Thursday February 17 2022
Biogas is the product of anaerobic digestion, the breakdown of organic matter by microbes in anaerobic conditions in an AD reactor. The process can also create a nutrient-rich digestate which can be used as an agricultural fertiliser.
However, the gas produced by anaerobic digestion is of variable quality, often dependent on the type of feedstocks used and the efficiency of the AD plants.
This means if biogas is to be used to heat and power homes and businesses beyond the immediate locality of the plant it was produced in, either via the national gas network or using a virtual network, it must be upgraded by removing impurities.
But what does that mean and how is it done? In this article, we take a look at upgrading biogas so it can be exported to the National Grid’s gas transmission network, irrespective of whether the biomethane is injected into a gas distribution network owned by Cadent Gas, Northern Gas Networks, SGN or Wales & West Utilities.
But before we look at how biogas is cleaned, it is important to understand what biogas consists of and which elements need to be removed.
Biogas is predominantly a mixture of methane (CH4) and carbon dioxide (CO2). The relative amounts typically vary from between 50 per cent to 70 per cent CH4 and 30 per cent to 50 per CO2, depending on the feedstock.
It also contains a number of other gases in smaller concentrations, including nitrogen (N2), hydrogen (H2), hydrogen sulphide (H2S), oxygen (O2), carbon monoxide (C0), ammonia (NH3), siloxanes (Si-O-SI), VOCs (Volatile Organic Compounds), and water.
Upgrading biogas is the process of removing the impurities to create biomethane that is suitable to be used in the national gas network. Due to its similarity to natural gas, biomethane is often called Renewable Natural Gas (RNG).
RNG can be used for all the purposes of conventional natural gas, such as heating and powering homes and businesses, as well as transport fuel in the form of compressed natural gas (bio-CNG) and liquified natural gas (bio-LNG).
To qualify to be exported to the national gas network, the methane content of biomethane must satisfy the gas grid network specification by being a minimum of 95 per cent.
Achieving this is divided into three processes – removing water vapour, biogas upgrading and biogas cleaning. Biogas upgrading deals specifically with the removal of CO2, the second most common element of biogas.
Biogas cleaning is the process of removing other impurities such as NH3 and H2S.
The primary reason for doing this is to optimise the calorific value of the gas for heating and power. By having impurities in the gas such as CO2 and N2, the gas will not burn as efficiently as needed. Removing them also prevents damage to equipment and appliances.
The process also removes harmful gas such as CO, and those that create bad odours such as H2S, to ensure the biomethane produced burns as cleanly and safely as possible.
So, how is biogas upgraded and cleaned? There are a number of ways.
The process of turning biogas into biomethane starts with removing water vapour. Water vapour dramatically reduces the net calorific value (NCV) of the gas, reducing its performance and its value as a fuel.
Moisture in the gas can also lead to condensation in the gas line, possibly creating corrosion and clogging of the pipes, so it is vital to remove it as soon as possible.
This is commonly removed via cooling which places condensate traps at strategic locations along a pipeline leading away from the digester. As the biogas cools on its journey, the moisture naturally condensates and collects in the traps. Done effectively, this can remove up to 95 per cent of the water vapour.
Historically, water scrubbing was the preferred method of upgrading biogas and separating out the multiple impurities, as it was seen as a simple and robust process. However, due to more efficient and cost-effective upgrade system options, primarily membrane-based, it has fallen out of favour.
Water scrubbing works by forcing biogas, under high pressure, up through a reactor column in which chilled water is flowing down. During this process, soluble gases like CO2 and H2S are dissolved in the water, leaving the methane to be collected at the top of the reactor column.
There is a second tower involved in the process where the CO2 rich water is depressurised to allow the CO2 to be removed and safely treated.
Water scrubbing is effective at meeting the stringent tests required to produce RNG and often results in biomethane of up to 98 per cent purity. However, O2 and N2 are only sparingly soluble in these conditions, meaning an extra polish might be required if these need to be removed separately.
A disadvantage of water scrubbing technology is that the cooling towers may often contravene planning applications due to visibility.
As its name suggests, this method of cleaning biogas uses membranes and pressure to separate CH4 and CO2.
The membranes used are made up of long, thin fibres – between 0.5mm and 1mm thick – and pressurised biogas travels along the length of their core. The CO2 permeates the membrane due to its ionic charges, whereas the CH4 remains in the core of the fibre to be collected at the end of the process.
Thousands of these fibres are bundled together into larger tubes and the biogas is filtered through them two or three times before it achieves the required level of purity.
As well as removing CO2, the membranes are also effective at taking out H2S and water vapour and the permeability of the membrane can be altered with temperature. Cooler temperatures make the membrane pores smaller, selectively removing smaller molecules such as CO2. Higher temperatures open the holes up, allowing a great number of molecules to permeate.
Older designs of this method saw as much as a 25 per cent loss in methane during in the process, however, modern designs have been developed that lead to little or no methane loss and are now the benchmark for upgrading system efficiency.
Another significant advantage of membrane technology is the ability to capture the CO2 effluent stream as a valuable asset for food-grade and/or industrial grade process applications and it helps minimise carbon emissions to atmosphere.
Membrane systems can be easily installed and commissioned as it is seen as a ‘plug and play’ configuration with reduced installation and commissioning costs. The ability to add CO2 capture and bio-LNG modules are also significant benefits.
Pressure swing absorption (PSA) separates gases based on molecule size and weight. The process uses several vessels running in parallel under pressure and an absorptive media.
When the biogas enters the process, it is first compressed, then cooled and dried by removing moisture. It then moves into the main PSA vessels where it comes into contact with the absorptive media. The CO2 and H2S permeate the media, whereas the CH4 moves through the absorber vessels relatively untouched to be collected at the top.
The waste gases are then removed from the PSA vessels by reducing the pressure.
PSA is the only gas upgrading system that can successfully remove O2 and N2 from biogas making it useful as both a primary upgrader and a polisher when other upgrading processes have been used. It also makes PSA technology the logical upgrade process when used for landfill operations where higher levels of N2 are encountered.
PSA generally creates biomethane with a gas quality of between 96 and 98 per cent.
Amine gas treatment is similar to water scrubbing, however, instead of passing through a reactor column along with chilled water, an amine solvent is used to remove CO2 and other gases.
A popular solvent is mono di-ethanol amine (MDEA) which chemically reacts with the CO2 and retains it in a solution, while the CH4 passes through the solvent untouched.
This is a very effective process that can generate very high quality RNG more than 99.9 per cent purity.
Amine gas treatment is a two-stage treatment. In the second phase, the scrubbing solvent is boiled to reverse the chemical reaction and release the CO2 so the solvent can be reused.
The process also creates high purity CO2, meaning it too can be reused in other industrial processes.
All of these methods are effective at producing biomethane of the required quality to be exported to the national gas network, or to be used as transport fuel.
This is an important consideration to UK AD plant operators, because the government’s Green Gas Support Scheme (GGSS) only provides incentives for digester plants that generate biomethane capable of being exported, not lower quality biogas as they once did.
The actual process used by AD plant owners to upgrade biogas will depend primarily on the composition and quantity of biogas being upgraded, i.e. the necessity to remove O2 and N2 as well as water vapour, CO2 and H2S, and the feedstocks they employ. Capital (Capex) and Operating (Opex) costs must also be assessed in light of the lifecycle the plant is designed for (typically 15-20 years), and planning application factors.
For advice on which biogas upgrading process would be best suited for your plant, get in touch.