The novel OLGA technology for complete tar removal from biomass producer gas

Transcription

1 Paper presented at: Pyrolysis and Gasification of Biomass and Waste, Expert Meeting, 30 September - 1 October 2002, Strasbourg, France The novel OLGA technology for complete tar removal from biomass producer gas Patrick C.A. Bergman,Sander V.B. van Paasen, and Harold Boerrigter * Energy research Centre of the Netherlands (ECN), Unit ECN Biomass, P.O. Box 1, 1755 ZG Petten, The Netherlands ABSTRACT A new process for the removal of tar from biosyngas, OLGA, is presented. This gas cleaning technology is based on physical principles and is designed to eliminate tar problems that relate to fouling and wastewater pollution. The applied principles are confirmed by experimental validation. OLGA can be operated without suffering from harmful effects on important gasification characteristics as cold gas efficiency, gasifier simplicity, and feedstock flexibility. Some fundamental considerations in the development OLGA are discussed. Amongst these is a different approach to the tar problem. This approach does not consider the total tar content as decisive in the evaluation of gas cleaning equipment. Instead, evaluation is focussed on whether tar properties as condensation behaviour and water solubility can still occur. A biosyngas free of tar should therefore be synonymous to a biosyngas that is free of tar related problems. INTRODUCTION THE TAR PROBLEM Tar can be considered as the Achilles heel of biomass gasification as tars are the major technical obstacle in the implementation of this technology. Condensing tars dramatically foul gascleaning equipment and liquid tar droplets that enter prime movers hamper the operation of these end-use applications of the biosyngas [1]. In the last few years, it has become clear that tar also plays an important but negative role in wastewater management. In conventional water-based gas cleaning systems tars and condensed water are mixed, creating an often costly and difficult water treatment problem. Regarding the presence of tars in biosyngas, it may be stated that tars are equivalent to a major economic penalty in biomass gasification. Tar aerosols and deposits lead to more frequent maintenance and repair of especially gas cleaning equipment and resultantly lower plant capacity factors. This leads to a decrease of revenues or to higher investments, as some equipment will be installed in duplicate to overcome standstills. Furthermore, removal of tar components from the process wastewater requires considerable investments that can even be dramatic as some tar components show poisoning behaviour in biologic wastewater treatment systems (e.g. phenol). Several measures for tar removal have been studied or are under investigation. These measures can be divided in primary measures, measures inside the gasifier, or secondary measures, measures downstream of the gasifier. With the prospect of operating an integrated biomass gasification installation without struggling with tar anywhere downstream, many researcher focus on effective primary measures. This should make complex and expensive gas cleaning equipment obsolete. Although measures inside the gasifier may be fundamentally more ideal, they have not yet resulted in satisfactorily solutions. Some of the primary measures do result in low tar emissions, but suffer from disadvantages related to, for instance, limits in feedstock flexibility and scale-up, the production of waste streams, a decrease in cold gas efficiency, complex gasifier constructions, and/or a narrow operating windows. Although primary measures can reduce the tar content considerably, it is foreseen that complete removal is not feasible without applying secondary measures (see Figure 1). * Corresponding author; phone: ; boerrigter@ecn.nl.

2 Tar removal 100% Secondary measures sum of tars Primary measures State-of-the-Art today Technology development in time Figure 1. Illustration of the need of primary- and secondary measures versus technology development in time. Secondary measures that have been investigated till now, exhibit similar deficiencies. The measures are either not effective enough, too expensive, or the tar problem is shifted to the treatment of wastewater. However, a secondary measure can be feasible without needing primary measures. This becomes even stronger when the problems with wastewater treatment can be eliminated as well. A secondary measure should therefore form the basis for tar removal from biosyngas and primary measures could possibly be used for its optimisation. CURRENT SITUATION (STATE OF MIND) During the 12 th European Biomass Conference held in Amsterdam in 2002, some attendants ventilated great disbelieve in the future prospects of biomass gasification. Tar was tagged to be the major reason for this lack of confidence. If the tar problem could not be solved within the last few decades, after trying so many alternatives for tar removal, why should one believe in future systems wherein tar would not be the major obstacle? The authors believe that there are arguments enough to disprove this pessimistic view. ECN started mid 2001 with the development of a new technology called OLGA. The starting point of the development of the patented OLGA technology was to establish tar removal from syngas on the basis of a scrubbing liquid other than water (a secondary measure). This paper will outline some of the features of OLGA, especially with respect to the design approach and performance characteristics. TAR DEFINITION AND PROPERTIES Tar comprises a wide spectrum of organic components, generally consisting of several aromatic rings. Reported tar concentrations are strongly dependent on the tar definition used and the measurement method applied. The large amount of tar definitions and measurement methods, as well as the wide spectrum of organic compounds, makes it almost impossible to capture "tars" with a clear definition. This already has resulted in unrealistic tar specifications for end-use applications, as such, that in many cases practically complete removal of tar is demanded. For the development of OLGA, a totally different approach was chosen. This approach does not concentrate on the tar content, but on the behaviour (i.e. the properties) of the tar. A feasible tar removal system is not a system that totally removes tars, but is a system assuring that tar related problems do not occur anymore. Hence, a biosyngas free of tar is in this approach synonymous to a biosyngas that is free of tar related problems

3 CLASSIFICATION SYSTEM According to the ECN definition, tar comprises all organic components having a higher molecular weight than to benzene. Benzene is not considered to be a tar. ECN uses a tar classification system comprising six classes (see Table 1). This classification system is in particular developed to provide easy insight in the general composition of tar. Trends are easier recognised on the basis of these classes. However, for more specific problems or issues the detailed data will remain necessary. Table 1. Tar classification system. Class Type Examples 1 GC undetectable tars. biomass fragments, heaviest tars (pitch) 2 Heterocyclic compounds. These are components that generally exhibit high water solubility. phenol, cresol, quinoline, pyridine 3 Aromatic components. Light hydrocarbons, which are important from the point view of tar reaction pathways, but not in particular towards condensation and solubility. toluene, xylenes, ethylbenzene (excluding benzene) 4 Light polyaromatic hydrocarbons (2-3 rings PAHs). These components condense at relatively high concentrations and intermediate temperatures. 5 Heavy polyaromatic hydrocarbons ( 4-rings PAHs). These components condense at relatively high temperature at low concentrations. naphthalene, indene, biphenyl, antracene fluoranthene, pyrene, crysene 6 GC detectable, not identified compounds. unknowns From the practical viewpoint, the classification comprises only tar components that can be measured. Classes 2 to 6 are sampled using the solid phase adsorption (SPA) method and measured by gas chromatography (GC). Although class 6 tars are sampled and measured (a peak is found in the chromatogram), it is unknown what the individual components are. In principle components in this class belong to the other classes, but are here lumped to a single concentration representing the unknowns. Class 1 represents the heavy tar fraction (roughly 7-ring PAHs). These components cannot be determined by the combination of SPA and GC. The components are measured by weight and thus represent the gravimetric tars. TAR CONDENSATION: THE TAR DEWPOINT Tar leads to fouling once the gas becomes (over) saturated with it. This leads to aerosol formation and depositions inside the installation. These fouling phenomena are not of concern as long as all the tar is present in the gas phase. It is therefore believed that the tar problem is fundamentally not concerned with the tar quantity, but is with the properties and the composition of the tar. The condensation behaviour of tar is an integral effect of all tar components that are present in the syngas. The components their individual contribution to the total tar vapour pressure is therein decisive. When the tar vapour pressure exceeds the saturation pressure of the tar, the gas becomes (over)saturated according Raoult s Law [2]. Thermodynamically, this state leads to condensation of the saturated vapour. The tar dewpoint is the temperature at which the real total partial pressure of tar equals the saturation pressure of tar. Hence, in condensation related issues, the tar dewpoint is a powerful parameter to evaluate the performance of gas cleaning systems. It

4 is believed that, when the dewpoint of tar is reduced to levels below the lowest expected temperature, fouling related problems by condensation or tar aerosols are solved. To use this approach in design issues, a calculation tool has been developed to predict the tar dewpoint on basis of the concentration of the individual tar components in the syngas. An illustration of the relation between the tar dewpoint and tar concentrations is provided by Figure 2. Condensation curves are given for the individual tar classes (as defined in Table 1), e.g. the dewpoint curve for class 5 is calculated including only class 5 tars. Furthermore, each tar component is contributes equal to the total concentration on mass basis. The dewpoint calculation excludes tar class 1, as the components are not known. For a CFB gasifier it is believed that tars that belong to class 1 start to condense around ºC. Temperature ( C II III IV V tar-class concentration (mg/m n 3 ) Figure 2. The tar dewpoint of the different tar classes in relation to the concentration. Leaving out class 1 in this discussion, it can be derived from Figure 2 that class 5 tars dominate the dewpoint of tar. Even for very low concentrations of class 5 tars (e.g. <1 mg/m n 3 ) a dewpoint below 100ºC cannot be obtained. The graph clearly points out that, dependent on the concentration in the syngas, classes 2 and 4 need to be partially removed for a proper tar dewpoint of about 25ºC. The class 3 tars play an unimportant role in this matter. WATER SOLUBILITY The pollution of wastewater is in strong relation to the type of tar components being present in the biosyngas. The (poly)aromatic non-polar components will practically not dissolve, however, small non-polar components may still form a problem as they dissolve in small amounts that can exceed allowable concentrations. In general this will not cause a problem as due to the volatility of these components they are easily removed from water. Polar components on the other hand, in particular phenol, dissolve in large quantities and are very difficult to remove. Waste process water from biomass gasification must be clean, which is much easier to accomplish when pollution of tar can be avoided. A similar tool as for the calculation of the tar-dewpoint is in development for the calculation of the water solubility of the tar classes. This tool was not available for this work. The class 2 tars are most important with respect to water solubility. This class comprises the oxygen, nitrogen, and sulphur hetero-atoms containing components that dissolve well. Also the class 3

5 (and benzene) can be important with respect to wastewater treatment. These components may dissolve in large quantities but they are readily removed. Other classes are typically insoluble and form a two-phase liquid/liquid system of tar and water with a rather low mutual solubility. DESIGN OF OLGA PROCESS OBJECTIVES The (ambitious) objective in the design of OLGA was the creation of a new process that eliminates issues involved with tar condensation and water solubility. The process to develop should be competitive with competing technologies. PROCESS TASKS The approach to design a process for complete and selective tar removal in a controlled way, started with a definition of the required tasks for such a process. Primary and secondary tasks are distinguished (see Table 2). Primary tasks deal directly with the objective. Secondary tasks are additional and need to be accomplished to obtain a system that also meets specifications from, for instance, the economic and legislation points of view. In contrast to the primary tasks, the secondary tasks are only indirectly responsible for the technical feasibility of the system. Primary tasks Table 1. Primary and secondary tasks of OLGA. 1. Selective tar removal (viz. no water removal). 2. Deep removal of tar components resulting in a syngas quality for which no tar condensation or tar desublimation occurs, and simultaneously absence of tar aerosols, while applying the desired operating conditions. 3. Specific removal of heterocyclic tar components (in particularly phenol), to avoid water contamination in the wet syngas cleaning that is necessary to remove contaminants like NH 3 and HCl. Secondary tasks 4. Avoiding waste streams 5. Avoiding a (too) high scrubbing liquid consumption. This is in particularly important with respect to process economics, but surely also with respect to de sustainable image of biomass gasification. 6. Removing dust and/or fines that have not been removed by dust separators upstream of OLGA. 7. Preventing a high gas-side pressure drop over de gas cleaning system. BASIC PROCESS STRUCTURE The biggest challenge is to remove tar selective from the syngas (task 1). In particular water must not be absorbed by the applied scrubbing liquid, as that would still lead to the pollution of process water. Similarly, the permanent gas components in the biosyngas (e.g. CO, H 2, and CO 2 ) should not dissolve in, or be absorbed by, the scrubbing liquid. This would not contribute to the simplicity of the process. Deep removal of classes 1 and 5 tars is desired in order to decrease the tar dewpoint and to eliminate condensation problems (task 2). Complete collection of these tar classes yields a dewpoint below 100ºC (cf. Figure 2). Furthermore, to operate end-use applications that require syngas temperature below 50ºC, without the risk of tar condensation, classes 2 and 4 tars need to be removed partly. The required collection efficiency depends on the actual amount and composition of the tars in the syngas (cf. Figure 2). Although the collection efficiency of class 2

6 tars (the heterocyclic tars) needs not to be complete from the condensation point of view, essentially quantitative removal is required to avoid of pollution of process water (task 3). The elimination of all condensation-related issues means also that no tar condensation may occur upstream OLGA. Hence, the syngas inlet temperature of OLGA must be higher than the tar dewpoint of the raw syngas. As a consequence of task 1, water present in the syngas may not condense simultaneously with tar. Therefore, the exit temperature of OLGA must remain above the water dewpoint of the syngas. Figure 3 illustrates this further by positioning OLGA with respect to both the tar and the water dewpoint. gasifier exit syngas temperature tar condensation OLGA tar dewpoint water condensation end-use application water dewpoint downstream gasifier Figure 3. The position of OLGA with respect to the syngas temperature and to the dewpoints of water and tar. Explanation on zones 1, 2, and 3 downstream the gasifier: (tar phase/ water phase) 1: (G/G), 2: (L/G), 3: (L/L) Upon cooling of the biosyngas, the temperature decreases below the tar dewpoint and tar condensation gradually takes place until syngas is not cooled further. At the resulting temperature, between the dewpoint of tars and water, a liquid/gas (L/G) phase system is obtained (L represents liquid tar). The scrubbing liquid acts as the medium to collect these liquid tars. The remaining tars are collected into the scrubbing liquid by absorption (i.e. the scrubbing liquid acts as absorption medium). This is illustrated by Figure 4. The degree of absorption latter can be controlled by changing the operation conditions and will be determined by the desired tar dewpoint of the outlet syngas. In the regeneration of the scrubbing liquid the tar is removed upon which some scrubbing liquid may evaporate.

7 tar + unrecovered scrubbing liquid syngas Liquid tar collection Gaseous tar absorption syngas make-up scrubbing liquid Figure 4. Basic structure of OLGA. CONCEPTUAL PROCESS STRUCTURE Figure 5 depicts the general concept of the air-blown biomass gasification process using OLGA. The produced syngas is first cooled and de-dusted upstream of OLGA. Downstream OLGA, the main (inorganic) non-tars impurities NH 3 and HCl are removed by wet scrubbing and water is condensed out due to further cooling of the gas. The syngas is then suitable for most of end-use applications as it is free of condensable tars, tar aerosols, as well as inorganic impurities. Tar and unrecovered scrubbing liquid are fed to the gasification process and also the separated NH 3 is recycled to the gasification process. Similarly to the tar recycling, separate experiments at ECN have shown that NH 3 is converted to nitrogen in the gasifier [3]. tar + unrecovered scrubbing liquid water biomass Gasification Cooling De-dusting tar removal OLGA water removal NH 3 /HCl removal syngas air make-up scrubbing liquid NH 3 Figure 5. General concept of air-blown biomass gasification using OLGA for tar removal. Although the major part of dust and/or fines will be collected upstream OLGA, it is inherent to the use of a scrubbing liquid as medium that fines will collected in the scrubbing liquid. The removal of small particles in OLGA is considered as a secondary task (task 6) that is optimised as much as possible so that small particles do not have to be dealt with further downstream. It is considered as one of the interesting and economical attractive optimisation options of OLGA to remove the full dust load from the biosyngas, making the separated dust removal step upstream obsolete. Intrinsically related to the use of a scrubbing liquid as process utility, is the consumption of liquid due to bleed streams and volatilisation upon stripping the tar. Even if the scrubbing liquid is very effective and the losses minimal, the process may not create another waste stream (task 4) as this causes an economic penalty for the waste handling. dust

8 It is furthermore undesirable as the consumption of scrubbing liquid creates an image for the process, which is in contradiction with the green nature of the biomass gasification. The necessity to minimise scrubbing liquid losses motivated the inclusion and design of a regeneration step and has been a major selection criterion for the scrubbing liquid to be applied (task 5). The collected tar and the unrecovered scrubbing liquid from the regeneration step are recycled to the gasifier, preventing a waste stream. Separate experiments carried out at ECN have shown that tars can be recycled and gasified without accumulation [3]. APPLIED EQUIPMENT A simplified flow sheet of OLGA is provided by Figure 6. OLGA consists of scrubbing towers interacting with each other in a classical absorption-regeneration mode. Syngas is fed to the tar collector in which tars are removed from the gas to the desired level. The scrubbing liquid with the dissolved tars is regenerated in the stripper. Part of the scrubbing liquid exiting is purged and charged to the gasifier. In case of air-blown gasification, air is used to strip the tar. Subsequently, the air with the stripped tars is used as gasifying medium. syngas air tar collection scrubbing liquid recirculation scrubbing liquid regeneration syngas liquid purge air Figure 6. Simplified flow sheet of OLGA. A practical reason for the selection of a scrubbing tower for the removal of tar is that the gas-side pressure drop can be limited (task 7). The first application of OLGA is foreseen for atmospheric gasification processes and it is important to limit the total gas side pressure drop over the whole installation from gasifier to end-user of the syngas. The typically used simple solid feeding systems, which encounter the highest absolute pressure, used can generally only function at small pressure drops. Therefore, equipment in OLGA with inherent high-pressure drop, such as venturi scrubbers, is avoided. The selected equipment for OLGA is mature, a lot of operational experience is available, and moreover, it is well known how to scale-up this type of equipment. EXPERIMENTAL VALIDATION OLGA was built on lab-scale to demonstrate its feasibility (Proof-of-Principle). The lab-scale OLGA was designed for this capacity and was built according general scale-up rules valid for the selected equipment. This ensured that the generated data could be used for scale-up of OLGA. The experimental program consisted of two series of experiments. The first series dealt mainly with the primary tasks (cf. Table 1). The second series dealt with the generation of design data

9 for scale-up of the OLGA to bench-scale. The ECN atmospheric air-blown bubbling fluidised bed (BFB) gasifier WOB was used to generate biomass-derived syngas. The WOB has a capacity of 1 kg/h of fuel producing 2 m n 3 /h wet biosyngas. The syngas typically contains 18 g/m n 3 of tar (including toluene and xylenes; on dry basis). Dust was removed from the biosyngas with a high efficiency filter before the syngas was fed to OLGA. Figure 7 shows a typical result obtained from the proof-of-principle experiments. The outlet gas of OLGA is free of heavy tars (class 1 and 5), also free of class 4 tars and nearly free of tars that are classified as unknown (class 6). The collection efficiency of the class 3 tars is about twothird and nearly all tar present in the syngas exiting OLGA are class 3 tars (viz. toluene and xylenes with a concentration of 1,534 mg/m n 3 ). A few milligrams of class 2 tars are still present in the cleaned biosyngas (removal efficiency 97%). Permanent gas components (e.g. CO and H 2 ) do not dissolve in the scrubbing liquids and also water is not absorbed. Class 6 Class 5 Class 4 Class 3 Class 2 Class 1 65% 99% 97% Inlet tar concentration: 18,000 mg/m n 3 Outlet tar concentration: 1,550 mg/m n 3 Tar dewpoint: -16ºC 0% 25% 50% 75% 100% collection efficiency The dewpoint corresponding to the presented tar collection efficiencies is -16ºC. In considering water solubility, the class 2 tars were not completely removed. However, the generated design data clearly indicate that the system can be easily optimised towards deep removal of class 2 tars, especially phenol. This was confirmed in an additional experiment in which the optimal conditions for phenol removal were applied. This experiment revealed that 99% of the phenol can be removed. The design data that was generated from the experimental results reveal that OLGA is also capable of complete tar removal if that would be necessary. The second series of experiments showed that the applied scrubbing liquid can be easily regenerated without significant losses by evaporation. The tar collection and stripping of the scrubbing liquid proved to be very effective and without unacceptable losses of scrubbing liquid. The syngas conditioned by OLGA did also not contain any particulate matter anymore. Although the concentration of particulate matter before OLGA was not measured, small amounts of dust have been found in the scrubbing liquid. This indicates that fines are removed as well. CONCLUSIONS Figure 7. Typical results obtained from the lab-scale OLGA facility. Based on the experimental results it can be concluded that with the OLGA technology a new and effective secondary measure is introduced for tar removal in biomass gasification. The approach of eliminating the problems due to tar (condensation and water solubility) instead of eliminating tar itself has resulted in a structural solution to the major tar problems. Despite the high tar collection efficiency of OLGA, no concessions need to be made to other important gasification characteristics, for instance, the cold gas efficiency remains unaffected. Also complex gasifier

10 constructions as a consequence of primary measures are not required. Consequently, a simple and robust gasifier can be deployed. This assures the desired fuel flexibility and flexibility in operation. OUTLOOK The future for biomass gasification is prosperous now the tar issue has been resolved and this major (technical) obstacle for implementation is removed: Achilles is no longer vulnerable. Together with the Dutch company Dahlman Industrial Group, ECN Biomass is working on the further development of the OLGA technology and demonstration on bench-scale. Engineering has already started, construction is foreseen to start in December 2002, and the first tests are scheduled for June After this Proof-of-Concept phase the OLGA technology is ready for commercialisation. ACKNOWLEDGEMENTS The work described in this paper was partly sponsored by Novem-NECST. Furthermore, the authors acknowledge Herman Bodenstaff, Ruud Wilberink, and Johan Kuipers for carrying out the experimental work and gas analyses. REFERENCES 1. Swaaij, W.P.M. van; Aarsen, F.G. van den; Bridgwater, A.V.; Heesink, A.B.M. (1994), A review of Biomass Gasification, A report to the European Community DGXII Joule Programme. 2. Reid, R.C.; Prausnitz, J.M.; Polling, B.E. (1988) The properties of gases & liquids, McGraw- Hill, 4 th edition. 3. ECN Biomass, Publication in preparation.