**1. Introduction**

Conventional fuels such as coal, oil, and natural gas have significant negative environmental impacts and cannot be used on sustainable basis. These concerns have helped shift energy consumption towards renewable and environment friendly sources like biomass-derived energy [1]. Energy from biomass materials are obtained either by direct burning or conversion into liquid or gaseous fuels for practical applications. Technologies such as combustion, gasification, pyrolysis, hydrothermal liquefaction, and fermentation are successfully developed, while few are either commercialized or are under development that convert biomass into useful forms of energy. Among these technologies, gasification offers flexibility in feedstock selection (e.g., agricultural and forest residues, byproducts of food industry and bio-refineries, organic municipal waste), and the product gases (syngas) can be used as fuel to produce heat and electricity. Hence, the gasification is considered one of the most promising technologies [2,3] that utilizes biomass for energy production [4]. Compared to direct combustion, gasification has distinct advantages, such as increased power generation efficiency (up to a 60% increase) and the ability to use the syngas for products other than electricity [5,6].

Biomass gasification is a proven technology and converts carbonaceous materials into syngas, which consists of hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2) with methane (CH4), water (H2O), and higher hydrocarbons (HC) in minor quantities [4,7]. The application of syngas includes electricity generation using internal combustion (IC) engines and gas turbines and production of chemicals and liquid fuels [8]. Regardless of many advantages of biomass gasification and wide application of syngas, commercial acceptability of the technology still faces challenges due to difficulty in cleaning the produced syngas [8]. Impurities—such as tars, particulate matters, sulfur, and ammonia—are produced during gasification and entrained in the syngas. In most applications, these impurities must be removed before using syngas. Among these contaminants, tar is a big challenge.

Tar forms during condensation of syngas, and is classified as primary, secondary, and tertiary. Formation of primary tars, in the pyrolysis step, occurs at low temperatures (below 500 ◦C). In the oxidation step, with increase in temperature (above 500 ◦C), the primary tars transform and begin to rearrange as secondary tars. Further increase in temperature (above 800 ◦C) leads to formation of tertiary tars [4]. Tar consists of a wide range of hydrocarbons that can cause blockage in the downstream equipment, engine, and compressors [9–11]. The composition and quantity of tar in syngas depends upon gasification conditions and feedstock properties [2]. The acceptable limit varies depending on the application as shown in Table 1 [9–12].

**Table 1.** Tar acceptance limit for different devices. IC: internal combustion.


Removal of tar from the syngas is done either by chemical methods, such as catalytic and thermal cracking [5,13,14], which are expensive to operate [5,15] or by physical methods, such as wet scrubbers and water spray, which have issues with disposing tar mixed solvents [15–17]. Besides disposal of tar-mixed solvent, major disadvantages of using a wet cleaning system include decrease in heating value of the producer gas and the net energy efficiency. Therefore, development of tar removal technology with low cost, minimal disposal issues, and high efficiency are instrumental for future of gasification-based technologies [5,16]. Biomass-based dry filters could be an economical and environmental friendly option. Biomass-based filters using corn cobs, wood shavings, and dry coconut coir, charcoal and saw dust have been explored [17–19]. Corn cobs and woodchips reduced the tar content to 2 g/Nm<sup>3</sup> [18]. To further improve tar removal, other studies have used forms of direct cooling system, including wet scrubbers and water spray towers, before a dry biomass filter [15–17]. However, wet cleaning systems require expensive waste water treatment, so safe discharge of the tar mixed solution is a challenge [15–17]. A series of heat exchangers [20–22] and oil scrubber [23–27] have shown to be effective in reducing tar to as low as 10 mg/Nm3. The tar collected from the heat exchanger and used oil from the oil scrubber can be recycled back to the gasifier [28,29], which eliminates waste effluent treatment. A cleaning system equipped with an indirect cooling system that eliminates waste effluent treatment process and filter mediums which can be reused as gasifier feedstock is a promising alternative to conventional cleaning technologies. However, dry biomass-based filter systems are not tested in combination with indirect cooling system (heat exchanger) or oil scrubber.

The goal of this study was to improve tar removal efficiencies of a dry biomass-based filter in combination with a heat exchanger and oil bubbler. The tar removal efficiencies were evaluated with three filtering methods: (i) wood shavings filter; (ii) wood shavings filter after cooling with a heat exchanger; and (iii) wood shavings filter after cooling with an oil bubbler.
