Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

Biogas from anaerobic digestion and landfills consists primarily of CH 4 and CO 2 . 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 H 2 S from biogas. Air dosing to the biogas and addition of iron chloride into the digester tank are two procedures that remove H 2 S during digestion. Techniques such as adsorption on iron oxide pellets and absorption in liquids remove H 2 S 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, CH 4 must be separated from CO 2 using pressure swing adsorption, membrane separation, physical or chemical CO 2 -absorption.

Techniques for transformation of biogas to biomethane

Biogas is a valuable renewable energy carrier. It can be exploited directly as a fuel or as a raw material for the production of synthesis gas and/or hydrogen. Methane (CH4) and carbon dioxide (CO2) are the main constituents, but biogases also contain significant quantities of undesirable compounds (contaminants), such as hydrogen sulfide (H2S), ammonia (NH3) and siloxanes. The existence and quantities of these contaminants depend on the biogas source (i.e., landfills, anaerobic fermentation of manure). Their presence constitutes a major problem because (i) they can be detrimental to any biogas thermal or thermocatalytic conversion device (e.g., corrosion, erosion, fouling); and (ii) they generate harmful environmental emissions. It is therefore important to include biogas purification steps upstream of its final use processes. This review is aimed at presenting the scientific and technical state-of-theart in biogas purification processes. Both mature, already-applied and promising, under-development technologies are reported and described here. © 2008 Society of Chemical Industry and John Wiley & Sons, Ltd

A review of biogas purification processes

Biogas as a renewable energy fuel – A review of biogas upgrading, utilisation and storage

The lack of tax incentives for biomethane use requires the optimization of both biogas production and upgrading in order to allow the full exploitation of this renewable energy source. The large number of biomethane contaminants present in biogas (CO2, H2S, H2O, N2, O2, methyl siloxanes, halocarbons) has resulted in complex sequences of upgrading processes based on conventional physical/chemical technologies capable of providing CH4 purities of 88–98 % and H2S, halocarbons and methyl siloxane removals >99 %. Unfortunately, the high consumption of energy and chemicals limits nowadays the environmental and economic sustainability of conventional biogas upgrading technologies. In this context, biotechnologies can offer a low cost and environmentally friendly alternative to physical/chemical biogas upgrading. Thus, biotechnologies such as H2-based chemoautrophic CO2 bioconversion to CH4, microalgae-based CO2 fixation, enzymatic CO2 dissolution, fermentative CO2 reduction and digestion with in situ CO2 desorption have consistently shown CO2 removals of 80–100 % and CH4 purities of 88–100 %, while allowing the conversion of CO2 into valuable bio-products and even a simultaneous H2S removal. Likewise, H2S removals >99 % are typically reported in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerophilic conditions. Even, methyl siloxanes and halocarbons are potentially subject to aerobic and anaerobic biodegradation. However, despite these promising results, most biotechnologies still require further optimization and scale-up in order to compete with their physical/chemical counterparts. This review critically presents and discusses the state of the art of biogas upgrading technologies with special emphasis on biotechnologies for CO2, H2S, siloxane and halocarbon removal.

Marc Ajhar

Papers

Siloxane removal from landfill and digester gas – A technology overview

Suitability of Tedlar gas sampling bags for siloxane quantification in landfill gas.

Siloxane removal with gas permeation membranes

Siloxane removal using silicone–rubber membranes

Membranes producing nitrogen-enriched combustion air in diesel engines : assessment via dimensionless numbers