Technologies for Tar Removal from Biomass-Derived Syngas

Introduction

Global warming and climate change have encouraged researchers to look for an alternative to replace fossil fuels. Replacing biomass with fossil fuels reduces greenhouse gas environmental impact and offers a solution to waste [1]. Energy from biomass is created either by direct burning or thermochemical or biochemical conversion into liquid or gaseous fuels. An advantage of thermochemical processing instead of biochemical processing and direct burning is that it can more readily break down biomass in a controlled manner to produce high concentrations of desired intermediates [2]. The two primary thermochemical approaches to convert biomass into fuels are gasification and pyrolysis. Biomass gasification technology that converts biomass to synthesis gas (syngas) has been investigated thoroughly to convert the low value and highly distributed solid biomass to a uniform gaseous mixture mainly including hydrogen (H2), carbon monoxide (CO), methane (CH4), and carbon dioxide (CO2) [3]. The application of syngas includes power generation using internal combustion (IC) engines, gas turbines, especially in remote areas with no electricity supply, fuel cells, and the synthesis production of liquid fuels and chemicals. Although many advantages of biomass gasification and wide application of syngas, commercial acceptability of technology still faces challenges due to the difficulty in cleaning the unwanted by-products (Figure 1), such as particulate matter, condensable hydrocarbons (i.e., tars), sulfur compounds (H2S and SO2), nitrogen compounds (NH3), alkali metals (primarily potassium and sodium), and hydrogen chloride (HCl) (Table 1) to meet process requirements and pollution control regulations (Table 2) [4, 5].

( ) ( ) ( )

$$ s + O _ {2} \left(o r H _ {2} O\right)\rightarrow C O, C O _ {2}, H _ {2} O, H _ {2}, C H _ {4} + c $$ , , , , Biomass O or H O CO CO H O H CH other hydrocarbons

2 2 2 2 2 4

$$ \rightarrow t a r + c h a r + a $$

tar char ash organic contaminants $$ \begin{array}{l} \rightarrow H C N, N H _ {2}, H C I, H _ {2} S + c \\ \rightarrow p a r t i c u l a t e s \\ \end{array} $$ , , , HCN NH HCl H S other sulfur gases inorganic contaminants particulates

2 2 Figure 1: Scheme of biomass gasification [6].

Among them, tars that constitute 0.1–10% of produced gas recognized as a universal challenge of gasification system and gain priorities for removal, because of condensation and subsequent plugging — it can potentially foul filters, lines, and engines, as well as deactivate catalysts in cleanup systems or downstream processes, resulting in serious operational interruptions [4, 7, 8, 9]. They also cause ineffective energy conversion since tar can polymerize into more complex structures which contain significant amounts of unused chemicals that could have otherwise been transformed into useful fuel gases such as H2, CO, CH4, etc. Additionally, formation of tar aerosol is toxic since it contains compounds with carcinogenic character. Up to now, a great amount of work on tar removal or reduction and comprehensive reviews have been reported. The present review updates and complements those past reviews by extending the review to discuss tar removal technologies at demonstration and pilot scale.

Tar Definition and Classification

Biomass in the presence of oxygen or steam is gasified and produces syngas, organic and inorganic contaminants and particulates (Figure 1). Tar forms during the condensation of syngas, which is a progress from highly oxygenated compounds of moderate molecular weight to heavy, highly reduced compounds. Hundreds or even thousands of different tar species are generated during the gasification and condensation, because of different operating parameters, thermochemical conversion processes. Feedstock composition, processing conditions, especially temperature (Figure 2), pressure, type and amount of oxidant, and feedstock residence time strongly affect the tar composition [12], which results in difficulties in collecting, analyzing, and even defining them. A recent intergovernmental effort has produced an explicit definition of “tar” as “all hydrocarbons with molecular weights greater than that of benzene” [13]. There are “tar standard” methods that are developed to take the sample and analysis the tar sample [14, 15].

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Based on the reactivity of the compounds, tars can be classified into four product classes: (1) primary products — cellulose-derived, hemicellulose-derived and lignin- derived products. Primary tars form at the pyrolysis step and low temperatures (<500oC); (2) secondary products — phenolics and olefins. In the oxidation step, the primary tars transform and rearrange as secondary tars at a temperature higher than 500oC and reach its maximum at 750 ͦC; (3) alkyl tertiary products — methyl derivatives of aromatic compounds such as methyl acenaphthalene, methyl naphthalene, toluene, indene, phenol and benzene form at temperatures above 650oC and reach to the maximum at ~900 ͦC; and (4) condensed tertiary products— poly- aromatic hydrocarbons (PAH) series without substituents like benzene, naphthalene, acenaphthalene, antheracene/ phenanthrene and pyrene form at a temperature higher than 750 ͦC [9]. By this token, primary tars are released from devolatilizing feedstock; higher temperatures and longer residence time result in secondary and tertiary tars. Overall, the severe conditions of thermochemical processes produce an array of tarry compounds with diverse properties [16]. Based on the chemical, solubility and condensability of these tar components, tars can be classified into five classes (Table 3) [6, 17, 18].

The tar dew point is as important as the total tar concentration in the application of biomass-derived syngas. Tar dew-point is a powerful parameter to evaluate the performance of gas cleaning systems [19]. Typical tar dew points are between 150°C and 350°C, which is usually far above the lowest process temperature (∼30°C). Class 1, 4, and 5 tars influence the tar dew point considerably. They readily condense even at high temperatures and can cause major severe fouling and clogging problems in biomass- derived syngas power systems and efficiency loss. Class 2 and

3 tars include heterocyclic aromatics and benzene/toluene/ xylene (BTX) compounds; however, have a low effect on the tar dew point, compete with heavier tars for active sites, and makes problems in the catalytic tar removal method [16]. They are water-soluble, which means they create pollution problems in the aqueous phase of downstream wet gas cleaning equipment [4, 20, 21].

have claimed to reduce tar amount significantly. However, the method must be efficient in terms of tar removal, economically feasible, but more importantly, it should not affect the formation of useful gaseous products and leave a less environmental impact. The available methods to reduce tar concentration in syngas down to the desired level can be classified into two types according to the location where tar is removed: primary method (in the gasifier) and secondary method (outside the gasifier) [4, 22] (Figure 3). The following sections describe both methods, with emphasis on the secondary method.

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In biomass gasification, system configuration and operating parameters, such as temperature, pressure, equivalence ratio (ER, O2 content of air supply/O2 required for complete combustion), type of feed, gasifying medium, residence time and etc. play key roles in the product distribution, as well as tar generation [24]. In order to remove tar efficiently, numerous techniques related to primary method treatments have been investigated. The current issues of primary treatments are (1) the proper selection of the operating conditions, (2) the use of a proper bed additive or a catalyst during gasification, and (3) a proper gasifier design [22].

Temperature Residence time, temperature, and gasifying agents strongly influence the efficiency of thermal cracking. Lower temperature gasifiers are known to produce excessive tar emissions, which can be reduced through various efficient ways: (1) increasing residence time, such as using a fluidized bed reactor freeboard is partially effective in most cases; (2) using heat exchangers which directly contact tar with an independently heated surface. It required a significant energy supply and decreases the overall efficiency since heat exchangers have the disadvantages of higher energy input and good gas mixing [25] and (3) partial oxidation by adding air or oxygen that could increase CO levels at the

expense of a decrease in conversion efficiency and increase the operational cost [4, 26].

Increasing the temperature could significantly change the product distribution and reduce the tar content. Tar content, gas heating value, char conversion, and the risk of sintering critically depend on the gasification temperature range [22, 27]. In order to get low tar content in the resulting syngas, high operating temperatures (>750°C) are applied to get carbon conversion of the feedstock at a high level. Higher temperatures can lead to the lower yield of tar and higher yields of gaseous products [28, 29]. Hence, high-temperature gasifiers are designed to operate at thermal treatment conditions that promote tar decomposition. Temperature not only affects the amount of tar formed but also the composition of tar by influencing the chemical reactions involved in the whole gasification network. Usually, if gasification temperature exceeds 850°C, oxygen-containing compounds such as phenol, cresol, benzofuran, and other 1-ring, and 2-ring aromatics decrease and the formation of Class 4 and 5 (3- and 4-ring) aromatics increase quickly with other unwanted by-products. An increasing temperature also promotes the formation of gaseous products [20, 30]. Increasing the temperature inside fluidized-bed gasifiers will reduce tar concentration in the produced gas [22, 31]. An intensive cooling system is required to cool down the produced gas in down steam. Ash melts at higher temperatures is another challenge [16, 32]. There are several other factors that limit the operating temperature. Tar content, gas heating value, char conversion, and the risk of sintering critically depend on the gasification temperature range [22, 27].

Pressure An increase in pressure could noticeably affect tar concentration and composition. For example, Knight observed at T= 824 ± 8 oC, steam/feed ratio of 0.76±0.09 (w/w) almost complete removal of phenols for Wisconsin whole tree chips at 21.4 bar compare to 8 bar, but the polycyclic aromatic hydrocarbon (PAH) fraction increased [33]. Wang, et al. reported a decrease in the number of light hydrocarbons (LHC, lower than naphthalene) as well as that of tar in the fuel gas with an increasing equivalent ratio (ER, the required O2 for gasification to required O2 for full combustion of the given biomass) for pressurized gasification with 100% carbon conversion [33].

Residence time Residence time affects either tar yield or tar composition. Residence time has little influence on the tar yield, but it significantly influences the concentration of O2-containing compounds and 1, 2, 3, 4-ring compounds that depends on the superficial velocity of the wet producer gases in the gasifier. With increasing the residence time, O2-containing compounds and 1- and 2-ring compounds (except benzene and naphthalene) decreased, whereas that of 3- and 4-ring compounds increased [34]. Corella, et al. observed in biomass gasification with a catalyst, increasing the residence time decreases the total tar content [35].

Gasifying agents Gasifying agents, such as steam, air, O2, and CO2 strongly affect gasification reactions, resulting in different tar compositions and concentrations. Pure steam or mixed steam were used to speed up tar cracking and avoid lower heating value or poor gas composition. The pure steam gasification product is more or less free from N2 and more than 50% H2 in the syngas. It was reported SB ratio (steam–biomass ratio (SB); H2O/biomass [(kg h-1) (kgdaf h-1) -1]) affect the gasification products: in spite of the sharp reduction in tar (8% yield at 0.5 SB decreased to almost nil at 2.5), there was a sharp decrease in the lower heating value that was attributed by the decrease in CO [36]. Many researchers used steam–oxygen mixtures for biomass gasification because steam gasification is endothermic and sometimes requires heat supply in the gasifier. Oxygen used as a gasifying medium can provide the necessary heat for gasification, and then the gasifier works as an auto-thermal reactor without complex design. Although the operating parameters and gasifiers are not the same, the researchers report similar results: the effect of gasifier bed temperature on tar content is significant at lower GR values (gasifying ratio: (H2O + O2)/biomass [(kg h-1) (kgdaf h-1) -1]). Under selected conditions, more tar is formed with pure steam, than that with a steam–O2 mixture and less with air as gasifying agent [22, 37, 38]. Although, the results of using gasifying mediums are promising; choosing the proper ratio of steam/biomass and compromising on gas quality is still a big challenge [39, 40].

O2 and air can help to reduce tar concentration. According to the Kinoshita et al. report, tar yield and tar concentration will be reduced when the ER increases because extra oxygen reacts with volatiles in the flaming pyrolysis zone. ER affects both tar concentration and tar composition. At 700◦C, increasing ER from 0.22 to 0.32 reduced tar concentration around 30% but increased PAH fraction in total tar. At ER of 0.27, all phenols are converted but benzene, naphthalene, 3- and 4-ring compounds concentrations increase. The effect of ER can be more considerable at higher temperatures [34]. A similar trend was proved by various experiments with varying ER carried out by other researchers [41, 42]. Higher ER value leads to a lower concentration of H2, CO, and higher CO2 content in the syngas and decreases the heating value. This effect of ER is more significant at higher temperatures [34].

CO2, as a promising gasifying medium, also can help with tar reduction. With or without the presence of a catalyst, CO2 can convert into other products. Steam–CO2 mixture produced the highest activity char, which resulted in high ash content produced gas [40]. CO2 transforms tar to CO and H2 in the presence of a catalyst, and reduces the CH4 and C2- fraction (C2H2; C2H4; C2H6) [43].

Bed Additives and Catalysts: The ‘ideal’ bed additives have high efficiency with high selectivity and low cost. Although, catalysts for tar reduction have been extensively investigated, only a few have been tried as active bed additives inside the gasifier itself during gasification. These bed additives act as in situ catalysts, not only influence the gas composition but also affect the heating value of the producer gas. Dolomite attracted more attention among all the bed-active materials. Steam and dry reforming reactions eliminate tar over calcined dolomite. Many researchers have been done using this catalyst to remove tar in gasifiers. It was reported that 1-10 wt% of calcined dolomite is sufficient to improve the quality of the producer gas (reduced 40-80% tar ) without changing the heating value since the reduction of CO is compensated by the increase in H2 production; and CH4 and C2Hn concentration do not change considerably in the producer gas [35, 44, 45, 46, 47]. In addition, using dolomite in proper operating conditions can improve the production of a clean gas significantly. There are other in-bed additives, such as limestone, olivine, Ni-based catalyst, transition metal oxides, potassium carbonate, zeolite in silica–alumina matrix, and char. Bed additives have the following general functionalities [9, 22, 48]:

1. Change gas distribution,

2. Reduce tar, 3. Increase hydrogen production, 4. Decrease CO slightly and increase CO2, 5. No variation in the amount of CH4, The heterogeneous nature of these additives cause coke formation and deactivation. Attrition and carryover of fines are other problems of these heterogeneous additives which may overload the Particulate Matter removal system downstream of the gasifier. Sometimes adding these low- cost additives could cause the auger jam which is proved to be another major problem to the real-world operation.

Gasifier Design: Reactor design improves the efficiency, heating value of the syngas and reduces tar concentration. Gasifiers are categorized as either fluidized-bed or fixed-bed [49]. Fixed-bed gasifiers produce gas with less particulate content including ash, tar, and char compared to fluidized- bed reactors. Depending on the airflow direction and its feeding point, gasifiers are known as updraft, downdraft and cross-draft (Figure 4) [22, 50]. In updraft gasifiers, biomass enters the gasifier from the top, while air is supplied from the bottom. The produced gas in the updraft gasifier contains lots of tar and moisture. In downdraft gasifier, both wood and air are supplied from the top and move downward. The syngas contains a low concentration of tar and particulate. Overall thermal efficiency is relatively low. In cross-flow reactor, the biomass fed on the top and move downward while air enters from the side of the reactor and move upward. So, syngas leaves the gasifier from the top. The produced gas has high tar content and the overall energy efficiency is low [50].

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The most popular designs are the secondary or guard bed and the two-stage gasifier. A guard bed can be a fixed, bubbling fluidized, or moving bed that produced syngas pass through that and mixed with a secondary airflow that is injected into the system. The bed can be calcined dolomites, magnesites, and calcites from different quarries and the temperature are 800-900 ͦC [42]. Secondary air injection could increase the temperature which led to a significant tar reduction [42, 51]. Secondary to primary air ratio, gasifier size, and temperature are the key parameters for improving the tar removal efficiency. Usually, effective ways are to control secondary to primary air ratio and sufficiently reduce the total tar at certain ranges of gasification temperature; design longer size gasifiers to raise the temperature in the freeboard and bring a higher possibility of lower tar content in the gas.

A two-stage gasifier has been reported to be very effective to remove tar. The gasifier includes the pyrolysis zone and the reduction zone. A secondary air supply is introduced to the second stage (reduction zone) and its temperature is kept as high as possible to crack the tars formed during the first stage (pyrolysis zone). Successful operation of this type of gasifier mainly depends on how stable the pyrolysis zone is [51]. The stabilization of this zone depends on the balance between downward solid movement and upward flame propagation [22, 52]. Upward flame propagation should exceed wood consumption to prevent shrinkage of flaming pyrolysis zone and reaching to the second air inlet. Adjustments of the airflow by changing the width of the second air inlet, could keep flame propagation upward. There are several projects running, mainly focused on small scale biomass CHP (Combined Heat and Power) applications and industrial syngas applications Asian Institute of Technology (AIT), Thailand, designed a gasifier which involves two levels of air intakes and could reduce tar about 40 times less than a single-stage reactor under similar operating conditions [51, 53, 54, 55].

Several attempts have been made to develop two- stage gasification techniques: for example the same reactor design — a combination of pyrolysis of the biomass feed with subsequent partial oxidation of the volatile products in the presence of a charcoal bed was reported by Henriksen, et al. and Brandt et al respectively [52, 56]; Susanto, et al. developed a co-current moving bed gasifier with internal recycling and a separate combustion zone for the pyrolysis gas. Their aim was to produce a design suitable for scaling up a downdraft gasifier while maintaining a low tar content in the producer gas [57].

Although most of the attempts made to modify the gasifier design claimed to produce a clean gas, so far the primary treatments are not yet fully understood and have yet to be implemented commercially. These methods involve complex process considerations including the selection of optimized operating parameters, include the effects of active bed materials and the design of biomass feeding and dust removal systems [19]. In fact, there is a reluctance to implement these improved designs on a commercial level. This lack of enthusiasm is also linked to the complex gasifier constructions and/or the decreased gas heating value due to the partial oxidation [23, 58].

Gas Cleaning Technologies-Economical View, Technical Barriers, and Future Opportunities

Syngas from biomass gasification can be potentially used for different applications: compressors, IC engines, gas turbines, fuel cells and liquid hydrocarbon fuels. For each application, the syngas needs to meet specific gas quality requirements. Biomass gasifiers have the potential to provide electricity in rural regions especially in the third world countries where biomass can be supplied from abundant agricultural industries and does not have access to the grid i.e. bubbling fluidized bed gasifiers (BFBG) has the potential to supply 100kWe energy that is enough to supply electricity for 100 households [4]. In waste-to-energy cogeneration plants, the cost-effectiveness of the plants depends on parameters such as plant capacity, a charge levied upon a waste provided (the gate fee), waste calorific value, including the percentage of moisture and biodegradable matter, syngas cleaning cost, hazardous waste disposal cost, revenue from the selling of electricity and heat, and loan condition. Ash and residue disposal could increase the cost considerably [158]. The biggest challenge in the commercialization of large- scale biomass gasification is the lack of cost-effectiveness and efficient remediation technology that removes the contaminants are formed during the gasification [58] include particulates (ash, char, condensing compounds), alkali metals (especially sodium and potassium in vapor phase), fuel-bound nitrogen (forming NO_x_ during combustion), sulfur, chlorine, and tar [4]. Producer gas cleaning system is very expensive and can cost up to half of the total system capital cost [85].

The syngas cleanliness depends on the required specification of different applications. However, no single clean method succeeds easily to meet the application requirement. Although many reports have provided information about the configuration and performance of the clean system for biomass-derived syngas cleaning; only a few literature studies have reported tar concentration in the cleaned syngas [75, 90, 159, 160]. Hence, it’s unclear whether the final tar content in the syngas is reduced to acceptable ranges or not. Usually, the cold gas cleaning process includes a combination of physical-mechanical gas purification systems. A combination of oil scrubber and char-bed filter can eliminate tar up to 98% [161], however, pressure drop should be considered on a larger scale. Unyaphan, et al designed a cleaning system that include cyclone, ceramic filter, air cooler, water coolers, venturi scrubber, and packed bed adsorber in series. Injection of produced gas in the form of microbubbles inside the venture oil scrubber increased the tar removal efficiency up to 97.7% [162]. Quench coupled with Adsorption Technology (QCAT) filling with ceramic rings and adsorbents with a mesh size of less than 24 were able to remove tar up to 99.9% and reach the tar concentration to less than 20 mg/m3. Absorber regeneration could be a challenge [163]. Mixing wet vegetable oil scrubber as a physical removal method and dolomite guard bed as chemical tar removal technology reduced the tar 97% and reduced its dew point to 17 ͦ C. Pallozzi, et al. reported vegetable oil scrubber saturation time is very low and ~ 1.5h. The saturation time for dolomite guard bed is more than 14h but it is not able to reduce the dew point to less than 56 ͦ C. Mixing these two technologies can improve both saturation time to more than 12h and tar removal efficiency [164]. Up to date, the syngas clean technologies have been significantly developed for qualified gas production; however, commercialization of biomass energy is still challenging, mainly because of the tar issue. It needs a synergistic combination of developed efficient technologies for removing tar and other toxic gases and particles. Hence, syngas clean processing design and system configuration/optimization play an important role in syngas usage. The most concerns in syngas cleaning systems towards commercialization are total energy efficiency, cost, and environmental impact and the important parameters include gas composition, lower heating value (LHV), cold gas efficiency, waste product treatment, health hazard, and danger of fire and explosion, etc [67].

Primary treatments suffer from system complexity, limitation in the flexibility of feedstock and scale-up, compromised heating value, plugging of feeding, the production of waste streams, and a narrow operating window [20, 165]. Hence, the suitable scenario for real-life experience is to reduce tar to a certain level by thermal cracking or catalytic cracking and simplify the system design, rather than to provide for very low tar emissions solely through primary treatments. In the gasifier, catalytic cracking is more advantageous in terms of energy efficiency compared to thermal cracking.

Among chemical and physical methods in secondary treatment to remove contaminants especially tar, chemical conversion methods including catalytic cracking, thermal cracking, and plasma gasification are more reliable and have higher efficiency but dry and wet mechanical/ physical gas cleaning methods are simpler and less expensive. Cyclone, rotating particle separators (RPS), electrostatic precipitators (ESP), bag filters, baffle filters, ceramic filters, fabric/tube filters, sand bed filters, absorbers, etc. are considered as dry cleaning methods and spray towers, packed column scrubbers (wash tower), impingement scrubbers, venturi scrubbers, wet electrostatic precipitators, OLGA, wet cyclones, etc. constitute wet cleaning methods [59].

Physical methods of secondary treatments produce disposals that reduce energy conversion efficiency [4]. Technical barriers for dry gas cleaning include the higher cost of temperature tolerable materials, tar removal efficiency, and plugging issues. Low-cost catalytic filters applied towards processing at a large scale are a promising direction for further technical development. Scrubbing with an absorbent liquid is a well-known technique for treating low-concentration, high- volume gas streams [4]. Wet gas cleaning processes remove various contaminants that are soluble in the chosen solvent. Different solvents such as water, oil, diesel fuel, engine oil, or fatty acid methyl esters derived from vegetable oil are used. Solvent efficiency, solvent price, effluent treatment cost, and process complexity are barriers in gas cleaning. Water as a cheap solvent is able to remove NH3, HCl, H2S, and light and oxygenated tar compounds but has low efficiency in removing non-polar heavy and heterocyclic tar [126].

Conventional wet scrubbers using water have disadvantages such as clogging, salt formation inside and outside of the nozzle, soaping, fouling, expensive sludge disposal and water treatment, and reducing the heating value of the gas [4]. Hence, although the tar removal efficiency of water scrubbers is acceptable, it is not a recommended technology for tar removal in the future. Effluent solvent treatment in a cold gas system causes the system to be more amenable to large scale applications in terms of cost, especially wastewater treatment [58]. Conventional wet scrubbers use water as a scrubbing medium; although, they are cheap they also consume a high amount of water. Providing cool water especially in summer and the necessary wastewater treatment make this technology expensive and complex. Water is only able to remove hydrophilic components (~ 30% of tar); so non-polar components that constitute the main part of the tar still remain in the syngas and cause clogging and fouling [4]. Organic solvents such as washing reagents that have high efficiency (>60%) in removing non-polar compounds are more expensive compared to water. Their tar removal efficiency depends on the kind of solvent, the initial tar concentration, and residence time. Organic solvents remove light tars that reduce the heating value of syngas. Removing light tars from the scrubber media by stripping and recycling them to the syngas can improve the heating value. The net energy efficiency of a wet scrubber is low. In a wet scrubber, salt forms in the tips, nozzle, tubes, and walls of a scrubber. Currently, OLGA is the only cold gas cleaning system that is claimed to be economically promising, highly efficient, without fouling and tar-free technology; however, the feasibility, durability, and reliability for the large-scale process with low cost are still under investigation. OLGA by removing tar without water condensation and recycling heavy and light tar consequently recovering all the energy and omitting tar waste stream minimized the water treatment cost and reduced the operating cost [166, 85]. OLGA removes 99% of heavy and light tar. Cooling down the RME scrubber and OLGA high production cost may affect the cost feasibility of OLGA at a lower scale [73]. On a scale higher than 2000 mn 3/h, the total specific costs are reduced to ~1.5 €ct/kWhe. 99% of phenol and 97% heterocyclic tar are removed from wastewater by stripping [166]. So, there is no need to treat wastewater further [4]. OLGA technology is favorable at scales higher than 4000 mn 3/h (corresponding to 10 MW biomass input); Boerrigter, et al demonstrated investment and operational cost at various scales (Figure 8) [166]. OLGA was originally built to remove tar from high-temperature gasifier (> 800°C). Condenser and absorber work with oil and water, respectively. At gasification temperatures less than 750°C, oil and water are replaced with two different oils to avoid tar condensation and polymerization, keep oil viscosity in a certain level, and guarantee polar tar removal [167]. A cyclone and a filter are used downstream to remove the tar [74]. Table 6 demonstrates some large-scale systems with the methods that have been used for water treatment and water recycling and Figure 8 demonstrates total investment and running cost to upgrade produced gas.

Catalytic filters are another novel mechanical treatment method used to minimize the cost of wastewater treatment due to the pollution by tar compounds [4]. Thermal and catalytic conversion methods completely destroy the tar; so, they have higher energy conversion efficiency. Also, they reduce waste streams and convert contaminants to environmentally benign or useful products [58]. Thermal cracking needs a supply of energy; to make it economical, the reaction conditions should be optimized [4, 58]. Thermal tar cracking imposes a heat penalty [74].

In terms of thermal cracking methods, in order to improve the tar removal efficiency, theoretically, the effort can be made by increasing residence time, increasing temperature or promoting partial oxidation by adding air, or oxygen. However, it will largely increase the system and operational cost and decrease energy efficiency. Hence, the thermal cracking method is not the best choice for tar removal. The system design by applying thermal cracking methods should be based on the different gasifiers and take advantage of the high temperature of the gasifier or exit gas. If partial oxidation by adding air or oxygen is used, the balance between energy expense and whole process cost need to be considered.

6

Operational costs= 0.67€ct/kWhe (0.07 €ct/kWhe (utility) plus 0.6 €ct/kWhe (scrubbing liquid consumption)) Figure 8: OLGA system- investment costs vs. produced gas [78].

From an economic and technical point of view, the catalytic process is a promising alternative. The great advantage of this strategy is that a high degree of purity can be achieved at a lower temperature compared to thermal cracking. In addition, it has the potential to increase energy conversion efficiencies while simultaneously eliminating the need for the selection and disposal of tars [5]. Using a catalyst reduces the required energy, increases the tar conversion efficiency, and eliminates the need to collect and dispose of the tar [130]. Its major limitation is fast catalyst deactivation, catalyst disposal, and catalyst regeneration cost [58]. Ideally choosing a cheaper catalyst with high activity, stability at high temperature, high level of poisons acceptance, and less coke formation can improve syngas quality and lower system and operational cost [168]. Up to date, impregnation of natural catalysts such as olivine, dolomite, and zeolite with nickel provide a relatively inexpensive catalyst with higher stability and less coke formation [4]. Char-supported metal catalysts are also an encouraging way to achieve the goal of low cost and high efficiency [20]. Co-impregnation of nickel on natural catalysts (olivine, dolomite, and zeolite) provides a relatively inexpensive catalyst with reduced coke deposition on the catalyst and increased stability. Dolomites are the popular cheap catalysts for tar elimination. But they are very fragile and quickly eroded in fluidized beds with high turbulence. Olivine, similar to dolomite is cheap but has a higher attrition resistance. Its mechanical strength is comparable to sand, even at high temperatures. The catalytic activity of olivine is less than dolomite [169]. Other kinds of catalysts still need modification to be used in the catalytic cracking system. Non- nickel metal catalysts especially rhodium-based catalysts have high tar conversion efficiency but they are more expensive than nickel catalysts. Extending their lifetime is very necessary to make them more commercially viable. Acid and base catalysts produce lots of ash and the formation of coke deactivates the catalysts fast. Char and activated carbon are produced inside the gasifier and are cheap; but, coke blocks their pore too [4]. However, char could be replaced by the newly generated char in the same system regularly. Catalytic tar cracking is suitable for large-scale systems with high-temperature gasifiers and high tar levels [72]. In both thermal and catalytic cracking methods, waste stream can be an environmental barrier, and energy consumption affects the cost of the system [20].

Vecchione et al. compared a catalytic tar reforming system using a Nickel-based catalyst with a scrubbing system using oil and water as the scrubber media. Catalytic tar removal showed reliable performance (> 90%) at 800 ͦ C. However, catalyst deactivation due to sulfur, chlorine and alkali metals, coke formation and catalyst sintering due to

repeated regeneration cycles at a high-temperature increase the catalyst consumption and disposal cost. Using water as a scrubber media brings down the purification treatment cost but the used oil in the scrubber can be used as a fuel in the same gasifier or transferred to another system and reduce the disposal and treatment cost considerably [85] (Figure 9).

Figure 9: Total Investment and Running Cost to Upgrade Produced gas [170]. †Data for 130 – 161 Nm3/h producer gas output rate Two-stage hot air dustributor Fuel Hopper Bag house filter Heat exchanger Heat exchanger Heat exchanger Gasification zone Flare line Ash removal Ambient air blower Ambient air blower Vibrator motor Water cooling tank Hot air to dry feedsticks Producer gas

Raman, et al. [88] designed a dual-fired downdraft gasifier to reduce the tar content of the produced syngas. The dual system helps to produce low tar syngas, with tar level reaching less than 100 mg/Nm3. Moreover, producing the low tar content syngas eliminated the wet scrubber cleaning system and reduced the maintenance cycle 5-8 times. To remove the tar, the system was equipped with dry gas cleaning (filter) and indirect gas cooling (heat exchanger) (Figure 8). In this system, in the first stage, 70% of the required oxygen enters the reactor and produced moisture content charcoal, volatile matter, fixed carbon, and ash. In the second stage, 30% of the air is introduced to the bed vertically to provide the constant temperature. Air at high temperature inters this stage and its temperature was between 935 ͦ C and 1100 ͦ C that burn gas partially and cracks tar at high charcoal bed temperature. Using a dry gas cleaning system eliminates a large quantity of water usage and its disposal (Figure 10).

The Güssing system was developed for a small town in eastern Austria. Although the biomass is provided through the regional forest and provides energy for this town, still it was very expensive to use a gasifier as a source of energy. The system needs to be improved to become more reliable economically. In the Güssing method, biomass gasifies in a fluidized-bed steam gasifier at 850°C. A water heat exchanger reduces the temperature from 600°C to 150°C. Then the syngas passes through a fabric filter to remove dust. The removed dust is then recycled to the combustion zone. The downstream scrubber, which used biodiesel as a scrubber media, reduces the concentrations of tar, ammonia, and acidic gas components. The gas temperature reached 40°C after passing through the scrubber which is suitable for gas engine application. Part of the scrubbing agent is replaced with the fresh one continuously and the removed media is introduced to the combustion zone. The condensate is used to produce the steam needed in the gasifier. Accordingly, no residue or wastewater remains apart. The pre-coat material removed the chlorine. The system produces 2MW electricity and 4.5MW heat per year and its efficiency is 80%. Table 16 demonstrates the costs [68, 171, 172].

The Wiener Neustadt shaft gasification plant uses sedimentation and filtration to clean water or scrubber media. They recycle the sedimentation and filtration residues to the combustion zone [76, 173]. The TU Graz system gasifies biomass in a double-fire fixed bed gasifier. In downstream a cyclone separates coarse particles. Then, the syngas is cooled down to 180°C in a heat exchanger to pass through a fabric filter for further particle removal. A scrubber which is operated with a mixture of water and RME removes tar. A thermal conditioning unit with stripping/desorption and adsorption treats the accumulated wastewater. The treated water is subsequently pumped into the sewer system [68]. Pyroforce fixed-bed gasifier plant stores the scrubbing media temporarily in a condensate tank and is then disposed of. DTU (Danish Technical University) Graz gasifies biomass in two stages. First, the biomass is pyrolyzed, and then the produced tar and volatiles oxidize partially. The tar concentration in the syngas is as low as 25 mg/Nm3.

Producing syngas with less tar (primary method) and using chemical and physical methods (secondary method) to remove tar are both required to reduce provided syngas for further application. One method cannot be efficient enough to reduce the tar to the required level. In both cold and hot gas cleanup, multi-contaminant removal would be desirable to drive costs down [58]. Accordingly, a mixture of various methods is required to reach the needed level of tar economically. Besides tar, other contaminants exist that need to be removed to some extent like particulates, soot, NOx, sulfur and chlorine, and alkali metals. In removing the alkali salts electrostatic and bag filters or wet scrubbers are very efficient [157]. Nitrogen components such as NH3 and HCN that have a high solubility in water and are difficult to remove them are better to be removed before combustion or wet scrubbing. Standard catalytic methods for NOx treatment are able to remove nitrogen compounds easily [157]. In choosing a suitable technology, besides efficiency, the global cost plays an important role. Equipment investment costs decrease by increasing the produced gas scale; so, small- scale produced gas (0 – 100 Nm3/h raw produced gas) is usually very expensive. Installing a large central processing facility brings up the piping cost and introduces the technical challenges with regards to raw produced gas transportation such as pipe corrosion. However, one or more bulk units that include the entire upgrading system that only requires piping and wiring connections are the best solution to make the system economically. Increasing energy efficiency and improved contaminant removal and resistance can improve the system. In addition, more research work needs to be done in the area of design and optimization of syngas cleaning systems. Gas cleaning conditions and systems configuration to control the tar levels need to be continuously modified for better efficiency and cost-effectiveness.

Conclusions

As a renewable energy source, biomass has the potential to displace petroleum and other fossil fuels via generating syngas for fuel, energy, and products through gasification. Although significant improvement has been made, it seems still challenging to achieve commercial deployment of biomass gasification at a large scale which is reliable and durable in real integrated gasification (IG) based system environment, because of the syngas contaminants. The importance of syngas cleaning, especially targeting tar removal has driven research that aims to develop gas cleanup strategies that will improve efficiency and decrease cost for many decades. Many technologies have been developed to remove virtually all contaminants down to desirable lower concentration limits. Primary treatment cannot be counted on solely for tar removal mainly because of system complexity and inconvenient features. Physical methods especially wet gas cleanup technologies are effective, but not beneficial to reduce the overall efficiency due to syngas cooling, cost efficiency, and environmental sustainability due to post-treatment of toxic effluent streams. Based on this review, catalytic cracking of secondary treatment is more advantageous in terms of energy efficiency compared to thermal cracking. The dominant challenge in hot gas cleanup at demonstration or pilot scale is rapid catalyst deactivation and coking formation. This challenge is persistent due to the lack of thoughtful consideration to deactivation in many catalytic hot gas cleanup studies and design/optimization of syngas cleaning systems.

Recommendations

1. It is recommended that serious attention be devoted to evaluating catalyst resistance to deactivation in hot gas cleanup and also to strategic system configurations.
2. Using hybrid method can provide a solution to reach the needed level of tar, economically.
3. Beside total energy efficiency, and cost, the environmental impact of the system should be considered.
4. On developing the tar removal method, there should be concentration on the methods that produce less/no waste.
5. Scale of the gasifier should be considered in choosing the gas clean-up method.
6. Removal of contaminants such as particulates, soot, NOx, sulfur and chlorine, and alkali metals should be studied. Acknowledgement The authors are grateful to Energy, Mining and Environment research center of National Council of Canada (NRC-CNRC) for in-kind supports.