About Biogas

An introduction to Anaerobic Digestion

Anaerobic Digestion is the microbiological process of the decomposition of organic matter, in the absence of oxygen. It is common to many natural environments and widely applied today to produce biogas in air proof reactor tanks, commonly named digesters. A wide range of micro-organisms are involved in the anaerobic process which converts waste materials in the form of sludge into two principal products, biogas and nutrient-rich digestate.

Biogas

Biogas is a combustible gas consisting of methane (CH4), carbon dioxide (CO2) and small amounts of other gases and trace elements. The Biogas can be used as a renewable fuel source for heating, Combined Heat & Power (CHP), or enriched into biomethane (Renewable Natural Gas – RNG) for injection into natural gas pipeline (Gas to Grid).

Digestate

Digestate is the solid product remaining from the anaerobic digestion process. This is used in agriculture as a fertiliser or compost.

Feedstock

Anaerobic Digesters can process sludge & slurries from various sources and on a commercial basis. These include;
  • Animal manure and slurry
  • Agricultural residues and by-products of crop production
  • Digestible organic wastes from the food production industry (vegetable and animal origin)
  • The organic part of municipal waste and from catering (vegetable and animal origin)
  • Sewage sludge
  • Dedicated energy crops (e.g. maize, miscanthus, sorghum, clover).

Biomethane

Biogas becomes biomethane when it is enriched to pipeline quality gas. It is identical in property to natural gas. Biogas starts with 60 – 70% methane (CH4) but contains some unwanted additions such as hydrogen sulphide (H2S), carbon dioxide (CO2), water and possibly siloxanes (synthetic silicone derivatives), dependent on the feedstock.


To meet UK gas pipeline specifications, it must go through several processes which removes these unwanted compounds producing an almost pure (98%) methane gas.

If the calorific value of the gas falls below a minimum threshold, propane can be added to bring it up to acceptable levels. The resulting biomethane then can be injected into the gas network or compressed for use in natural gas vehicles.

Hydrogen sulphide (H2S)

H2S is a colourless gas known for its pungent “rotten egg” smell, even at low concentrations. With the chemical formula H2S, it consists of two hydrogen atoms bonded to a single sulphur atom. This compound is naturally produced during the decay of organic matter and can also be a byproduct of various industrial processes, such as sewage treatment. Despite its natural occurrence, H2S poses significant risks due to its toxic and highly corrosive nature.


Wastewater treatment streams contain differing levels of sulphur compounds. Under atmospheric conditions (in the presence of oxygen), the sulphur is mainly bound as sulphate (SO4) and safely dissolved in the wastewater.

In the anaerobic digester, the organic matter is degraded by anaerobic bacteria. In the absence of oxygen, the sulphate is converted to hydrogen sulphide (H2S) gas.

Thus, after the anaerobic digestion process, the sulphur compounds present in the inlet stream leave the digester through the biogas as H2S. Hydrogen Sulphide is well known and understood in the Oil & Gas sector and has long been taken very seriously given its highly toxic & corrosive nature. Typically, the H2S in the biogas will be in the range from 0.1% to 3.0% (1,000-30,000 ppm), which is quite comparable to the levels found in Oil & Gas wellheads, where it’s common to find concentrations ranging from 0-5% (0-50,000 ppm).

It is quite common for anaerobic digester plants to accept sludges from a variety of external sources, where the content can vary greatly and the H2S levels are unpredictable.

Sulphuric acid (H2SO4)

The Hydrogen Sulphide found in the anaerobic digester and the resultant Biogas is a gas. When present in a saturated environment, as is always the case in a digester, the H2S forms Sulphuric Acid, H2SO4. Since the feedstock sludge can vary greatly, so can the levels of H2S and the resulting concentrations of H2SO4.


The challenge is that H2SO4 corrodes most materials, especially yellow metals, low and high grades of steel and iron. This problem of saturated Biogas, unrecognised by general valve suppliers to the Water Industry, has led to utilisation of low-cost products often with inappropriately selected materials being frequently used. This almost always results in a range of failures, costly repairs and extensive downtime. Damage from corrosion also results in atmospheric leaks of toxic Hydrogen Sulphide along with the highly flammable Biogas, in breach of Environment Agency regulations.

Care must be taken to select appropriate materials for the valves being installed in a Biogas plant. MGA Controls has worked closely with a number of key suppliers and industry bodies such as WIMES & IGEM, to develop a range of valves suitable for the highly corrosive Biogas environment.

Explosive atmospheres & ATEX

Biogas becomes biomethane when it is enriched to pipeline quality gas. It is identical in property to natural gas. Biogas starts with 60 – 70% methane (CH4) but contains some unwanted additions such as hydrogen sulphide (H2S), carbon dioxide (CO2), water and possibly siloxanes (synthetic silicone derivatives), dependent on the feedstock.


To meet UK gas pipeline specifications, it must go through several processes which removes these unwanted compounds producing an almost pure (98%) methane gas.

If the calorific value of the gas falls below a minimum threshold, propane can be added to bring it up to acceptable levels. The resulting biomethane then can be injected into the gas network or compressed for use in natural gas vehicles.

DSEAR

The Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) 2002 require the elimination or reduction of risk of fire and explosion from substances connected with work activities and are applicable to Biogas plant.


In addition to the regulations, there is an Approved Code of Practice and guidance (ACOP) and guidance available to provide practical advice on how to comply with the DSEAR regulations. The key aspects of DSEAR in regard to Anaerobic Digesters are;

  • Complete risk identification and evaluation for DSEAR compliance.
  • Hazardous Area Classification – Identify and classify hazardous zones to minimise risk.
  • Equipment Specification for ATEX ratings – Verifying that the Biogas equipment meets ATEX standards for DSEAR compliance.

Summary

It has been common practice to supply standard industrial butterfly valves with painted cast iron bodies, NBR, EPDM, PTFE or standard viton liners, or stems exposed to the media and standard elastomer O-ring stem seals. Whilst low cost and availability are often the primary drivers, there is considerable scope for the selection of inappropriate valves resulting in;
  • Valves seizing open/closed
  • Liners swelling, decaying, becoming embrittled and disintegrating
  • Valves leaking across the seat & unable to provide isolation
  • Valves leaking toxic & flammable gases and acid through the stem into the atmosphere
  • Metallic & non-metallic, wetted and non-wetted materials corroding, internally & externally
Biogas Valve Solution

The MGA engineered solution

Biogas facilities present the plant designers and operators with a number of interesting challenges to ensure safe operation and compliance with regulations. Key to this is the selection of valves and instrumentation that meet the challenge of the hazardous & corrosive nature of Biogas.

MGA Controls has developed a range of Biogas valves (Threaded & Flanged Ball Valves, Flanged & Lugged Butterfly, Pressure Vacuum Relief Valves, Pressure Relief Valves)

Biogas Ball Valves
Ball Valves -CF8M Stainless Steel, with NACE MR0175 Standard compliant materials, capable of meeting the UK Biogas, IGEM & WIMES standards, with appropriate leak-tight sealing and proven long term performance.
Biogas Butterfly valves
CF8M Stainless Steel, Butterfly Valves with a Viton-Bio Acid Resistant Liner based on gas service approved materials and capable of meeting the UK Biogas, IGEM & WIMES standards, with appropriate leak-tight sealing and proven long term performance.
The base model is a Gas Service valve certified to EN 13774 – ‘Gas Fittings: Shut-off valve for local distribution’ and EN161 – ‘Gas Fittings: Pneumatically operated automatic shut-off valve’ with leak-tight ‘Dry-Shaft Liner Technology’, preventing stem leakage.
Supplied with a Liner material of ‘FPM-006, Fluorelastomer (Viton-BIO)’, suitable for a media of Sulphuric Acid, Sour Gas, Natural Gas and other oils, hydrocarbons, chemicals & solvents. Suitable for 20% H2S (≤200,000 P/M @ ≤ 40oC) and H2SO4 at any concentration @ ≤80oC.
Our solution includes an undercut disc reducing torque, strain and wear, thereby increasing longevity.
Our ball and butterfly valve range can be supplied with actuation and instrumentation which meets the demanding Biogas application requirements.
To give the industry confidence in the Biogas Butterfly Valves being offered, MGA are also providing a 2 year manufacturer’s warranty.
For critical areas and assured isolation, Double Block and Bleed arrangements can be provided.

Pressure Vacuum Relief Valve

Motherwell Tank Protection (MTP) is a UK manufacturer specialising in high-precision, critical safety tank protection. With a heritage dating back to 1859, the company has evolved into a leading provider of advanced control technologies for hazardous area storage tanks and are widely adopted in oil terminals, refineries along with Biogas facilities in Industrial and Wastewater treatment plants.

Motherwell Tank Protection’s product portfolio includes:

  • Pressure Vacuum Relief Valves (PVRV)
  • Pressure Relief Valves (PRV)
  • Flame Arresters

These products are designed and certified in accordance with hazardous area industry standards such as ATEX, BS EN ISO 28300 & API 2000 and to be suitable for use in highly corrosive Hydrogen Sulphide (H2S) and Sulfuric Acid (H2SO4) applications (in accordance with NACE standards). MTP products are supplied with full 3.1 Certificates, assuring wetted parts fully comply with the appropriate material and quality standards.

MTP’s safety critical products maintain system integrity by precisely regulating internal pressures and preventing structural damage to tanks by releasing excess pressure and allowing inflow to relieve below atmospheric pressure conditions. Advanced manufacturing techniques and the use of ‘zero memory’ materials, provides the best-in-class shutoff performance, minimising emissions.

Motherwell Tank Protection’s ATEX certified End-of-Line flame arrestors serve as critical safety components, stopping flame propagation (deflagration, Detonation) and reducing the risk of explosions in gas-rich environments.

MGA Controls works in alliance with Motherwell Tank Protection to provide a comprehensive Biogas offering to the Wastewater treatment market, which fully comply with the requirements set out in WIMES and IGEMS standards. This partnership enhances the delivery of comprehensive solutions by combining MTP’s expertise in tank protection with MGA’s presence as a key partner to the Water Utility companies, and through MGA Control’s extensive experience supplying hazardous area (ATEX & NACE certified) equipment to the process industries.

MGA’s extensive technical knowledge of the Biogas process (and associated Combined Heat & Power CHP and Gas to Grid) allows us to provide a consultative approach to the Water Utilities and Contractors, both for those building new plant and refurbishing existing facilities – Managing potentially explosive and highly corrosive applications with high quality, low maintenance equipment and free technical support to contractors and to those responsible for the identification, evaluation & management of risk in accordance with the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR).

Sizing Relief Valves

Pressure Vacuum Relief Valves and Pressure Relief Valves are Safety Critical Valves for a plant that is designed to produce and contain corrosive and flammable chemicals.

As a safety feature they must be correctly sized in relation to the:

  • Maximum Allowable Working Pressure (MAWP) of the digester
  • Operating Pressure of the digester
  • Test Pressure of the digester
  • Flow Rate of the digester gas output
  • Over-Pressure of the digester

The ‘Duty’ & ‘Standby’ Relief Valve Set-Points must also be determined by these Pressure Vessel characteristics and are governed by standards API 2000 / ISO 28300, the venting of atmospheric or low-pressure storage tanks.   These are summarised as follows:

Pressure VessetPercentageTypical Relief Valve
116%Maximum relieving pressure, multiple reliefs – Standby Valve
110%Maximum relieving pressure, single relief – Duty Valve
105%Maximum allowable set pressure, multi-reliefs – Standby Valve
Maximum Allowable Working Pressure – MAWP100%Maximum allowable set pressure, single relief – Duty Valve
Typical maximum allowable operating pressure90%

The ‘Typical maximum allowable operating pressure’ is set at 90% of MAWP as it typically takes the Digester pressure to drop a minimum of 10% to allow a Relief Valve to re-seat following a pressure trip. Failure to re-seat will result in ‘chattering’ of the valve where the Pallet and Diaphragm cannot settle back into position and gas continuously escapes into the atmosphere. For an ATEX zoned area, this must be avoided at all costs to avoid potential ignition of the gas. Atmospheric venting must also be minimised for environment reasons as Methane is a more potent greenhouse gas than Carbon Dioxide. This is of course of special interest to the Environmental Agency.

The ‘MAWP’ is dictated by the Pressure Vessel Test Pressure which is 1.5 greater than the MAWP figure.

For example:

If the digester has a membrane roof, the test pressure is typically restricted to 30mBar.  Therefore, MAWP at 100% is 20mBar.
If MAWP is 20mBar, then the maximum operating pressure of 90% is 18mBar and the Relief Set Pressures and Maximum Relief Pressures for a typical 2 x PVRV multi relief valve system, such as a Cow Horn assembly, are not permitted to be greater than the following (as outlined above):

PVRV1

  • Relief Set Pressure 20mBar
  • Max Relieving Pressure 22mBar

PVRV2

  • Relief Set Pressure 21mBar
  • Max Relieving Pressure 23.2mBar

The valve is sized to ensure that before the Maximum Relief Pressure is achieved, there is sufficient flow through the Pressure Relief Valve to match the digester gas output. An over-pressure may be specified as being less than the maximum additional over-pressure of 10% but excessively oversizing of the valve will result in a Pallet that will also not re-seat and chatter when tripped.

A Vacuum Relief Valve is also sized by a 10% Over-Pressure, using a set point commonly -2.5mBar, but a minimum Vacuum Flow Rate is seldom provided.

The standard documentation pack includes all calculations for pressure and vacuum relief valve flowrates for both PVRV1 and PVRV2. An Over Pressure Relief Valve Flow Rate graph is also provided.

Sizing for a Vented Port

Many Pressure and Pressure Vacuum Relief Valves are simply fitted with a Weatherhood.  This disperses the Biogas into the atmosphere immediately off the top of the valve.  However, recent investigations have identified a risk to Service Engineers, working in the vicinity of PVRV’s that may vent flammable and corrosive gases.  As such, Vented Ports are now being specified by some sites to allow a Vent Stack to be fitted, venting to atmosphere a minimum of 3m from the working ground level.  Depending on the number of pipework turns, diameter of the pipe, length of the pipe and the design of the weather hood, an increase in head loss is applied to sizing calculations to ensure the specified flow rate is achieved.

Materials

Materials include FEP diaphragms, PTFE seats, all 316SS wetted parts to NACE MRO175 certified by 3.1 material certs.

To include:

Sizing calculations (eg CNC 380), over pressure graphs, set-point test certification and leakage test certification

ATEX certificates

Atex certification provide, suitable for Zone 1 & Zone 2 applications

Biogas Ball Valves – Specification DN15-DN100

  • 2 way, 3-piece, full bore Floating Ball Valve
  • Threaded connection BSPP
  • CF8M/316 Stainless Steel Body, Ball & Trim
  • PTFE Seat & Stem Seals
  • Viton O-Rings
  • Body design EN 12516
  • PED 2014/68/EU
  • ATEX Directive 2014/34/EU Ex II 2 GD
  • NACE MRO175
  • EN 10204/3.1B Material Certification
  • Pressure & Seat test acc. API 598, equivalent to ISO 5208 [EN 12266-1]
  • Lockable Manual operation through Lever & T-Bar
  • ISO 5211 Direct Mount for Electric Actuator or Pneumatic Actuator
  • Slam-shut or controlled closure

Biogas Ball Valves – Specification DN50-DN200

  • 2 way, 2 piece, full bore Floating Ball Valve 
  • Flange connection Raised Face PN16 EN 1092-1
  • Face to face EN 558 Series 27 – Short Pattern 
  • CF8M/316 Stainless Steel Body, Ball & Trim
  • TFM1600 PTFE Seats
  • Grafoil Packing & Viton O-Rings
  • Body design EN 12516
  • Fire Safe certified acc. API607
  • PED 2014/68/EU
  • ATEX Directive 2014/34/EU Ex II 2 GD
  • NACE MRO175
  • EN 10204/3.1B Material Certification
  • Pressure & Seat test acc. EN 12266-1
  • Rated for ‘Safety Integrity Level’ SIL 2 applications
  • Lockable Manual operation through Lever, T-Bar or ATEX Gearbox & Handwheel & padlock flange
  • ISO 5211 Direct Mount for Electric Actuator or Pneumatic Actuator
  • Slam-shut or controlled closure
  • General Biogas usage, industry proven & acid resistant 
  • Biogas 3mm full face flanged gaskets available
  • Independently laboratory tested to be gas tight @100mb, field tested to be ‘gas tight’

Biogas Ball Valves – Specification DN250-DN300

  • 2 way, 2 piece, full bore Semi-Trunnion Ball Valve 
  • Flange connection Raised Face PN16 EN 1092-1
  • Face to face EN 558 Series 27 – Short Pattern 
  • CF8M/316 Stainless Steel Body, Ball & Trim
  • PTFE Seats
  • SPW 316 + Graphite Packing & Viton O-Rings
  • Body design EN 12516
  • Fire Safe certified acc. API607
  • PED 2014/68/EU
  • ATEX Directive 2014/34/EU Ex II 2 GD
  • NACE MRO175
  • EN 10204/3.1B Material Certification
  • Pressure & Seat test acc. EN 12266-1 Leakage Rate A
  • Rated for ‘Safety Integrity Level’ SIL 3 applications
  • Lockable Manual operation through ATEX Gearbox & Handwheel, padlock flange
  • ISO 5211 Direct Mount for Electric Actuator or Pneumatic Actuator
  • Slam-shut or controlled closure
  • General Biogas usage, industry proven & acid resistant 
  • Biogas 3mm full face flanged gaskets available
  • Independently laboratory tested to be gas tight @100mb, field tested to be ‘gas tight’

Biogas actuated ball valves

All of our ball valves can be:
  • actuated electrically or pneumatically
  • open/closed or modulating
  • controlled or slam shut (<1 sec)
  • Atex Zone 1 or Zone 2 rated
Actuators can be A5 corrosion resistant paint specification, fail safe, spring return or ‘stay put.’

Biogas Butterfly valves

All of our ball valves can be:
  • 2 way, Centric Rubber Lined Butterfly Valve
  • Fully lugged & tapped to suit PN16 flange connection (2.5 Bar MWP)
  • Face to face EN 558-1 Series 20
  • CF8M/316 Stainless Steel Body & Disc AISI420 Stainless Steel Stem
  • FPM-006 Viton-Bio Liner (replaceable)
  • Liner extended onto the flange (no gasket required)
  • Undercut disc for low breakout torque, reduced wear & increased longevity
  • ATEX earthing connection
  • Body design EN 593
  • PED 2014/68/EU
  • ATEX Directive 2014/34/EU
  • EN 10204/3.1 Material Certification
  • Operability test acc. EN 12266-2
  • Pressure & Seat test acc. EN 12266-1 P12 Leakage Rate A
  • Rated for ‘Safety Integrity Level’ SIL 2 applications
  • Lockable Manual operation through Lever (Gas Service yellow) or ATEX Gearbox with handwheel & padlock flange
  • ISO 5211 Direct Mount for Electric Actuator or Pneumatic Actuator
  • Slam-shut or controlled closure – certified EN161 Gas Fittings: Automatic shut-off valve
  • open/closed or modulating
  • controlled or slam shut (<1 sec)
  • Atex Zone 1 or Zone 2 rated

Manual & actuated butterfly valves

All of our butterfly valves can be:
  • actuated electrically or pneumatically
  • open/closed or modulating
  • controlled or slam shut (<1 sec)
  • Atex Zone 1 or Zone 2 rated
Actuators can be A5 corrosion resistant paint specification, fail safe, spring return or ‘stay put.’

Standards

The National Association of Corrosion Engineers (NACE) is recognised globally as the premier authority for corrosion control.
NACE International (‘National Association of Corrosion Engineers’) give guidance on the corrosion resistant metals and elastomers for the prevention of Sulphide Stress Cracking caused by the presence of Hydrogen Sulphide in a ‘Sour Gas environment’.
MR0175 was developed for the most aggressive corrosion areas within the Oil Industry, the Well-Head and Extraction applications, where the highest concentrations of H2S are found.
The MGA Biogas solution is assembled in accordance with the following standards
  • WIMES 9.01 – CHP Installations
  • IGEM (Institution of Gas Engineers & Managers) Technical Gas Standards
  • WIMES 8.09 i2 – Valves
  • Water Industry Asset Standard: AM-DES-MEC-F23-M148 Valves (Iss. 3.0)
  • Valves for biogas applications shall comply with the specific requirements outlined in WIMES 8.09. All materials shall be compatible for use with saturated biogas containing H2S.
Statutory guidance
SR2021 No 7: Anaerobic digestion facility, including use of the resultant biogas – waste recovery operation
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