity for N2 and O2. If the gas to be treated contains oxygen, it is beneficial to remove the H2S prior to the Selexol unit since O2 will react with the H2S to form elemental sulfur causing operational problems within the unit. One disadvantage of the Selexol process is the relatively high cost of the solvent. The Selexol process has been successfully employed in a number of landfill gas applications. The process will be further described in the example section of this report.

Acid gases can also be separated from landfill gas by utilizing membrane separation techniques, which utilizes the differential permeability of components through polymeric membranes to affect separation. For landfill gas applications, CO2 and H2S have high permeabilities and will pass through the membrane (permeate), while CH4 has a very low permeability and will remain on the outside of the membrane. A single stage membrane will produce a gas stream containing approximately 88% CH4. To achieve a higher CH4 concentration, multiple stages at high pressure (approximately 575 psig) are required. All gaseous components have some permeability; consequently, the permeate stream will contain various amounts of all components contained in the landfill gas. Membranes have been successfully used to treat landfill gas since the early 1980’s. To ensure long membrane life it is beneficial to process the LFG through activated carbon for removal of NMOC’s prior to processing through the membranes.

Adsorption techniques (gas being adsorbed onto a solid) can also be employed for treating landfill gas.  A molecular sieve consisting of a zeolite material is generally the adsorbent of choice for treating landfill gas. These materials are very porous and have very high surface areas, the majority of which is internal to the particles.  Under pressure, the molecular sieves preferentially adsorb CO2, H2S and water, although all gaseous components will be adsorbed to some degree. When a bed of molecular sieves approaches saturation, it is taken off line and depressured releasing the adsorbed material. Because of this processing technique, these systems are called Pressure Swing Adsorbers or PSA’s. To achieve smooth operation, multiple vessels must be operated in parallel. Molecular sieves have been successfully employed for treating landfill gas for sometime.

It is important to remember that all of these methods simply separate the acid gas components (CO2 and H2S) from the landfill gas producing an acid gas stream that will require further processing before being released to the atmosphere. At a minimum, incineration in a flare will be required and possibly H2S removal prior to incineration.

Heavy Hydrocarbon Removal

A common method of removing heavy hydrocarbon including halogenated organics is with activated carbon. The heavy hydrocarbons are preferentially adsorbed onto the activated carbon and when the carbon bed becomes saturated the carbon is replaced with fresh material. Generally, at least two carbon vessels are employed in a lead-lag arrangement to ensure that one vessel is always online. Activated carbon has been used to treat a variety of gas streams including landfill gas for quite some time.

Siloxane Removal

Cosmetics, detergents, paper coatings and textiles all contain silicon compounds, which when deposited in a landfill degrade to volatile organo-siloxanes.  Problems occur when these compounds are burned since their product of combustion is silica, which will deposit on cold surfaces in the combustion equipment such as turbine blades and boiler tubes. Many treatment schemes are available for removing siloxanes. The more common methods consist of passing the gas over a bed of activated carbon or activated graphite. Both methods have been employed on landfill gas with good success.

Nitrogen/Oxygen Removal

Landfill gas will always contain some amount of air due to the infiltration of air into the landfill as landfill gas is withdrawn. The amount of air infiltration depends on the position of the wells, with deeper wells having less air infiltration than shallow wells and on the diligence of the operators balancing the gas gathering system. Unfortunately, nitrogen and oxygen are very difficult to remove from any gas. Oxygen is generally removed by passing the gas at elevated temperatures over a catalyst, which uses the oxygen to oxidize some methane to CO2 and water. Membranes and PSA’s can only remove approximately 45% of the oxygen.

Nitrogen is also very difficult to remove mainly because its molecular size is very close to methane. In the natural gas industry, nitrogen is generally removed by cryogenic processes, which are very complicated and expensive and are not adaptable to landfill gas applications. Membranes and PSA’s can only remove approximately 10% of the nitrogen. Consequently, the nitrogen content of landfill gas is generally controlled by adjusting the wells to minimize air intrusion into the landfill.

Recently the Engelhard Corporation developed a new molecular sieve called The Molecular Gate®. This molecular sieve is unique in that the exact size of the pores can be controlled such that molecules with similar sizes in the 3.0 to 4.0 range (N2/CH4, Ar/O2 and N2/O2) can be separated. This technology is relatively new; however, it has been successfully employed on natural gas and coal bed methane. Nitrogen and oxygen removals approaching 80% can be expected with this technology.

Physically, the Molecular Gate system is operated like a pressure swing adsorber. The gas to be treated is passed over the Molecular Gate media until the media becomes saturated with the adsorbed components. The adsorber is then taken off line and depressured, releasing the adsorbed components. As with a standard PSA, multiple vessels operating in parallel are required to achieve smooth operation.

Although the molecular gate technology has not been employed in processing landfill gas, the proposed processing scheme as described later includes a great deal of pretreatment of the gas prior to entering the molecular gate unit; consequently, the technical risk associated with a beta unit of this technology is greatly reduced. One common complaint of the molecular gate technology is cost.

PSA/Membrane/Molecular Gate System

Figure 1 illustrates a processing scheme employing a PSA, a membrane and a molecular Gate to achieve high BTU gas from landfill gas. Landfill gas is removed from the landfill via a landfill gas blower. In the illustration a constant draft is maintained on the landfill well system by controlling the speed of the blower with a variable frequency drive (VFD). Other control methods can be utilized such as maintaining a constant flow of landfill gas through the system. Since the landfill gas will contain varying amounts of oxygen, it is best to remove the H2S prior to further processing to prevent the formation of elemental sulfur in downstream processing equipment. For this application, a H2S scavenger is being employed in a lead—lag arrangement, which in essence will remove all of the H2S. The gas is then compressed to approximately 215 psig, which is controlled via spillback to the suction of the compressor. The gas is then cooled to less than 125 oF and then passed through coalescing filters to remove any entrained liquid droplets.

The gas then passes through a standard PSA which removes a large portion (approximately 68%) of the CO2 and all of the moisture. Methane loss is minimal (approximately 7.5%). The permeate stream from the PSA is the only outlet for the removed CO2. The permeate is directed to the flare, and the residue is passed through a lead-lag activated carbon system which removes nearly all of the siloxanes and heavy hydrocarbons. The effluent gas is then filtered to remove carbon dust before entering the membrane system. The membrane has a high selectivity for removing CO2; however, it also has relatively high methane lost (approximately 15%). To recover the lost methane, the permeate stream is recycled back to the suction of the compressor. 
This initial processing scheme (PSA/Activated Carbon/Membrane) is the MEDAL process as marketed by Air Liquide.

The methane rich effluent from the membrane is still contaminated with N2 (~16%), O2 (~1.7%) and CO2 (~3.8%) which lowers the heating value of the gas to approximately 785 BTU/CF. To increase the heating value to approximately 950 BTU/CF, the gas is then passed through a Molecular Gate which removes all of the remaining CO2 and approximately 80% of the N2 and O2 at the expense of a 10% lost of methane. To reduce this loss, 70% of the permeate stream is recycled back to the compressor suction. The effluent tail gas from the Molecular Gate has a heating value of approximately 300 BTU/CF. Consequently, this gas stream may be utilized in a gas engine or it can be sent to flare. The final product gas has a heating value of approximately 950 BTU/CF and the overall system has a methane recovery of approximately 88%. 

Selexol Process 

A schematic flow diagram of a Selexol system treating landfill gas is illustrated in Figure 2. As previously described, the Selexol solvent is a physical solvent which means that components are absorbed by their relative solubility in the solvent. For landfill gas applications, all of the components contained in landfill gas have high solubilities in the solvent with the exception of methane, nitrogen and oxygen. Consequently, much of the processing to upgrade landfill gas to pipeline quality can be done in the Selexol process. 

As in the previous example, the landfill gas is transferred from the landfill through a gas gathering system via a landfill gas blower. The gas is then passed through a H2S scavenger system and then compressed to approximately 215 psig. The gas then passes counter currently to the Selexol solvent in a Stage 1 Selexol absorber. The contaminant laden solvent from this absorber is then directed into an air stripper for contaminant removal. The contaminant laden air is sent to an incineration system prior to exhausting to the atmosphere. The gas then passes through a second stage absorber before being directed to N2 and O2 removal. The spent solvent is regenerated by three depressurization steps.

Summary

In conclusion, there are several technologies which have been demonstrated to be capable of upgrading landfill gas to pipeline quality gas. The difficulties with upgrading landfill gas are not associated with the technical aspects of the project but rather with the operational and maintenance aspects of the systems.  As illustrated in Figure 1 and 2, the required processing schemes involve equipment and complexity that are common to the petroleum, gas processing and chemical industries but are not common to a landfill operation. This is the main reason why some systems have failed in the past. However, with today’s energy prices, the recovery of landfill gas is no longer an environmental nicety that must be subsidized with tax credits. It presents a substantial amount of income and thus can support a higher grade of operating personnel. For example, recovering 5,000 scfm of landfill gas and upgrading it to pipeline quality gas represents approximately $8,000,000 a year in revenue, if it is sold at $8/MMBTU. 

Another situation, which may become a problem, is the operation of the landfill gas gathering system. All processing systems operate better if the composition and flowrate of the stream to be process remains relatively constant. Variability in composition and flowrate will complicate the operation to some degree. There needs to be some coordination between the landfill operator and the operator of the gas recovery system.


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