Nitrogen Rejection and C02 Removal Made Easy

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MOLECULAR GATE® ADSORPTION SYSTEM FOR THE REMOVAL OF
CARBON DIOXIDE AND / OR NITROGEN FROM
COALBED AND COAL MINE METHANE (page 2 of 3)

by Michael Mitariten, P.E., Engelhard Corporation

Western States Coal Mine Methane Recovery and Use Workshop
Two Rivers Convention Center
Grand Junction, CO
April 19 - 20, 2005

Updated January 2009

Continued from Page 1

Gas Production from CO2 Contaminated Coal Bed Methane

Coal bed methane is produced from various basins in the USA, typically from shallow mines with gas produced at the surface at low pressure.  Contamination with CO2 is common and the prolific San Juan basin requires the operation of many amine plants for removal of CO2.  Other basins have some level of contamination, though less widespread, including coals from the Black Warrior, Powder River, Illinois and Appalachian basins.
 
While amine plants are a traditional solution for the removal of CO2, there are drawbacks for the technology and for certain applications, Molecular Gate adsorption systems can offer an attractive alternative.  In amine system, the amines solvents can degrade and cause corrosion, especially from oxygen contamination that is a concern with low wellhead pressures, with resulting downtime and operational challenges. 

Amine systems generally operate at high pressure requiring compression of a wet, CO2 rich feed.  This presents challenges in the compression design and operation with corrosion and on-stream factor issues for the multi-stage compressors commonly used.  The amine plant also produces a water saturated product stream that requires glycol dehydration to meet pipeline water specifications.  In comparison the Molecular Gate adsorption system operates at a relatively low pressure, typically at the discharge of a screw compressor, for which corrosion is a minimal concern and the dry, low CO2 product stream presents no concern for a product reciprocating compressor.

Gas Production from GOB and Abandoned Coal Mines

Abandoned coal mines must be evaluated for their composition and gas production.  Both of these items are difficult to accurately establish, since the shut-in wells are typically at substantial pressures, as high as 40 psig.  The flow rate of gas will decrease and its composition may change as the gas is withdrawn.

The gas capacity of the abandoned mine is a function of the coal characteristics, volume of remaining coal and size of the mine.  We have seen wells under fairly high pressures of 20-40 psig, whose capacity would be misleading if one simply expected the initial flows from depressurizing wells to be maintained.  In general, it is prudent to install upgrading equipment of a capacity that can be fully utilized while including the ability to expand capacity if the gas production proves to be greater than initially assumed. This is the typical approach for Molecular Gate adsorption system design and takes advantage of the ease and cost effectiveness by which the system can be expanded, typically to twice the initial rate.

The feed gas composition can change, as the pressure of the mine is reduced, and a larger portion of the produced gas comes from gas desorbing off the coal rather than that from the mined out area of the mine.  The composition change is not well defined. And in this consideration, the ability to remove both carbon dioxide and nitrogen along with flexibility for changes in composition is critical for continual production of pipeline quality gas.

Where the gas is produced from the GOB from active mining, the flow and composition variations are larger due to changing production patterns.  The Molecular Gate adsorption system is flexible to such changes and can always produce pipeline quality gas regardless of the feed level of CO2 or N2.

A small 1 MM SCFD unit designed to upgrade CMM

Figure 5.. A small 1 MM SCFD unit designed to upgrade CMM

Figure 5 shows a small Molecular Gate unit that was installed in the Illinois basin.  This feed gas contained a few percent CO2 and up to 10% N2. As with most units in low-pressure applications, the feed gas was compressed to 100 psig in a gas engine driven screw compressor burning the tail gas from the process.  The product gas after the Molecular Gate unit was further compressed from 90 psig to 600 psig in a two-stage electric driven, reciprocating compressor for sale into an interstate pipeline.  Since both the vacuum pump and product compressor were electric driven, a gas engine driven genset (also burning tail gas as fuel) was used to provide power.  Due to the use of tail gas as fuel there was no direct loss of methane from the system.

The small unit allows for the mounting of the vessels and vacuum pump on a single skid.  To minimize the installation costs, the system was mounted on timbers and gravel and moved as a single unit.

Equipment of the Molecular Gate Adsorption System

The basic equipment required for the removal of the impurities consists of multiple adsorber vessels filled with adsorbent and a valve and piping skid that is placed alongside the adsorber vessels and serves the purpose of switching flows between adsorber vessels as they cycle between the process steps of adsorption, depressurization, regeneration and repressurization. 

It is common to regenerate the system with a single stage vacuum compressor to maximize the regeneration of the adsorbent.  This vacuum pump is generally electric driven and, depending on its size, can be on the main valve and piping skid or on its own separate skid for installation alongside the Molecular Gate unit in the field.

The overall system control system, often including feed and product compression, is controlled by an integrated control system that provides for push-button start-up. The control system operator interface can be supplied with the ability to remotely monitor the system via a modem connection.

A 2.5 MM SCFD unit designed to upgrade CMM (plus contaminated natural gas)

Figure 6.  A 2.5 MM SCFD unit designed to
upgrade CMM (plus contaminated natural gas)

A photo of a larger 2.5 MM SCFD unit in the Illinois basin is shown in Figure 6.  This unit started operation n 2002 and upgrades gas from a series of wells from abandoned coal mines and nitrogen contaminated natural gas.  Since many wells feed this unit and the sources vary, the system sees a range of feed compositions.  In general, the feed contains up to 6% CO2 and up to 15% N2 with a pipeline specification of 4% inerts. 

The system consists of an oil flooded screw compressor and two-stage product compressor, tail gas vacuum pump, four adsorber vessels and a valve and piping skid.  Buffer tanks to smooth the fluctuations in the flows of certain streams are also provided.
 
Unlike the smaller 1 MM SCFD unit in Figure 5, the four adsorber vessels for this unit are mounted on a concrete pad and placed alongside the valve and piping skid.  The vacuum pump is next to the skid on its own skid package.

At this site, drives are electric and tail gas is burned in gensets to meet the power needs of the site.

Operating Features

The Molecular Gate adsorption system operates unattended and can be monitored remotely.  Where maintaining an inert level within a small window is critical, a product analyzer can be used to adjust the operating conditions of the system.  Such an analyzer allows the unit to automatically compensate for changes in the feed composition or pipeline requirements.

From a zero pressure condition, start-up can be conducted with delivery of product gas to the pipeline within 15 minutes.  Control, operation and monitoring of the unit can be conducted locally and monitored through a remote station and a modem connection. 

The system can deliver a high on-stream factor.  For nitrogen rejection units this is, in part, because it eliminates the need for a separate carbon dioxide removal unit, has limited critical items, and continues operation in the event of the failure of certain components.   This reliability, combined with the unattended operation and occasional monitoring by the pumper, results in minimal operation and maintenance costs.

Commercial Molecular Gate adsorption systems are offered as a complete unit with maximum skid mounting of equipment for minimal installation cost.  The shop-fabricated, modular systems are delivered with minimal coordination required from the user.

Tail Gas Use

Because the Molecular Gate adsorption system does not recover all the methane and loses a portion into the tail gas, the use of the tail gas is a process optimization for each project.  Where the coal mine methane feed gas contains relatively low levels of inerts, the tail gas from the Molecular Gate adsorption system has a sufficient heating value to provide fuel to the feed compressor and/or pipeline compressors. 

The feed compressor, product compressor and vacuum pump can consume in the range of 8-12% of the methane in the feed.  This fuel demand permits the balancing of the methane recovery by the Molecular Gate unit with the fuel demand (where the tail gas is burned as fuel).  In this manner, there is essentially no loss of methane from the system.  Burning the low heating value tail gas requires air/fuel control adjustments in the compressor.  An automatic control system tailored to the application has been incorporated into several systems to allow robust operation.

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Further Information:

If you would like an evaluation of how the Molecular Gate technology can solve your gas treatment needs simply complete and email back the Estimate Request Form or contact Paul Baker at 614-760-8013 or by email info@moleculargate.com.

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