Techniques for transformation of biogas to biomethane

doi:10.1016/j.biombioe.2011.02.033

Biomass and Bioenergy

Volume 35, Issue 5, May 2011, Pages 1633-1645

https://doi.org/10.1016/j.biombioe.2011.02.033 Get rights and content

Abstract

Biogas from anaerobic digestion and landfills consists primarily of CH₄ and CO₂. Trace components that are often present in biogas are water vapor, hydrogen sulfide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen. In order to transfer biogas into biomethane, two major steps are performed: (1) a cleaning process to remove the trace components and (2) an upgrading process to adjust the calorific value. Upgrading is generally performed in order to meet the standards for use as vehicle fuel or for injection in the natural gas grid.

Different methods for biogas cleaning and upgrading are used. They differ in functioning, the necessary quality conditions of the incoming gas, the efficiency and their operational bottlenecks. Condensation methods (demisters, cyclone separators or moisture traps) and drying methods (adsorption or absorption) are used to remove water in combination with foam and dust.

A number of techniques have been developed to remove H₂S from biogas. Air dosing to the biogas and addition of iron chloride into the digester tank are two procedures that remove H₂S during digestion. Techniques such as adsorption on iron oxide pellets and absorption in liquids remove H₂S after digestion.

Subsequently, trace components like siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen can require extra removal steps, if not sufficiently removed by other treatment steps.

Finally, CH₄ must be separated from CO₂ using pressure swing adsorption, membrane separation, physical or chemical CO₂-absorption.

Highlights

► Biogas can be cleaned to remove trace components : review of techniques. ► Biogas can be upgraded to adjust the calorific value : review of techniques. ► Conclusions are focused on removal of water, H₂S and CO₂.

Introduction

Biogas is generated by micro-organisms in the absence of air by a so called anaerobic metabolism. Industrial biogas is produced at (1) sewage treatment plants (sludge fermentation stage), (2) landfills, (3) sites with industrial processing industry and (4) at digestion plants for agricultural organic waste, both mesophilic (35 °C) and thermophilic (55 °C) [1].

The nature of the raw materials and the operational conditions used during anaerobic digestion, determine the chemical composition of the biogas [2]. Raw biogas consists mainly of methane (CH₄, 40–75%) and carbon dioxide (CO₂, 15–60%). Trace amounts of other components such as water (H₂O, 5–10%), hydrogen sulfide (H₂S, 0.005–2%), siloxanes (0–0.02%), halogenated hydrocarbons (VOC, < 0.6%), ammonia (NH₃, <1%), oxygen (O₂, 0–1%), carbon monoxide (CO, <0.6%) and nitrogen (N₂, 0–2%) can be present and might be inconvenient when not removed (Table 1) [3], [4], [5], [6], [7].

The treatment of biogas generally aims at: (1) a cleaning process, in which the trace components harmful to the natural gas grid, appliances or end-users are removed, (2) an upgrading process, in which CO₂ is removed to adjust the calorific value and relative density in order to meet the specifications of the Wobbe Index. This latter parameter is dependent on both the calorific value and the relative density (Fig. 1) [2], [5].

After transformation, the final product is referred to as ‘biomethane’, typically containing 95–97% CH₄ and 1–3% CO₂. Biomethane can be used as an alternative for natural gas. In general, the type of end use of the biogas sets its quality demands [3]. An overview of the currently available and used biogas transforming techniques, operational conditions, efficiencies and drawbacks, is given below. This overview is structured according to the components that need removal or conversion.

Section snippets

Removal of water

Pipeline quality standards require a maximum water content of 100 mg m⁻³ water and compressed natural gas (CNG) vehicle fuel standards require a dew point of at least 10 °C below the 99% winter design temperature for the local geographic area at atmospheric pressure [8]. Untreated or raw biogas is usually saturated with water and the absolute water quantity depends on the temperature. For example, at 35 °C the water content is approximately 5% [2]. The lower the temperature, the lower the water

Removal of H₂S

Due to the damage that H₂S can cause in piping and motors, it is typically removed in an early state of the biogas upgrading process. Several techniques are applied: (1) removal of H₂S during digestion and (2) removal of H₂S after digestion (Table 3) [[2], [3], [5], [7], [8], [9], [10], [11]].

Organic silicon containing compounds (siloxanes)

Siloxanes are a group of components that contain a Si–O bound and organic radicals (methyl, ethyl and other organic groups) bound to the silicon atom. Siloxanes are used in cosmetics, pharmaceuticals and as anti-foam products and share useful properties like high compressibility, low flammability, low surface tension and water repelling properties, high thermal stability, low toxicity (non allergenic) and biodegradability. Both linear and cyclic siloxanes can be present in biogas [12].

The

Halogenated carbon hydrates

Higher and halogenated carbon hydrates are mainly found in landfill gas. They cause corrosion in engines and can be removed with activated carbon. Little molecules like CH₄, CO₂, N₂ and O₂ can migrate through the pores, while larger molecules are adsorbed. Generally, two tubes are used in parallel: one for treatment and one for regeneration. Regeneration is done by heating the activated carbon to 200 °C, thus evaporating the adsorbed components which are thereafter removed by an inert gas flow

Removal of oxygen/air

Oxygen and in part also nitrogen indicate that air has intruded the digester or landfill gas collector. This occurs quite often in landfills where the gas is collected through permeable tubes by providing a slight vacuum. Small concentrations (0–4%) of oxygen are harmless. Biogas in air with a methane content of 60% is explosive between 6 and 12%, depending on the temperature. A lower methane content in the biogas will increase the share of biogas needed in air for explosion to occur. Oxygen

Removal of NH₃

In industrial large scale cleaning processes, NH₃ is often removed from gas by a washing process with diluted nitric or sulfuric acid. The use of these acids demands installations made of stainless steal that can be expensive for small scale applications like biogas cleaning. NH₃ can also be removed with units filled with activated carbon and is also eliminated in some of the CO₂-removing units, like adsorption processes and absorption processes with water [5].

Removal of CO₂

Upgrading biogas to natural gas quality is a multiple step procedure. After removal of water (vapor), H₂S, siloxanes, carbon hydrates and NH₃, the removal of CO₂ is necessary in order to obtain the quality that meets the Wobbe Index. As the CO₂ of the upgraded gas is removed, is the relative density decreased and the caloric value increased, increasing the Wobbe Index (Fig. 1) [5].

Depending on its intended use (pipeline or vehicle fuel), biomethane consists typically of 97–99% methane and 1–3%

Removal of water

Generally condensation methods are mainly used as a first step in a biogas cleaning process. This step prevents contact of compressors, pipes, activated carbon beds and other parts of the process with water. In this way, corrosion is avoided.

In order to meet the standard for injection in the grid or vehicle fuel, water must be removed through an adsorption or absorption technique at higher pressure. The most common technique here is adsorption on alumina or zeolites.

Removal of H₂S

Commonly FeCl₂/FeCl₃ is

Acknowledgments

This project was funded by a grant from the Institute for the Promotion of Innovation by Science and Technology in Flanders within the scope of the TETRA project (IWT nr. 060168) titled: “Biomethane: processing of biogas to natural gas quality”. The authors are very grateful to the following companies: Thenergo NV, Desta NV, BiogasTec NV, Vanacker Johan bvba, Biogas-E vzw, Desotec NV, Bio-Energy, Belconsulting NV, Beke Environmental Technologies bvba, Global Water Engineering, Febem-Fege, Karel

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ReviewTechniques for transformation of biogas to biomethane

Abstract

Highlights

Introduction

Section snippets

Removal of water

Removal of H2S

Organic silicon containing compounds (siloxanes)

Halogenated carbon hydrates

Removal of oxygen/air

Removal of NH3

Removal of CO2

Removal of water

Removal of H2S

Acknowledgments

Energy Conv Managem

J Hazard Mater

Desalination

Separ Purif Techn

Desalination

Chem Eng J Biochem Eng J

Utvärdering av uppgraderingstekniker för biogas

Adding gas from biomass to the gas grid

Biogas upgrading upgrading to vehicle fuel standards and grid injection

IEA Bioenergy

Review
Techniques for transformation of biogas to biomethane

Removal of H₂S

Removal of NH₃

Removal of CO₂

Removal of H₂S