Biogas upgrader

Last updated

A biogas upgrader is a facility that is used to concentrate the methane in biogas to natural gas standards. The system removes carbon dioxide, hydrogen sulphide, [1] water and contaminants from the biogas. One technique for doing this uses amine gas treating. This purified biogas is also called biomethane. It can be used interchangeably with natural gas.

Contents

Raw biogas produced from digestion is roughly 60% methane and 29% CO2 with trace elements of H2S; it is not high quality enough to be used as fuel gas for machinery. The corrosive nature of H2S alone is enough to destroy the internals of a plant.

ComponentRangeAverage
Methane45–70%60%
Carbon dioxide25–55%35%
Water vapour0–10%3,1%
Nitrogen0,01–5%1%
Oxygen0,01–2%0,3%
Hydrogen0–1%< 1%
Ammonia0,01–2,5 mg/m30,7 mg/m3
Hydrogen Sulphide0–30'000 mg/m3500 mg/m3

The solution is the use of biogas upgrading or purification processes whereby contaminants in the raw biogas stream are absorbed or scrubbed, leaving more methane per unit volume of gas. There are four main methods of upgrading: water washing, pressure swing adsorption, selexol adsorbtion, and amine gas treating.

Water washing

The most prevalent method is water washing whereby high pressure gas flows into a column in which the carbon dioxide and other trace elements are scrubbed by cascading water running counter-flow to the gas. This arrangement can deliver 98% methane with manufacturers guaranteeing maximum 2% methane loss in the system. It takes roughly between 3% and 6% of the total energy output in gas to run a biogas upgrading system

Pressure Swing Adsorption

A typical PSA system for biogas will have four stages, one each for water vapor, carbon dioxide, nitrogen and oxygen. [2] Gas to be upgraded enters each vessel, is compressed to a high pressure whereby the gas to be removed is adsorbed on to the surface of the adsorbent, and is then decompressed allowing the methane to leave. The adsorbent is then regenerated. For oxygen, molecular sieve is used, for nitrogen a zeolite, for carbon dioxide and water a zeolite or activated carbon.

Selexol

In the Selexol process (now licensed by UOP LLC), the Selexol solvent dissolves (absorbs) the acid gases from the feed gas at relatively high pressure, usually 300 to 2000 psia (2.07 to 13.8 MPa). The rich solvent containing the acid gases is then let down in pressure and/or steam stripped to release and recover the acid gases. The Selexol process can operate selectively to recover hydrogen sulfide and carbon dioxide as separate streams, so that the hydrogen sulfide can be sent to either a Claus unit for conversion to elemental sulfur or to a WSA Process unit for conversion to sulfuric acid while, at the same time, the carbon dioxide can be sequestered or used for enhanced oil recovery.

Selexol is a physical solvent, unlike amine based acid gas removal solvents that rely on a chemical reaction with the acid gases. Since no chemical reactions are involved, Selexol usually requires less energy than the amine based processes. However, at feed gas pressures below about 300 psia(2.07 MPa), the Selexol solvent capacity (in amount of acid gas absorbed per volume of solvent) is reduced and the amine based processes will usually be superior.

Amine gas treater

H2S or both H2S and CO2 can be removed with this technology.

The chemistry involved in the amine treating of such gases varies somewhat with the particular amine being used. For one of the more common amines, monoethanolamine (MEA) denoted as RNH2, the chemistry may be expressed as:

RNH2 + H2S RNH+
3 + SH−

A typical amine gas treating process includes an absorber unit and a regenerator unit . In the absorber, the downflowing amine solution absorbs H2S and CO2 from the upflowing sour gas to produce a gas stream free of hydrogen sulfide and carbon dioxide as a product and an amine solution rich in the absorbed acid gases. The resultant "rich" amine is then routed into the regenerator (a stripper with a reboiler) to produce regenerated or "lean" amine that is recycled for reuse in the absorber. The stripped overhead gas from the regenerator is concentrated H2S and CO2.

Membrane-based Gas Permeation Systems

Membrane-based biogas upgrading systems utilize the different permeabilities of gases through a membrane fiber. As biogas passes through a dense polymeric membrane, CO2 is prevented from through-flow and removed, while CH4 passes through. Membrane-based gas permeation systems consume only electrical power, but do not require any chemicals or water. In order to achieve higher methane contents (up to 99% methane) in the final gas, the gas passes through serial groups of membranes. Since membranes are sensitive to water and other impurities in biogas, gas permeation/membrane systems require efficient pre-treatment (especially H2S and water removal).

Objectives and variants

biogas and natural gas pipelines Biogas pipes.JPG
biogas and natural gas pipelines

A distinction can be drawn between the basic treatment of raw biogas, which is necessary for example for use in a biogas CHP plant, and the more elaborate treatment needed to obtain natural gas quality (biomethane). The above table shows the composition of raw biogas after primary treatment and biomethane. The fractions of crude biogas can vary greatly, depending on substrate, plant design, and other factors. The nature of the biomethane is adapted to the corresponding qualities of natural gas.

Biogas is used mostly directly in a biogas cogeneration plant. This requires desulfurization and drying in order to avoid corrosion in the CHP. To be able to feed biogas into the natural gas network or for fuel use, a more comprehensive treatment is necessary. In addition to drying and desulfurization the carbon dioxide must be removed and chemical conditioning to obtain properties meeting the specifications for natural gas. This biomethane can be injected into the natural gas network and converted to electricity and heat through CHP at a place where the heat can be used, such as a swimming pool, which has a year-round high heat demand.

Use of the natural gas 'grid' also permits retail customers to purchase a certain proportion of biomethane gas in their gas supply contracts.

See also

Related Research Articles

<span class="mw-page-title-main">Biogas</span> Gases produced by decomposing organic matter

Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, wastewater, and food waste. Biogas is produced by anaerobic digestion with anaerobic organisms or methanogens inside an anaerobic digester, biodigester or a bioreactor. The gas composition is primarily methane and carbon dioxide and may have small amounts of hydrogen sulfide, moisture and siloxanes. The methane can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used in fuel cells and for heating purpose, such as in cooking. It can also be used in a gas engine to convert the energy in the gas into electricity and heat.

<span class="mw-page-title-main">Hydrogen sulfide</span> Poisonous, corrosive and flammable gas

Hydrogen sulfide is a chemical compound with the formula H2S. It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. Swedish chemist Carl Wilhelm Scheele is credited with having discovered the chemical composition of purified hydrogen sulfide in 1777.

Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as a fuel. Historically, it has been used as a replacement for gasoline, when gasoline supply has been limited; for example, wood gas was used to power cars in Europe during WWII.

<span class="mw-page-title-main">Ethanolamine</span> Chemical compound

Ethanolamine is a naturally occurring organic chemical compound with the formula HOCH
2CH
2NH
2 or C
2H
7NO. The molecule is bifunctional, containing both a primary amine and a primary alcohol. Ethanolamine is a colorless, viscous liquid with an odor reminiscent of ammonia.

<span class="mw-page-title-main">Anaerobic digestion</span> Processes by which microorganisms break down biodegradable material in the absence of oxygen

Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.

Acid gas is a particular typology of natural gas or any other gas mixture containing significant quantities of hydrogen sulfide (H2S), carbon dioxide (CO2), or similar acidic gases. A gas is determined to be acidic or not after it is mixed with water. The pH scale ranges from 0 to 14, anything above 7 is basic while anything below 7 is acidic. Water has a neutral pH of 7 so once a gas is mixed with water, if the resulting mixture has a pH of less than 7 that means it is an acidic gas.

Sour gas is natural gas or any other gas containing significant amounts of hydrogen sulfide (H2S).

<span class="mw-page-title-main">Molecular sieve</span> Filter material with homogeneously sized pores in the nanometer range

A molecular sieve is a material with pores of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecules migrates through the stationary bed of porous, semi-solid substance referred to as a sieve, the components of the highest molecular weight leave the bed first, followed by successively smaller molecules. Some molecular sieves are used in size-exclusion chromatography, a separation technique that sorts molecules based on their size. Another important use is as a desiccant. Most of molecular sieves are aluminosilicate zeolites with Si/Al molar ratio less than 2, but there are also examples of activated charcoal and silica gel.

<span class="mw-page-title-main">Claus process</span> Gas desulfurizing process leading to the formation of elemental sulfur

The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the chemist Carl Friedrich Claus, the Claus process has become the industry standard.

Amine gas treating, also known as amine scrubbing, gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines (commonly referred to simply as amines) to remove hydrogen sulfide (H2S) and carbon dioxide (CO2) from gases. It is a common unit process used in refineries, and is also used in petrochemical plants, natural gas processing plants and other industries.

<span class="mw-page-title-main">Pressure swing adsorption</span> Method of gases separation using selective adsorption under pressure

Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperature and significantly differs from the cryogenic distillation commonly used to separate gases. Selective adsorbent materials are used as trapping material, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed gas.

Renewable natural gas (RNG), also known as biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and has a methane concentration of 90% or greater. By removing CO2 and other impurities from biogas, and increasing the concentration of methane to a level similar to fossil natural gas, it becomes possible to distribute RNG via existing gas pipeline infrastructure. RNG can be used in existing appliances, including vehicles with natural gas burning engines (natural gas vehicles).

<span class="mw-page-title-main">Natural-gas processing</span> Industrial processes designed to purify raw natural gas

Natural-gas processing is a range of industrial processes designed to purify raw natural gas by removing contaminants such as solids, water, carbon dioxide (CO2), hydrogen sulfide (H2S), mercury and higher molecular mass hydrocarbons (condensate) to produce pipeline quality dry natural gas for pipeline distribution and final use. Some of the substances which contaminate natural gas have economic value and are further processed or sold. Hydrocarbons that are liquid at ambient conditions: temperature and pressure (i.e., pentane and heavier) are called natural-gas condensate (sometimes also called natural gasoline or simply condensate).

Selexol is the trade name for an acid gas removal solvent that can separate acid gases such as hydrogen sulfide and carbon dioxide from feed gas streams such as synthesis gas produced by gasification of coal, coke, or heavy hydrocarbon oils. By doing so, the feed gas is made more suitable for combustion and/or further processing. It is made up of dimethyl ethers of polyethylene glycol.

Rectisol is the trade name for an acid gas removal process that uses methanol as a solvent to separate acid gases such as hydrogen sulfide and carbon dioxide from valuable feed gas streams. By doing so, the feed gas is made more suitable for combustion and/or further processing. Rectisol is used most often to treat synthesis gas (primarily hydrogen and carbon monoxide) produced by gasification of coal or heavy hydrocarbons, as the methanol solvent is well able to remove trace contaminants such as ammonia, mercury, and hydrogen cyanide usually found in these gases. As an acid gas and large component of valuable feed gas streams, CO2 is separated during the methanol solvent regeneration.

<span class="mw-page-title-main">Methyldiethanolamine</span> Chemical compound

Methyldiethanolamine, also known as N-methyl diethanolamine and more commonly as MDEA, is the organic compound with the formula CH3N(C2H4OH)2. It is a colorless liquid with an ammonia odor. It is miscible with water, ethanol and benzene. A tertiary amine, it is widely used as a sweetening agent in chemical, oil refinery, syngas production and natural gas.

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

CrystaSulf is the trade name for a chemical process used for removing hydrogen sulfide (H2S) from natural gas, synthesis gas and other gas streams in refineries and chemical plants. CrystaSulf uses a modified liquid-phase Claus reaction to convert the hydrogen sulfide (H2S) into elemental sulfur which is then removed from the process by filtration. CrystaSulf is used in the energy industry as a mid-range process to handle sulfur amounts between 0.1 and 20 tons per day. Below 0.1 tons of sulfur per day is typically managed by H2S Scavengers and applications above 20 tons per day are typically treated with the Amine – Claus process.

Power-to-gas is a technology that uses electric power to produce a gaseous fuel. When using surplus power from wind generation, the concept is sometimes called windgas.

Sorption enhanced water gas shift (SEWGS) is a technology that combines a pre-combustion carbon capture process with the water gas shift reaction (WGS) in order to produce a hydrogen rich stream from the syngas fed to the SEWGS reactor.

References

  1. ↑ EVALUATION OF UPGRADING TECHNIQUES FOR BIOGAS, Margareta Persson, October 2003, School of Environmental Engineering, Lund University
  2. ↑ Zafar, Salman. "PSA System for Biogas Upgradation". Energy Consult. Retrieved 31 December 2013.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.