Proceedings World Bioenergy 2010

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78 world bioenergy <strong>2010</strong> D BIOfuELs fOR TRaNspORT – BIOGas, BIOEThaNOL aND BIODIEsEL
BIOGAS UPGRADING BY TEMPERATURE SWING ADSORPTION Tamara Mayer, Michael Url, Hermann Hofbauer Institute of Chemical Engineering, Vienna University of Technology Getreidemarkt 9/166, 1060 Vienna, Austria Phone: +43 1 58801 15901, Fax: +43 1 58801 15999, Mail: tamara.mayer@tuwien.ac.at ABSTRACT: This paper presents a novel process for biogas upgrading by means of temperature swing adsorption. Temperature swing adsorption process experiments were carried out in a laboratory test rig focusing on the process step of desorption. Desorption experiments were performed using three different variations of regeneration. Further on, performance and efficiency of the applied desorption variations were investigated. As a result, desorption by any combination of direct and indirect heating is considered as the best and most efficient way. Referring to the adsorption step, separation performance is excellent, carbon dioxide is fully adsorbed and pure methane can be obtained. Keywords: biogas, upgrading, adsorbents 1 INTRODUCTION Natural gas had a share of 24% in the gross energy consumption of the EU-27 in the year 2007. Therefore Europe’s dependence on natural gas can be considered as quite high. Due to the fossil origin of natural gas, it has two drawbacks. First, it will for sure run out at some point in the future and second, emissions deriving from natural gas are not carbon neutral but they affect the climate involving the whole issue of green house gas emissions and global warming. For these two reasons alternatives for natural gas have to be found and biogas represents one option thereby. Biogas is a renewable energy source deriving from anaerobic digestion of organic matter. If biogas should replace natural gas, it has to possess a certain quality similar to natural gas. In order to achieve this quality and to obtain gas which is suitable for replacing natural gas, biogas must be upgraded. This paper investigates a novel process for biogas upgrading based on the principle of temperature swing adsorption (TSA). 2 OVERVIEW OF BIOGAS UPGRADING TECHNOLOGIES Biogas typically consists of about two thirds methane balanced by about one third carbon dioxide and impurities such as hydrogen sulphide [1]. Moreover, biogas is usually saturated with water when leaving the anaerobic digestion plant. If biogas should be injected into the natural gas grid, it needs to exhibit a certain quality similar to natural gas which consists mainly of methane. Hence, gas components such as carbon dioxide and hydrogen sulphide have to be removed from biogas in order to obtain a high amount of methane. The entire process of biogas upgrading roughly comprises separation of water and hydrogen sulphide as well as methane enrichment. 2.1 Overview Among currently available biogas upgrading technologies, water scrubbing and pressure swing adsorption are clearly the two most applied ones [2] and they can therefore be classified as state of the art for biogas upgrading. Other technologies like chemical or organic physical absorption processes along with membrane based techniques are in an advanced state of development or even under demonstration. Although there are established technologies, which are already well developed and going to be continuously improved and optimized, innovative technologies find their way. These emerging biogas upgrading technologies include for instance cryogenic upgrading or in situ methane enrichment. Comprehensive reviews of biogas upgrading technologies and further related information are given elsewhere (e.g. in [3] or [4]). 2.2 Comparison of biogas upgrading technologies Biogas upgrading processes can be categorized into wet and dry processes depending on whether a liquid phase is required or not. Accordingly, all kinds of absorption processes are related to wet processes whereas dry processes involve adsorption processes and membrane processes like gas permeation. Wet biogas upgrading processes are characterized by the need of a liquid such as water or organic solvents in order to perform removal of carbon dioxide along with further undesired components. Disadvantages deriving from utilization of any kind of liquid are on the one hand the need of disposal and on the other hand possible treatment of the liquid before disposal. Moreover, wet processes require drying of the upgraded gas before it leaves the plant, what represents an additional process step compared to dry processes. However, pressure swing adsorption processes use activated carbon beds as guard filter for protection of the actual adsorbent which is a carbon molecular sieve. This process configuration causes the need of disposal of saturated activated carbon. Furthermore, biogas upgrading processes can be categorized according to the need of pressurization of raw gas. Correspondingly, the processes of pressure swing adsorption, water scrubbing, organic physical scrubbing as well as gas permeation require the raw gas to be compressed in order to perform the upgrading. On the contrary, chemical scrubbing with amines and the temperature swing adsorption process presented in this paper are so called pressure less processes. Further similarities of these two processes are utilization of amines as sorbent, low methane losses during the process and high methane content in the upgraded gas (98% or even above) because of excellent selectivity of the amine for carbon dioxide. However, there are differences concerning regeneration of saturated amines. Regarding world bioenergy <strong>2010</strong> 79
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