CONVERSION OF LANDFILL GAS TO VEHICLE FUEL: CURRENT STATUS

 

 

Jeffrey L. Pierce, P.E.

Vice President

SCS Engineers

3711 Long Beach Boulevard

9th Floor

Long Beach, California 90807-3315 USA

jpierce@scsengineers.com

 

 

 


Abstract

 

One of the more interesting potential uses for landfill gas (LFG) is the production of vehicle fuel.  LFG can be used to produce compressed natural gas (CNG) or liquefied natural gas (LNG).  CNG or LNG can be used to: 1) fuel vehicles at the landfill (water trucks, earthmoving equipment, light trucks, autos, etc.); 2) refuse hauling trucks (long haul refuse transfer trailers and route collection trucks); and 3) supply the general commercial market.

 

The paper will review the technologies now being used in an existing CNG project and at several now under construction LNG projects.  The paper will discuss: 1) technology; 2) performance; 3) LFG supply issues; 4) fuel quality issues; and 5) economics.

 

Gas Fuels Versus Liquid Fuels

 

Liquid fuel currently dominates the vehicle fuel market.  Diesel and gasoline are the liquid fuels now in use.  Diesel is generally used to fuel buses, heavy duty trucks and heavy equipment.  Gasoline is generally used in automobiles, vans and light trucks.  It is not uncommon, however, to see diesel used in light trucks, vans and automobiles.

 

Gas fuels are beginning to displace liquid fuels.  Gas fuels include compressed natural gas (CNG) and liquefied natural gas (LNG).  Gas vehicles burn fuel in a gaseous state and require a pressure of 75 psig to 125 psig.  If CNG is stored on the vehicle, its storage pressure is typically 3,000 psig.  Its pressure is reduced prior to delivery into the engine.  If LNG is stored on the vehicle, it is stored at 75 psig to 125 psig and the LNG is vaporized prior to delivery to the engine.  The performance of the engine is indifferent to whether the gas is supplied by CNG or LNG.

The principal advantage of gas fuels versus liquid fuels is in the area of air emissions.  Gas fuels have lower air emissions.  The actual reduction in air emissions depends on whether the displaced fuel is gasoline or diesel and on the specific engine the comparison is based upon.  Typical air emission reductions that can be expected for gas fuels versus diesel fuel are as follows:

 

Air Pollutant

Reduction

Nitrogen Oxides (NOx)

60% to 85%

Particulates

60% to 80%

Carbon Monoxide (CO)

10% to 70%

Non-Methane Organic Compounds (NMOC)

10% to 85%

 

Gas fuels have benefits beyond reductions in air emissions.  Natural gas is generally less expensive than liquid fuel (on an energy content basis).  To some extent this is due to taxation policies; however, natural gas is generally less expensive than liquid fuels prior to tax.  Gas fuels are arguably safer than liquid fuels since gas fuels are carried in stronger tanks, have a higher ignition temperature, have a higher lower flammability limit, and will dissipate to the atmosphere rather than spill on the ground.

 

Some proponents of gas fuels argue that gas fuels lower long-term engine maintenance costs; however, limited information is presently available in support of this claim.

 

The principal drawbacks to gaseous fuels are as follows:

 

·         The engines are more expensive; and

·         Liquid fuels are more readily available.

Liquid fuels can be purchased virtually anywhere.  Gas fuels fueling stations are difficult to find.  Gas fuels have made its greatest inroads in the fleet market (buses, taxis, etc.) where the fleet owner can establish his own fueling network.

 

LNG Versus CNG

 

The principal advantage of LNG over CNG is that it has a higher "energy density."  The same volume of LNG has 3.5 times the energy as CNG.  The actual benefit is, of course, that much less volume is required for LNG storage.  The reduction in volume has three benefits: 1) vehicles using LNG require less volume for on board fuel storage; 2) it is less expensive to store LNG than CNG at the point of production and at remote distribution stations; and 3) it is less expensive to transport LNG than CNG from the point of production to remote distribution stations.  The characteristics of the currently available vehicle fuels are as follows:

 

 

Gasoline

Diesel

CNG

LNG

Btu/ft3

16,710

18,980

3,230

11,310

Btu/gal

125,000

142,000

24,160

84,570

Btu/lb

20,000

19,410

23,890

23,890

Note: CNG as a gas at 3,000 psig.  Other fuels as liquids.

 

Because of its greater energy density, LNG is more often employed for larger, heavy use vehicles.  LNG is sometimes transported to fuel distribution stations where it is vaporized and pressurized to CNG.  LNG can be delivered in tank trucks containing 2,000 to 10,000 gallons.  CNG must be transported in tube trailers which can carry a maximum of 100,000 standard cubic feet of CNG.

 

Landfill Gas and Gas Fuel Quality Issues

 

The principal component of landfill gas (LFG) is methane.  The principal component of CNG and LNG is methane.  For this reason, LFG can provide a feedstock for CNG and LNG production.

 

Typical LFG composition is as follows:

 

 

 

 

 

 

 

Component

Range

Methane

35% to 55%

Carbon Dioxide

20% to 45%

Oxygen

0.5% to 5%

Nitrogen

2% to 45%

Carbon Monoxide

<10 ppmv

Hydrogen Sulfide

20 to 500 ppmv

Non-Methane Organic Compounds

200 to 2000 ppmv

 

Pure LFG has a methane content in the range of 55 percent to 57 percent, and contains virtually no oxygen or nitrogen.  Oxygen and nitrogen are introduced into LFG by air drawn into the landfill during LFG extraction from the refuse and/or due to air leakage into the LFG collection piping.

 

Fuel quality standards governing gas fuels are set by regulatory agencies such as the California Air Resources Control Board (CARB) or by independent organizations such as the Society of Automotive Engineers (SAE).  It is necessary for gas fuels produced from LFG to satisfy gas fuels standards in order for gas fuels to be considered interchangeable with gas fuels.  CARB1 standards as follows:

 

Methane

³ 88%

Ethane (C2)

£ 6%

C3 and higher

£ 3%

C6 and higher

£ 0.2%

Hydrogen

£ 0.1%

Oxygen

£ 1%

Carbon Monoxide

£ 0.1%

Inert Gas (CO2 + N2)

£ 4.5%

Sulfur

£ 16 ppm

 

Water content is controlled by a dewpoint temperature standard.  The dewpoint at vehicle fuel storage container pressure is to be at least 10º F below the 99.99 percent winter ambient air temperature for the region.

 

Methane contents of 95 percent to 97 percent are requested by many engine manufacturers and 88 percent should be considered the minimum target methane percentage.

 

Landfill gas contains low levels of ethane and C2+ compounds, low levels of hydrogen and low levels of carbon monoxide; thus, the CARB standards for these compounds are easily satisfied.  The virtual lack of ethane and C2+ compounds in LFG-produced gas fuels benefits engine performance, and, in this regard, LFG produced gas fuels are better fuels than gas fuels from natural gas.

 

The challenge in converting LFG to gas fuels is to reduce carbon dioxide, moisture, nitrogen, and oxygen to relatively low levels.

 

The gas fuels specification does not address other trace compounds present in LFG which may cause problems in engines, including siloxanes and chlorinated non-methane organic compounds (NMOC's).  It would be prudent to remove these substances, even if not required by published gas fuels specification.

 

Conversion of Landfill Gas to CNG

 

The principal task in converting LFG to CNG is to increase its methane content or, conversely, to reduce its carbon dioxide, nitrogen and oxygen content.  Three methods have been successfully employed in the United States, beyond pilot testing, to remove carbon dioxide from LFG:

 

·         Membrane separation;

·         Molecular sieve; and

·         Amine scrubbing.

 

There is only one LFG to CNG conversion plant operating in the United States.  It employs membrane separation.  Two projects, which produce high-Btu gas for pipeline sale, employ molecular sieve technology.  Four other pipeline sale projects employ amine scrubbing.  The product gas specifications governing pipeline gas sale are similar to those governing CNG. 

 

All three of the above methods are focused on carbon dioxide removal, and do not remove oxygen and nitrogen.  The oxygen and nitrogen limitations of the fuel specification must be satisfied by wellfield operation and design.  Air intrusion into the wellfield must be minimized.  In some instances, this may require that LFG from interior wells be directly sent to the vehicle fuel plant, and that LFG from perimeter wells be sent to another outlet for use or disposal.

 

Selexol is the most common amine used in LFG service.  A typical Selexol-based plant employs the following steps:

 

·         Landfill gas compression (using electric drive, LFG fired engine drive, or product gas fired engine drive);

·         Moisture removal (using refrigeration);

·         Hydrogen sulfide removal in a solid media bed (using an iron sponge or a proprietary media (such as Sulfatreat) ;

·         NMOC removal in a primary Selexol absorber; and

·         Carbon dioxide removal in a secondary Selexol absorber.

 

In the Selexol absorber tower, the LFG is placed in intimate contact with the Selexol liquid.  Selexol is a physical solvent which preferentially absorbs gases into the liquid phase.  NMOC's are generally hundreds to thousands of times more soluble than methane.  Carbon dioxide is about 15 more times soluble than methane.  Solubility is also enhanced with pressure.  The above principles are exploited to remove NMOC's and carbon dioxide from the landfill gas to yield a purified methane stream.  The Selexol vessels operate at pressures in the range of 500 psi.  The Selexol liquid is regenerated by lowering its pressure (flashing) and then running air through the depressurized Selexol to strip off the NMOC's and carbon dioxide.  The stripper air from the NMOC removal step is normally sent to a thermal oxidizer where all or part of the thermal energy required to support combustion is supplied by the NMOC's and methane in the stripper air.  The stripper air from the carbon dioxide removal step is normally vented to the atmosphere.

 

A typical molecular sieve plant employs the above-described compression, moisture removal and hydrogen sulfide removal steps, but relies on vapor phase activated carbon and a molecular sieve for NMOC and carbon dioxide removal, respectively.  The activated carbon removes NMOC's and protects the molecular sieve.  The molecular sieve is a vessel which contains a media which preferentially adsorbs certain molecules (in this case, carbon dioxide) when contacted with a gas stream which is under pressure.  When the media is exhausted, the vessel is brought offline and is regenerated through a depressurization and purge cycle.  The activated carbon can also be regenerated on site through a depressurization and purge cycle.  For this reason, the process is often called pressure swing adsorption.  The purge streams are generally disposed of in a thermal oxidizer.  The thermal oxidizer generally requires some supplemental energy which can be provided by LFG or product gas.

 

A typical membrane plant employs the above-described compression, moisture removal and hydrogen sulfide removal steps, but relies upon activated carbon and membranes for NMOC and carbon dioxide removal, respectively.  Activated carbon removes NMOC’s and protects the membranes.  The membrane process exploits the fact that gases, under the same conditions, will pass through polymeric membranes at differing rates.  A “fast” or highly permeable gas such as carbon dioxide will pass through a membrane approximately 20 times faster than a “slow” or less permeable gas such as methane.  Pressure is the driving force for the separation process.  The feed gas (LFG) and product gas (predominantly methane) enter and exit the membrane module at approximately the same pressure.  The permeate gas (predominantly carbon dioxide) exits at a lower pressure.  The operating pressures, number of membrane stages in the series, and provisions for gas recycle depends on desired methane recovery percentage and desired product gas methane purity.  In natural gas processing applications, both methane recovery percentage and desired product gas methane purity are highly important.  In LFG applications, product gas methane purity is of importance; however, methane recovery as a percentage of methane in the raw LFG is of less importance.  The membrane configuration employed for an LFG utilization project should strike the optimal balance between capital cost and methane recovery.  The activated carbon regeneration stream and the reject carbon dioxide stream from the membranes are typically directed to a thermal oxidizer for disposal.

 

The membrane process and the molecular sieve scales down more economically to smaller sized plants, and for this reason these technologies are more likely to be used for CNG production than Selexol.

 

The Los Angeles County Sanitation District (LACSD) owns the only operating LFG to CNG facility in the United States.  A few other projects are reportedly under development.  The LACSD facility is located at their Puente Hills Landfill in Whittier, California, and has been in operation for over 8 years.  It converts an inlet flow of 250 scfm at 55 percent methane to 100 scfm of CNG at 96 percent methane.  The product is equal to about 1,000 gallons of gasoline equivalent (GGE) per day.  While the facility is rather small from an LFG perspective, it is significant from a vehicle fuel perspective.  At a fuel economy of 20 miles per gallon, this facility could support 20,000 trip miles per day.  If a fleet vehicle traveled 200 trip miles per day, the LACSD facility could support 100 vehicles.  About 70 percent of the methane in the landfill gas is converted to product CNG, with about 30 percent of the methane lost in the waste gas stream.  Higher methane recoveries would be possible with a more expensive membrane configuration.  A 30 percent loss of feedstock would be unacceptable in other industries; however, LFG is a low or no cost feedstock and relatively poor methane recovery is an acceptable trade-off for lower construction costs.  LACSD’s experience with the operation of the Puente Hills facility and their experience with vehicles running on CNG has been summarized in two papers by Wheless, et.al2 3.  LACSD considers the CNG project to be a technical success.

 

The process chain for CNG production at Puente Hills is as follows:

 

·         Landfill gas compression and moisture removal.  Compression is undertaken in multiple stages to reach 525 psi;

·         Vapor phase activated carbon;

·         Gas heating to 140º F;

·         Three stages of membrane separation;

·         Multi-stage compression of the product gas to 3,600 psi;

·         Compressed gas storage facilities; and

·         A fuel dispenser to dispense 3,000 psi CNG.

 

The permeate gas (waste gas) from the first stage and part of the gas from the second stage of membranes contains 25 to 30 percent methane and is blended with other LFG now being fired in a steam cycle power plant, which is located on-site and which pre-existed the CNG facility.  At a typical CNG installation, the waste gas would be flared.  The permeate gas from part of the second stage and the third stage membrane is relatively rich in methane (about 80 percent) and is recycled back to the inlet of the LFG compressors.  The activated carbon is regenerated on-site.

 

The construction cost of the Puente Hills CNG facility was $1.4 million (cost escalated to 2002 dollars).  The Puente Hills project is a small demonstration project, and its costs are not representative of a larger project.  Wheless, et.al estimates the total cost of CNG production for a membrane facility to be as follows:

 

Inlet LFG

(scfm)

Plant Size

(GGE/day)

Cost

($/GGE)

250

1,000

$1.19

500

2,000

$0.95

1,250

5,000

$0.76

2,500

10,000

$0.68

5,000

20,000

$0.57

           Note: Wheless' costs escalated to 2002 dollars.  GGE means gallons of gasoline equivalent.

 

Conversion of Landfill Gas to LNG

 

If LFG is first converted to CNG, it could then be liquefied to produce LNG using conventional natural gas liquefaction technology; however, there are two considerations that must be addressed if this approach is employed.  The first is that carbon dioxide freezes at temperatures higher than methane liquefies.  To avoid "icing" in the plant, the product CNG must have as low a level of carbon dioxide as possible.  This preference would favor the molecular sieve over the membrane process or to at least favor polishing the membrane product gas with a molecular sieve.  The second consideration is that natural gas liquefaction plants have generally been "design to order" facilities which process large quantities of CNG.  Smaller, pre-packaged liquefaction plants have begun to be offered by a few manufacturers.  Even these "small" plants have design capacities of 10,000 gal/day or greater.

 

At least one manufacturer, Cryofuel Systems (Monroe, Washington, USA), has begun to manufacture an LFG to LNG processing unit which uses cryogenics to accomplish the gas separation function, and to directly produce LNG.  The details of Cryofuel’s system are proprietary, but the system generally involves the following steps:

 

·         LFG pretreatment to remove moisture and to remove other contaminants, including sulfur compounds and high molecular weight NMOC's.  Chilling and absorption are employed in LFG pretreatment;

·         Separation of carbon dioxide from the LFG through liquefaction of the carbon dioxide.  Carbon dioxide is the first of the principal gases in LFG to liquefy; and

·         The carbon dioxide-free LFG (now almost entirely methane) is then liquefied.  In addition to producing LNG, this step reduces the nitrogen and oxygen content of the product.  Nitrogen and oxygen can be “flashed off,” since the liquefaction temperature of nitrogen and oxygen are both below that of methane.

 

Waste gases from LFG pretreatment and carbon dioxide separation are generally flared; however, it is sometimes possible to apply some of this LFG to a beneficial use which is tolerant of lower methane content (<40 percent) LFG.

 

General performance of a standard system currently offered by Cryofuel Systems will produce 5,000 gal/day of LNG from 900 scfm of LFG.  The product LNG is 97 percent methane, 20 psig and -250º F.  Inlet LFG characteristics are as follows:

 

Components

Nominal

Limits

Carbon Dioxide

38%

45% max

Oxygen

1%

2% max

Nitrogen

13%

35% max

Methane

48%

35% min

Sulfur Compounds

125 ppmv

500 ppmv

Non-Methane Organic Compounds

1000 ppmv

2000 ppmv

 

Deviations from the nominal values will affect performance.

 

The 5,000 gal/day module requires about 750 kW of power, which can be purchased from the local utility or it can be self-generated.  The installed cost of a 5,000 gal/day module with 10,000 gallons of LNG storage, a four line fuel dispenser system, with self-generation of power, would be about $3,500,000.  Annual operating cost would be about $250,000.  At a 90 percent capacity factor, the total LNG cost would be $0.58 per gallon, assuming capital recovery over 10 years at 15 percent.

 

Cryofuel Systems has operated an 850 gal/day pilot plant on LFG at the Hartland Landfill in Vancouver, Canada.  Six of Cryofuel Systems 5,000 gal/day modules are to commence operation at four landfills between October 2002 and June 2003.

 

Conclusion

 

Long-term operational experience with the conversion of LFG to vehicle fuel is limited to one project in the United States.  The conversion of LFG into vehicle fuel does, however, appear to be technically and economically feasible.

 

1 California Code of Regulations, Title 13, Division 3, Chapter 5, Article 3 (Specifications for Alternate Motor Vehicle Fuels).

 

2 Processing and Utilization of Landfill Gas as a Clean Alternative Vehicle Fuel, SWANA 17th Annual Landfill Gas Symposium, March 22-24, 1994, Long Beach, California, USA.

 

3 Converting Landfill Gas to Vehicle Fuel: The Results of Over 30 Months of Operation, SWANA 19th Annual Landfill Gas Symposium, March 19-21, 1996, Research Triangle Park, North Carolina, USA.