*2.1. Experimental Set Up*

Schematic diagrams of the experimental set up are shown in Figures 1 and 2. Oklahoma State University patented downdraft gasifier (20 kW) [30] with an average feedstock consumption rate of 15 kg/h was operated at a low equivalence ratio (0.20) to generate syngas with high tar content. Operational conditions of gasifiers for each treatment are presented in Table 2. Switchgrass used as gasification feedstock was grown at the Agronomy Research Station of Oklahoma State University (Stillwater, OK, USA). A 10-layered cylindrical biomass filter system with total surface area of 1.26 m<sup>2</sup> (bed height of 0.96 m and diameter of 0.17 m) was designed. Syngas entered into the filter at the bottom at a flow rate of 35 Nm3/h. Syngas exited through the filter top (Figure 3). Micro data acquisition (DAQ) K-type thermocouples (Omega Engineering, Norwalk, CT, US.) were installed to measure the gas temperature at the inlet and outlet of the filter system. The pressure drop across the filter system was measured using U-tube manometers (Dwywe Inc, Wilmington. NC, US).

**Figure 1.** Schematic diagram of the three gas cleaning treatments: (**I**) wood shavings filter; (**II**) heat exchanger + wood shavings filter; and (**III**) oil bubbler + wood shavings filter.

**Figure 2.** Wood shavings filter column installed with a gasifier system at Oklahoma State University.


**Table 2.** Gasifier operating conditions.

**Figure 3.** 10-layered biomass-based filter system.

The wood shavings, screened through 2 mm wire mesh, with a particle size between 2 mm and 1.5 cm were used as filter medium (Figure 4). Wood shavings were purchased from a local sawmill in Stillwater. A total of 4.0 kg of wood shavings was used for each run to maintain the uniform packed bed density of the filter. Syngas was passed through the filter for 1.0 h after the gasifier was at equilibrium condition. Chilled water (5 ◦C, flow rate of 15 L/min) for single tube heat exchanger (1.19 m height and 0.02 m diameter; Figure 5) was supplied by a water chiller (Model 30070, Schreiber Engineering Corporation, Cerritos, CA, USA). The oil bubbler (0.48 m height, 0.38 m diameter and 10 mm bubble size with 1-inch oil level; Figure 6) used canola oil that was supplied by Jedwards International, Inc. (Quincy, MA, USA). The oil bubbler was also cooled with chilled water (5 ◦C, flow rate of 15 L/min). The inlet and outlet syngas tar contents, pressure drop across the filter, temperatures at inlet and outlet of the filter, heating values of syngas and gas temperatures at inlet and outlet of the heat exchanger were recorded.

**Figure 4.** Wood shavings used as the filter medium for all treatments.

**Figure 5.** A single tube heat exchanger used for cooling syngas for treatment II.

**Figure 6.** Oil bubbler unit for treatment III.

### *2.2. Tar and Syngas Sampling and Analysis*

The tar removal efficiency of the filter was determined by comparing tar content at inlet and outlet of the filter. Tar sampling protocol by Good et al. [31] was used. The method consists of a series of six impingement bottles in which the syngas was passed. The first one served as a moisture collector. The gas then flowed through a series of four impinger bottles filled with a solvent, i.e., acetone to dissolve the tars. The last bottle was kept empty to ensure the collection of final condensates. For tar analysis, syngas was sampled at the inlet of cleaning system approximately 30 min after the gasifier reached equilibrium condition. The gas at the filter outlet was sampled 30 min after start of filter operation. During each test, syngas was separately sampled every 20 min at the filter outlet for analysis of gas composition using a gas chromatographer (Model CP-3800, Varian, Inc., Palo Alto, CA, USA). The tar content in syngas was calculated as the ratio of the weight of tar in sampled gas (g) and volume of sampled gas (m3).
