SILOXANE REMOVAL SYSTEM – VENTURE ENGINEERING ANNOUNCES NEW OFFERING TO LANDFILL AND DIGESTER GAS CUSTOMERS
Due to significant market demand from our landfill gas and digester gas (biogas) customer base, Venture has designed a completely modularized gas conditioning skid, primarily intended for siloxanes removal, with added gas conditioning benefits as described herein.
Venture Engineering’s siloxanes removal system is based on a multitude of practical landfill gas processing experience and was founded on the principals of design of siloxanes/NMOC removal systems located at various client installations, all of which were stick built, non-proprietary, industry standard unit operations. The Venture siloxanes removal system encompasses the best of all of our designs into one modularized, shop fabricated system. Venture’s system includes regenerative selective adsorption followed by activated carbon adsorption.
Siloxanes removal systems have been in landfill gas service for 25 plus years. Until the more recent development of various molecular sieves, activated carbon was the primary removal device for siloxanes. A large enough activated carbon system will remove siloxanes to just about any level required. However, large activated carbon systems require space and energy to regenerate on-site. Furthermore, moisture, NMOC’s and hydrogen sulfide also compete for adsorption surface area inside the activated carbon system.
Venture uses several different types of selective adsorbants including activated alumina, silica gels, and in some instances, molecular sieves in the first stage adsorption skid. The ultimate combination depends on the individual characteristics of the LFG to be conditioned.
The basic system includes a dual-swing bed adsorption skid, followed by a multi-bed activated carbon skid. Both systems are regenerated on-site either using waste heat (via inert tail-gas), or via low watt density electric heating elements. For high BTU gas plant applications, regeneration using heated tailgas (from a TOU heat exchanger as an example) is the preferred method. However, for IC engine or turbine plants, no such inert tailgas stream exists. As such, a combination of cleaned LFG gas (slip stream) and electric heating elements provide the regeneration for the off-line vessels.
Benefits of Venture’s Siloxane Removal System:
Venture’s skid mounted gas conditioning system has been designed primarily to consistently remove siloxanes from biogas to levels acceptable for combustion or to meet a pipeline specification. In addition to siloxanes removal, Venture’s gas conditioning system will also reduce the levels of other contaminants such as hydrogen sulfide and halogenated organics, which has a tremendous benefit on downstream operations, as discussed below.
Siloxanes
During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or various other elements in the combustion gas. Deposits are formed containing mostly silica and silicates (SiO2 and SiO3), but can also contain calcium, copper, sodium, sulfur, and zinc. Most deposits caused by combustion of siloxanes are off-white to light brown in color and are of varying texture, some very smooth with a powdery-looking surface, while others are coarse and grainy. These deposits can ultimately build to a surface thickness of several millimeters and are difficult to remove by chemical or mechanical means. The propensity for silica/silicate deposition will vary based on flame front, heated surface area, rotation/tip speed, post combustion equipment, heat recovery and catalyst. The damage caused by siloxanes combustion byproducts and deposits can be severely limiting to operations.
Reciprocating piston engines experience fouling in the combustion chamber, on the valves, valve seats, piston crowns and cylinder walls. Sometimes deposits collect under the exhaust valves resulting in blowby and burnt valves. This can significantly reduce compression and engine efficiency. In gas turbines, deposits from siloxanes combustion form in the hottest areas, mainly on the first few rows of nozzles and blades. Prolonged operation of gas turbines where siloxanes are present in the biogas can lead to severe erosion of the turbine blades and a significant drop in operating efficiency.
Because of the difficulty in removing the silicon-based deposits and the cost to overhaul engines and turbines, many manufacturers have already set very low limits for siloxanes with warranty provisions that are linked to these siloxanes limitations. So either remove the siloxanes, or live without the manufacturer’s warranty if they’re not controlled. The following table lists current (as of 1/31/2009) siloxanes limits for some common manufacturers.
Siloxane
Engine Manufacturer mg/m3 (in Landfill Gas)
Caterpillar 28
Jenbacher 10
Waukesha 25
Deutz 5
Solar Turbines 0.1
IR Microturbines 0.06
Capstone Microturbines 0.03
Additionally, various emissions controls devices that use catalysts have been negatively affected by siloxanes. Siloxanes, as reduced silicon dioxide, coat the catalyst and creates an impermeable glass. This reduces the efficiency of the catalyst for removal of formaldehyde and other byproducts from combustion. By removing the siloxanes ahead of combustions, the life of the catalyst can be greatly extended.
Hydrogen Sulfide
Hydrogen sulfide is highly corrosive and can be just as detrimental to an IC engine or turbine, as siloxanes. Hydrogen sulfide is oxidized into sulfur dioxide which dissolves as sulfuric acid. Sulfuric acid, even in trace amounts, can make a solution extremely acidic. Extremely acidic electrolytes dissolve metals rapidly and speed up the corrosion process. This is particularly true in high temperatures, such as is the case with an IC engine. Studies have shown that the first part of a biogas engine to wear out is the cylinder liner at the upper position of the piston ring. Excessive wear in cylinder liners at this position is caused by the corrosion phenomena. Even if there is no oxygen present, biogas can corrode metal. Hydrogen sulfide can become its own electrolyte and adsorb directly onto the metal to form corrosion. If the hydrogen sulfide concentration is very low, the corrosion will be slow but will still occur due to the presence of carbon dioxide.
Venture’s gas conditioning system, while sized to remove siloxanes, will also significantly reduce hydrogen sulfide concentrations in the activated carbon process. Alumina, silica gels and molecular sieves do not remove hydrogen sulfide.
Halogenated Hydrocarbons
Halogenated hydrocarbons (primarily chlorinated and fluorinated) can also have a detrimental effect on an IC or Turbine plant. Like hydrogen sulfide, chlorinated and fluorinated hydrocarbons form their respective acids: hydrochloric acid and hydrogen fluoride. By reducing the levels of these halogenated hydrocarbons, corrosion problems are reduced and engine performance is increased.
Venture’s gas conditioning system employs two successful removal media’s for halogenated hydrocarbon removal: activated alumina and activated carbon. Silica gels and molecular sieves have little effect on heavy hydrocarbon concentrations.
Siloxanes Removal System Specs (LFG to Electricity Installation)
- Carbon steel with coal-tar epoxy interior coating (or stainless steel if preferred), is used for all pressure vessels, stainless steel is used for piping and valves. Vessels are designed to the requirements of ASME Section VIII, and piping is designed to the requirements of ASME B31.3 with Class 150 flanges.
- High efficiency 0.01 micron coalescing pre-filtration and 1 micron particulate after filtration is provided. Filter elements are contained in stainless steel housings designed to ASME Section VIII.
- Oxygen analyzer (for process and safety control)
- High temperature valves with metal seats (activated carbon system)
- Piping and vessel insulation (activated carbon system)
- Inlet blower, motor and VFD (where raw LFG feed pressures are below 5-7 psig)
- Air cooled heat exchanger (to remove heat of compression (175F to 95F) of the raw gas prior to selective adsorption)
- Heaters use low watt density elements with outlet temperature controls suitable for use in a Class 1 Div 2 environment and are fully insulated for personnel protection.
- Electrical enclosures are stainless steel, providing NEMA 4X ingress protection, and are purged and pressurized for a Class 1 Div 2 environment.
- Operation is controlled using a PLC with a full color interactive digital display on a panel mounted operator interface.
- Safety features include alarm contacts for connection to a client’s main PLC system, and cycle control and safety overrides interlocked to the PLC via valve limit switches, temperature controls and pressure transmitters.
The following requirements are the responsibility of the Owner/Installer (or optionally provided by Venture):
- Site location suitable for operations and maintenance
- Concrete pad for skids
- A single point 460/3/60 power supply via a suitable fusible disconnect
- Inlet & outlet connections including bypass
- Emissions exhaust connection to a local flare or thermal oxidizer, including a flame arrestor device
- Connection of the condensate drain ports to a local condensate treatment or storage facility
- Connection of alarm contacts and alarm monitoring
- Single point instrument quality air to the skid location
- Instrument air supply for pneumatic valve actuation
System Performance:
The system described herein is completely modularized and designed to remove siloxanes from raw biogas with an inlet flowrate of up to 4000 SCFM (modules in increments of 1000 SCFM) and inlet siloxanes concentration of 75 ppmv or less to an outlet concentration of 1.0 ppmv or less. This assumes that total gaseous non-methane organics (NMOCs) does not exceed 6000 ppmv (as ppm methane), and hydrogen sulfide ≤35 ppmv. Higher NMOCs and/or hydrogen sulfide will affect the size of the system. In instances where hydrogen sulfide concentrations are significantly higher (>100 ppmv), it may be more economical to employ a hydrogen sulfide removal step ahead of the selective adsorption skid. This will be determined on a case by case basis.
The initial investment (capital) for a Venture Siloxane Removal and Gas Conditioning skid can be as low as $180/SCFM (inlet) and up to $225/SCFM, depending on specific gas composition, flow rates, temperature and pressure requirements.
For information, call Mr. Bill Slatosky, Process Engineering Manager, at 412-231-5890.