High BTU landfill gas recovery utilizing pressure swing mdea process

Waste Management & Research (1987) 5, 133-139

HIGH BTU LANDFILL GAS RECOVERY UTILIZING PRESSURE SWING MDEA PROCESS Harold L . Dinsmore $ (Received 28 November 1986)

The extraction and processing of landfill gas to pipeline quality specifications is a viable method for recovering landfill gas . A plant operating at the Central Disposal Sanitary Landfill in Pompano Beach, Florida, employs a proprietary process involving a methyldiethanolamine (MDEA) based solvent . The process utilizes a combination pressure-swing and thermal solvent-regeneration procedure which dramatically reduces energy requirements relative to those of traditional amine processes . Much of the equipment is skid mounted and modularized which reduces field installation costs and facilitates future removal to other sites . Key Words-Landfill gas, pipeline quality, methyldiethanolamine (MDEA), pressure-swing, thermal regeneration . 1 . Plant design requirements The sales-gas contract at Pompano Beach is with Florida Gas Transmission Company . As such, the plant is required to produce high BTU, pipeline quality gas . The plant is designed to recover and process landfill gas to a demanding set of specifications which include the following : (1) high gas throughput, approximately 220 km 3 day- ' (8 M.M SCFD) of total raw landfill gas including processed gas and that used for fuel in engines and solvent regenerators ; (2) high sales-gas heating value 36 .6 MJ m -3 (980 BTU/SCF); (3) low sales-gas CO 2 content 1 .0 vol% ; (4) low sales-gas H Z S content 23 mg m -3 (1 .0 Grain/100 SCF) ; (5) low sales-gas moisture 80 mg m -3 (5 lbs/MMSCF) ; and (6) high sales-gas pressure 6800 kPa (975 psig) . 2 . Process description The gas recovery system selected for the Central Disposal landfill gas recovery project includes three major processing units which are shown schematically in Fig . 1 . These three processing units include a compression unit, an amine-treating unit (ATU), and a dehydration unit . The compression unit has several functions . It creates sufficient vacuum to extract the gas from the landfill, it pressurizes the landfill gas to enable downstream gastreatment processes to operate properly, it partially removes moisture from the landfill gas, and it provides sufficient pressure to enable sales gas to flow into customer's gas transmission line . The compressor unit actually consists of two parallel compressor trains each rated for 50% of the system design flowrate . Each compressor train (see * Based on a contribution to the GRCDA Landfill Gas Symposium, Newport Beach, CA, March 1986 . + Manager Resource Recovery Department, John Zink Company, Tulsa, OK 74170, U.S .A . 0734-242X/87/020133 + 07 $03 .0010

© 1987 ISWA



134

H . L . Dinsmore C02 vent

H20 vent

Sales gos Fuel gas Gas-well collection system

Raw I.f. gos 10

Compression units stages land 2

4-

Amine treating unit (ATU)

Stage 3 booster

Dehydration unit (DHU)

1 Fuel gos

Landfill

Hydrocarbon -+ condensate to disposal

Water to disposal

Fig .. 1 . Scheme to show overall process for landfill gas recovery plant producing pipeline specification gas .

Fig . 2) consists of three compression stages . The first stage is a flooded rotary screw compressor ; the second and third stages are a reciprocating compressor . Both the screw and reciprocating compressors of each train are driven by an internal-combustion gas engine fuelled by landfill gas . The raw landfill gas flows into a first-stage suction scrubber which removes any entrained liquid in order to protect the first-stage compressor . The first-stage com-

Fuel gos Scrubber Gas cool

r

2nd STG comp

Scrubber

Gas cool Feed Lgusto ATU I I °O

F L-

Engine

Ist STG comp

I

3rd STG comp Gas cool oc

Gas-well collection system

Fuel gas to ATU

-+ Water

Sales gas

Fig . 2 . Scheme to show compression unit process .

Treated gas from DHU



High BTU landfill gas recovery

135

pressor is a flooded rotary-screw type which compresses the gas to about 725 kPa (105 psia) . The discharge flows into an inter-stage cooler which removes the heat of compression and condenses water from the gas stream by cooling the gas . The condensed water and gas then flow into the second-stage suction scrubber which is designed to remove the condensed liquids and restrict entrainment in order to protect the second stage of compression . The second stage of compression is actually the first stage of a reciprocating compressor . In the second stage the gas is compressed to about 2000 kPa (300 psia) . The discharge then flows into an inter-stage cooler where the heat of compression is removed and water is condensed by cooling the gas . The gas then flows into the gastreating portion of the landfill gas recovery system where CO 2 and HZ O are removed . After the gas has been treated, it flows back to the compression unit's third-stage of compression which is actually the second stage of the reciprocating compressor . This gas is compressed to meet the sales-gas transmission line requirements of 6820 kPa (990 psia max .) . From the third compression stage, the gas flows into an after cooler which removes the heat of compression, it then flows into the sales-gas line . A single engine drives the screw and reciprocating compressors of each of the two compression units . The engine uses landfill gas for fuel which is taken from the secondstage suction scrubber . Fuel gas used in the amine treating unit is also taken from the second-stage suction scrubber . Compressed landfill gas flows from the second-stage of the compressor units to the ATU which removes the carbon dioxide from the landfill gas . If present in the landfill gas, hydrogen sulphide will also be removed by the ATU as well . The ATU is designed to utilize Gas Spec SS-JZ1 solvent which is produced by Dow Chemical Company for use in John Zink Landfill Gas Recovery Systems . The solvent has a methyldiethanolamine (MDEA) base and is used to absorb carbon dioxide from the landfill gas . For this application it has a number of advantages which include : (1) high carbon dioxide removal and methane recovery efficiencies ; (2) production of very high BTU sales gas ; (3) being a tertiary amine based solvent, it has much greater resistance to degradation, has lower corrosion rates, and requires less energy for regeneration than other primary or secondary amines ; (4) because it is utilized as a 50 wt% aqueous solution, it has a lower solubility for hydrocarbons than competitive physical solvents ; (5) it does not require extensive gas pretreatment to remove landfill-gas trace contaminants ; and (6) it is effective in the removal of hydrogen sulphide and carbon dioxide from the landfill gas . The ATU includes an amine-contactor column, a low-pressure flash drum, an amine stripper column and reboiler, solvent coolers, pumps and a reflux system . The basic setup of the ATU is depicted in Fig . 3 . The ATU is designed to remove acid gases (primarily carbon dioxide) from the gas . Carbon dioxide reduces the overall BTU (heat) content of the recovered gas and must be removed if higher BTU or pipeline-quality gas is to be produced . The gas to be treated flows into the lower section of the amine contactor and passes up through a series of trays . Counter-current to the gas flow, semi-lean and lean amine streams flow down the contactor making contact with the gas stream on the trays . The contactor column is effectively divided into two zones where the CO, is to be



1 36

H. L . Dinsmore

t

Co vent

t



~ Wet high-BTU gas to dehyd. unit

amine Semi-lean amine Ref l u drum

Low press flash

Landfill Gas from comp. unit

Contactar l

A

00

Reboiler

Rich amine

Fuel gas trom compression unit

Fig. 3 . Scheme to show the amine treating unit process .

removed . Bulk removal of CO 2 occurs in the lower section of the column as the C0 2 rich gas comes into contact with a semi-lean stream of the amine . Rich amine is circulated from the bottom of the contactor column to a flash drum where, by pressure reduction, a substantial quantity of the CO 2 is removed by venting . The partially regenerated solvent (semi-lean amine) is recirculated from the flash drum back to the lower section of the contactor column to be contacted with the C0 2 -rich gas stream . It is this procedure of partially regenerating the amine solvent, by utilizing the pressureswing technique, which differentiates the process from conventional amine gas treating processes and dramatically reduces solvent regeneration energy requirements . The gas flow continues up the contactor column where the gas stream is contacted with thermally regenerated solvent (lean amine) which reduces the CO 2 content of the gas stream to the levels in the design specifications . The lean amine is produced by taking a relatively small slip stream of semi-lean amine from the flash drum (discussed previously) into a stripping column where it is thermally regenerated . The semi-lean amine flows into the top section of the stripping column and down through the contacting medium . Hot vapours from the amine reboiler rise up the column counterflow to the amine, contacting the amine and stripping it of the CO 2 it has retained from the flash drum . A gas-fired amine reboiler, using landfill gas for fuel generates the hot vapours required for the stripping . The hot vapours are carried from the top of the stripping column through an overhead steam condenser . The condensate and the CO 2 are separated in the reflux drum and the condensate is pumped to the top of the stripping column as cool reflux . The CO 2 and some water vapour are vented to the atmosphere . The lean amine is pumped through a cooler and back to the contactor column completing the cycle .



High BTU landfill gas recovery

137 H20 vent

Gas/gas exch .

Gas chiller Cold sep .

Wet high-BTU gas from amine treatment unit

#2 4 0

3Phase sep .

Leon ethylene glycol

Pipeline-spec . gas to sales gas Booster

i Reconcentrator Ref rig . unit Rich ethylene glycol Fuel gas

•

Hydrocarbon liquids to disposal

Fig . 4. Scheme to show the dehydration unit process .

Activated carbon filtration is provided for the rich amine stream to remove contaminants picked up by the amine and any degradation products which could cause problems with the amine solution . The treated gas, now reduced in volume by the removal of the CO, flows to the dehydration unit for removal of moisture from the gas . The primary purpose of the dehydration unit is to reduce the moisture content of the gas to below the required maximum of 80 mg m -3 (5 pounds per MMSCF) . The process (schematically illustrated in Fig . 4) utilizes mechanical refrigeration to chill the gas to approximately - 18°C (0°F) . In order to prevent hydrate and/or ice formation at this temperature, a small quantity of ethylene glycol is injected into the gas before chilling ; the moisture condensed from the gas at the low temperature is absorbed by the glycol . The ethylene glycol is separated from the dry gas, reconcentrated, and recirculated . The dry sales gas is further pressurized and metered into the sales pipeline .

3 . Plant performance The plant at Pompano Beach was put on stream in October of 1985 . Since that time, operating data have proven that the plant has met performance expectations and has had no trouble producing sales gas with the required specifications . Table 1 lists a typical set of test data and compares the plant design specifications with expected performance . An analysis of the data leads to the conclusions given below .

3 .1 . Raw-gas feed rate The data reported are for a plant raw-gas feed rate of approximately 46% of design . The plant, to date, has not been operated at capacity for a number of reasons not



138

H. L . Dinsmore

TABLE I Comparison of actual and design plant performances, 28 October 1985 Design Raw landfill gas feed Flowrate, (dry basis), km 3 day - ' (MM SCFD) Composition (dry basis) Methane (CH 4 ), vol% Carbon dioxide (CO 2 ), vol% Oxygen (02), vol% Nitrogen (N A ), vol% Hydrogen sulphide (H 2 S), ppmv Moisture Temperature, °C (°F) Pressure, P (inches water) Sales-gas product Flowrate, km 3 day- ' (MM SCFD) Carbon dioxide, vol% Hydrogen sulphide, mg M-3 (GR/100 SCF) Moisture, mg m-3 (LBS/MMSCF) Oxygen, vol% Heating value (HHV) MJ M-3 (BTU/SCF) Temperature, °C (°F) Pressure, kP (PSIG) Fuel-gas consumption, kJ m- ' (MMBTU/HR) Electrical requirement, kW Methane-recovery efficiency, Overall Less engine fuel

218

(7 .69) max .

56 .0 40.0

Actual 100

(3 .52)

55 .2 42 .4 0 .2 1 .5 600

0 .1 2 .4 0 49 - 20

Saturated (120) max . (- 80) min .

105

(3 .71) max .

1 .0 max . 2 .3 (1 .0) max . 80 (5 .0) max .

1 .0 max . 36 .6 (980 min . 49 (120) max . 6700 (975) max . 32 .5 (30.8) 390

82 .6 93 .7

Saturated 43 -8 .6

(110) (-35)

48 (1 .70) 0 .78 0 .44 (0.19) 43 (2 .7) 0 .4 36 .5 (977) 27 (80) 3450 (500) 12 .7 (12 .0) 390

93 .2 94 .6

related to plant equipment . However, it is planned that plant throughput will be increased as additional gas wells are brought on-line . Based on projected plant performance, no operational problems are expected when this occurs .

3 .2. Sales-gas quality specifications The plant has had no trouble meeting required sales-gas quality specifications . This includes meeting the relatively stringent requirements for CO z , H 2 S and moisture . On the particular date (28 October 1985) that this test data was taken, which was soon after start-up, the heating value of the sales gas was 36,500 kJ m -3 (977 BTU/SCF) which was slightly below the specified 36,600 kJ m-3 (980 BTU/SCF) . This required a small amount of propane injection . However, propane injection has not normally been required because the heating value of the sales gas has been consistently in the 36,80037,200 kJ m -3 (985-995 BTU/SCF) range . The test data reflected a slightly lower heating value due to a higher than normal nitrogen concentration in the sales gas . The higher nitrogen concentration was due to overdrawing some of the gas wells and has subsequently been corrected .



High BTU landfill gas recovery

139

3 .3 . Plant utility consumption

Plant utility consumption is within performance expectations . In fact the test data listed in Table 1, show the plant to be operating at 46% of the design throughput while utilizing only about 39% of the design fuel-gas consumption and approximately 46% of the design electrical consumption . The lower than expected fuel-gas requirement is due to lower than design sales-gas pressure requirements meaning less compressor horsepower requirements. Also, less fuel gas is required for amine thermal regeneration than projected because the amine pressure-swing regeneration procedure has been more effective than expected . Thus, less lean-amine circulation has been required for proper CO2 removal. 3 .4 . Methane recovery efficiency

Plant methane recovery efficiency has been better than expected . The plant was designed to give an overall methane recovery efficiency of 82 .6% ; in actual operation the overall plant methane recovery efficiency has been 93 .2% . The better than expected recovery efficiency has been realized primarily because of the lower fuel-gas requirements . 4. Summary The technology described herein has been proven to be a viable process to extract and upgrade landfill gas to a high BTU product. Although there are certainly a lot of other competitive processes and technologies available, we believe that the MDEA process using pressure-swing regeneration has many desirable and proven features which may make it more attractive than other technologies particularly for larger landfill gas recovery projects and/or where stringent sales-gas quality is required .