This is the heart of the extraction process.
The landfill gas well. Diameter and depth
The optimum diameter for a landfill gas well is in the vicinity of 900mm to 1200mm in diameter. The diameter being dependant on the static pressure of the landfill. If the static pressure is low <1000 Pascals ( one inch gauge pressure = 250 Pascals approx. ) then the diameter of the well should be between 1100mm and as large as 1500 mm where static pressures measure lower than 100 Pascals. If the landfill has a high degree of organic matter and measures a high static pressure >1000 Pascals and other conditions are also favourable then the smaller 900mm diameter well is adequate.
Though initially I installed 600mm diameter wells in the first extraction system that I had designed; I now believe that such small diameter wells are best avoided if possible. Two of the initial 6 wells' production declined to near zero on a periodic basis even though well R1M2 had the initial highest static pressure (350 Pa ).
The well should be drilled out to the full depth of the landfill; or to within 2 metres of the top of the groundwater table: whichever comes first.
( Do not confuse perched water lenses with the natural water table ). With wells greater in depth than 20 metres a dual conversion design should be used. ( Due to the constricting of the suction centroid by the excessive compaction of the refuse ). As all of the landfill gas wells that I have drilled have been less than 20 metres I am yet to use a dual conversion design. The deepest well that I have had drilled is 24 metres ( only one ) whereas the average has been 12 - 16 metres.
Ideally the 1200mm diameter should be the accepted standard for landfill gas wells .
The landfill gas well : Well Pipe
If the standard size well diameter is used ( 1200mm ) then the optimum diameter for the landfill gas well pipe is 200mm and the optimum material is class 12 PVC socketed pipe. ABS square thread is better but substantially more expensive ( in some instances HDPE & MDPE are also used ). A 350mm diameter concrete epoxy coated pipe should be used where the extraction system is used to contain migration of landfill gas into buildings situated immediately above the landfill. ( Buildings or any habitable structure should not be built over a landfill until all the methane production is fully spent. ( No matter what precautions are taken there is always the risk that a discharge of landfill gas will happen at the most unexpected time and cause the most unexpected results).
The standard pipe should have alternating lines of slots 200mm long x 5mm wide at 75mm circumferential separation. For the 200 diameter pipe this equates to 8 lines of slots. A 300mm diameter pipe would have 12 lines of slots. The slots commence 500mm from each end.
When joining socketed or square screwed end pipe, after sliding the prepared surfaces together a circumferential line of 8-12 self-tapping 316 8gauge screws should be immediately inserted in the midband of the glued section. This draws the glued faces together with more force ( assisting the gluing process ) and also gives the final pipe length sufficient strength to withstand being raised by a crane and lowered into its working well without coming apart and injuring operational personnel; let alone smashing the pipe and requiring a whole new pipe assembly. ( There is always the risk of things going bump. All due care is the liability of the installation contractor ). With deep wells the amount of slots should be greater towards the bottom of the well as opposed to the top.
The top section of the well pipes that I install has a capability of coping with up to a 2 meter slump in the landfill depth. This expansion-joint section is a reduction from the 200mm to 150mm . Factory fabricated and assembled.
The prime reason for having the large well diameter and large extraction well pipe diameter is to minimize the amount of detrital matter that would be carried both into the well gallery and into the well pipe by strong suction forces. Well gallery surface vacuums should be as low as possible <100pa/M2. Once the landfill gas is out of the working section of the well gallery then it is important to maximize the gas velocity by way of reducing the diameter of the extraction pipework from the gallery head to the header lines. This is achieved, first by the expansion joint and then by the pipework of the mass flow measurement apparatus. The larger the diameter of the working well pipe in the well gallery the less detrital matter it will collect. The larger the well diameter the longer it will produce landfill gas before clogging. I am yet to have a well >900mm clog. An end cap is installed on the bottom of the pipe. This to stop migration of the gallery aggregate and landfill content from blocking the pipe. The dual conversion well would have an outer diameter pipe of 300mm and an inner of 200mm. The outer would service the upper gallery and the 200mm the lower gallery. A concrete plug seals the bottom gallery from the one above. Dual mass flow apparatus assist in tuning the performance of each gallery.
Well Gallery, Fill Aggregate and Cap.
The well gallery should be for as much of the well depth as possible. With well slotting commencing 2 - 3 metres below the base of the cap to within 1 - 2 metres from the base of the well.
Depending on the nature of the landfill cover the well cap is generally no more than 2 meters - 3 metres deep. The aggregate is first capped with a dry cement/bentonite/bricky sand mixture for a depth of 1 metre. On top of this is 500mm of bricky sand and then a concrete cap (20/20/100 is sufficient) to almost ground level. This being dependant on how the mass flow apparatus is to be installed. ( Bearing in mind the nature of the fill and compaction characteristics of the landfill; these parameters need to be determined on a site by site basis by the design engineer ).
Fill aggregate should be a 10mm - 25mm mix of quartzite or similar inert rock. Anything else will be eaten by the moist ( acid/alkali) agents from within the landfill.
At the Hulett St landfill the previous consultants' wells were, on average, 10 - 13 metres deep and 200mm in diameter. The extraction pipe being 65mm in diameter and the extraction gallery only the bottom 3 metres of the whole well depth. Wells clogged with detrital matter and also drowned in the smallest volume of water. Of the initial 30+ wells installed an additional 25+ of similar profile ( with larger extraction pipes and altered topology but still narrow bore <350mm in diameter wells and similar gallery design to before ) were installed and occasionally there can be up to 50%+ of wells off line at anyone time.
I installed 3 x 900mm diameter wells and 2 x 1200mm diameter wells. These wells produce over 20% of all the landfill gas used in the power station which is fuelled from the landfill. No more site work is possible until the legals are sorted out. Not only do narrow bore wells with shallow galleries flood and drown. They also clog for a host of other reasons.
Do not install narrow diameter wells.
Mass flow Control.
Mass flow control is one of the most important aspects of regulating the supply from the landfill. In one of my first installations at Roghan Rd. Fitzgibbon Brisbane, the mass flow control was designed by the client and incorporated orifice plates across which a pressure differential was taken. The gas composition being read at the same time and the mass flow was calculated.
It occurred to me that orifice plates were too great a constriction on the flow of the gas ( up to 4 -6 Kpa for high flows ) that I investigated the available technology for mass flow monitoring and control. Everything from Pitot tubes, ultrasonics to dedicated venturis was investigated. Eventually I settled on ISA S.75. This uses Butterfly valves with known Cv at specific openings and measuring the pressure drop as laid out in the standard. These valves offered numerous advantages. As the mass flow apparatus that I use is normally fitted in a horizontal position an orifice plate acts as a dam to both detrital matter and water; clogging the smooth flow of landfill gas.
( All passive constriction apparatus used for the measurement of fluid mass flow require long lengths of straight pipe both fore and aft of their location to minimize the effects of turbulence: generally 20 diameters upstream and 15 diameters downstream. Horizontal location of the mass-flow apparatus is therefore imperative otherwise the downstream side would stand up to 5 metres high above the orifice constriction device. Not complying with the relevant standards leads to incorrect and misleading results. )
The butterfly in the butterfly valve allows most of the detrital matter to pass and nearly all the water with minimal detrital matter held back. It is, in essence, self-cleaning.
The pipe emanating from the well head is 150mm diameter. This is reduced to 100mm for the mass flow apparatus which contains 2 butterfly valves. The first ( closest to the well head ) valve is the 'orifice valve' This places the necessary constriction ( by locking the valve opening on the respective angle on its notch plate ) for the measurement of the mass flow. The next butterfly valve is fitted with a gearbox and wheel.
Pressure differential is measured across the orifice valve and 'tuned' with the geared valve. Pressure drops are from 50 Pascals - 250 Pascals for whatever mass flow is required ( up to 600 cubic metres per hour per well can be measured with pressure drops of less than 100 Pascals ). This enables the full work of the extraction compressor to be transferred to the well gallery resulting in lower energy requirements for the extraction process.
Well groups are also controlled with other mass flow apparatii as part of the balanced mode operational paradigm. All field measured mass flows need to be calibrated against an accurate totaliser due to the heterogeneous and unstable nature of the composition of landfill gas.
Programs and manuals for the calculation of mass flow are available from the valve manufacturers namely: Keystone and Fisher.
The entire extraction process can be automated. Micro-gas lines from each well can be routed to a central control point. Control valves can be either electrically ( flame proof ) or pneumatically actuated and controlled and the entire well field operated automatically in balanced mode on a continuous basis rather than weekly tweaking from manually taken results.
The danger with automation is that the well field will not receive as many hands on inspections as would be done with a manual system. The automatic system is more expensive. I prefer and recommend the manual system.
Mass flow control should be placed as close as possible to the well head. The further from the well head the greater the risk of water clogging by virtue of condensate accumulating between the mass flow control and the well head.
With the mass flow control near the well head greater vacuums ( draws ) can be imposed on the extraction system. This minimizes the accumulation of condensate within the extraction pipework.
Condensate control is required irrespective of how great a vacuum is imposed on the field. Pipework low points should have barometric drip legs installed and multiple arrays should meet at common condensate drop-out pots. The final condensate control point should be as close as possible to the extraction compressor.
Condensate recirculation should be incorporated where possible as a matter of course.
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