 CaO amount of 10 wt% and various NiO contents of 2, 4, and 8 wt% were synthesized by co-impregnation on the ceramic supporter. The results of SEM analysis showed that the NiO and CaO were deposited and well dispersed on the porous ceramic supporters. The presence of an associated active NiO peak at a wavelength of 692 cm–1 was observed in the FTIR result. The durian and mangosteen peels residues were pyrolyzed at 400–600°C for 2 h to prepare biochar to be used as tar adsorbent. The BET surface area of obtained biochar was 0.9219–0.9989 m2/g with a range of adsorption pore size of 11.193–11.912 nm. The syngas samples were collected from the gasification unit at 15, 25, 35, 45, and 60 min during a 1-h period of processing time. The syngas compositions were analyzed by gas chromatography. The GC results indicated that increasing NiO contents in the catalysts tended to result in increasing methane and carbon dioxide concentration of the syngas, possibly contributed from the tar cracking reactions. With the presence of biochar, the amount of tar captured in the filter unit was significantly increased up to nine times compared to the system without biochar.


INTRODUCTION
Thailand is an important food-production country with abundant plantation areas to produce agricultural products. In southern Thailand, the palm-oil industry has been growing for decades. The government policy on increasing usages and production of renewable energy from high potential sources to overcome fossil energy crises has been one of the driving forces to promote planting of oil palm. In 2018, the Office of Agricultural Economics of Thailand (OAE) estimated the plantation area of oil palm agriculture to be roughly 2 million acres, yielding almost 16.8 million tons of oil palm crops to feed the refinery plants for palm oil production (Office of Agricultural Economics [OAE], 2019). Consumption for industrial uses leads to a massive amount of biomass residues left from agricultural activities and the refinery process. Typically, biomass residues from palm oil refinery process are approximately 78 to 82 percent, mainly composed of empty fruit bunch, fibers, and kernel shells. These biomass residues can be potentially used as feedstocks in thermochemical processes to produce biofuels or syngas (Lam et al., 2019). The syngas obtained from a biomass gasification process commonly consists of combustible components with high heat value that can be used as an alternative source of fuel for the direct heating or gas engines in power generation processes. Typically, the properties of the produced syngas depend on several affecting factors such as the type of biomass, setup and process details, and operating conditions. Besides palm kernel shell, many other types of biomass, including risk husk, coconut shells, cedarwood, and almond residues, have been demonstrated to be used as raw materials in some kinds of gasification to produce useful syngas as well (Chaiprasert and Vitidsant, 2009;Li et al., 2010;Koike et al., 2013;McCaffrey et al., 2019). However, one typical problem often found in such applications is the coagulation due to the deposition of condensed tar generated from the gasification process in the fuel line and engine. Consequently, these tar components result in less efficiency and internal damage to the engine. There are several approaches to remove tar from the produced syngas. Several studies showed that additional thermal cracking reactions of the hot syngas with the aid of steam or catalysts could significantly decrease the tar components from syngas (Bonilla et al., 2019). The process involves chemical reactions to transform tar components into high heating value gaseous components, such as oxygen/sulfur-containing hydrocarbon, aromatic compounds, and polynuclear aromatic hydrocarbons (PAHs) (Kostyniuk et al., 2019). The decomposition of tar involves complex reaction pathways associated with the presence of hydrocarbon compounds. Those reactions include partial oxidation, dry reforming, steam reforming, hydrogenation, thermal cracking, and carbon formation, which generally result in the formation of hydrogen, methane, carbon monoxide, and carbon dioxide as the end product species.
Several research works have suggested the uses of nickel or base oxides in the gasification process to improve the quality of syngas (Mohammed et al., 2011;Wang et al., 2012). A study by Sutton et al. (2001) reported that nickel oxide (NiO) effectively enhances tar cracking and hydrocarbon removal of the syngas, particularly at temperatures higher than 780 • C. Catalyst promoters, such as CeO 2 (Kimura et al., 2006), manganese oxides (MnO x ) (Koike et al., 2013), and metallic Pt or Co (Chaiprasert and Vitidsant, 2009), could be incorporated into the nickel-based catalysts to facilitate further involving watergas shift reforming reactions, resulting in increasing syngas yield with higher hydrogen content. For more economical approaches, other lower-cost but less effective oxides such as CaO or MgO could be used and incorporated into the process and still give acceptable gas yield results (Kong et al., 2011;Ismail et al., 2012). Many studies have also found the influence of CaO on promoting the reaction rate of partial oxidation and the water-gas shift reaction to enhance the H 2 throughput of the gasification process (Han et al., 2011;Chiang et al., 2012;Udomsirichakorn et al., 2013). As suggested from the literature reviews mentioned above, the implementation of combined NiO and CaO catalysts could synergistically improve the performance of the gasification process of the preferred choice of biomass, particularly in terms of high gas yields with low tar components. In addition, several materials, including wood chips, activated carbon, rice husk char, and corncob biochar, have demonstrated the capacity of adsorbing tar components from syngas produced by various gasification conditions to some extent due to their porous surfaces (Kong et al., 2011). The effect of functional groups of the adsorbent could also play an essential role in the adsorption mechanism of the tar species, particularly on the phenolic contents (Brandt et al., 2000;Li et al., 2010). Besides the type of adsorbent, the operating temperature of the adsorption unit, preferable below 250 • C, also affects the tar reduction performance (Ravenni et al., 2019).
The purpose of this research was to study the effects of NiO/CaO-based catalysts and biochar obtained from agricultural biomass residues on the syngas's quality improvement from the gasification process of palm kernels. Using the co-impregnation sol-gel method, NiO and CaO were doped onto the ceramic supports. The concentration of NiO was varied into three different dosages of 2, 4, and 8 wt%, whereas the CaO content was fixed at 10 wt%. To enhance the tar reduction, biochar, which was prepared from pyrolysis of durian and mangosteen peel residues, was utilized to adsorb excess liquid and gaseous tar from the produced syngas.

Catalyst Preparation
Nickel nitrate hexahydrate (Ni[NO 3 ] 2.6H 2 O), AR grade purchased from Ajax Finechem, and calcium nitrate (Ca[NO 3 ] 2.4H 2 O), AR grade obtained from Loba Chemie, were used as the precursors to prepare the solutions with three different weight ratios of 1:5, 2:5, and 4:5. Then the ceramic-ring supporters with the high surface area were soaked in precursor solutions in the hot basin at 70 • C until the gel solution was completely dried. The wet substrates were heated in the oven at 110 • C for 24 h before calcining at 950 • C for 6 h before being kept in the desiccator at room temperature.

Biochar Preparation
Durian and Mangosteen peels were sun-dried and chopped to be approximately 2 cm in diameter before putting into 18 L of a bucket. The bucket filled with the dried and chopped biomass was placed along with woodchips and palm fiber (used as fuel for the synthesis of biochar) into a 100 L Stainless Steel tank. The pyrolysis process of biochar was conducted at 400-600 • C for 2 h.

Experimental Setup of the Gasification Process
Palm kernel shell was dried in the oven at 80 • C for 24 h prior to being placed in the gasifier with the amount of 1 kg. Air was pumped at a constant rate of 25 L/min to the gasifier to maintain biomass to air equivalent ratio of 1:0.6. The process is comprised of a downdraft gasifier, catalytic reactor, tar condenser, and tar filter. The synthesized ceramic ring doped with catalyst was filled in the catalytic reactor with the ratio of PKS/catalyst of 1:0.25. Cracked tar from the catalytic reactor was then condensed in the tar condenser. Syngas was passed to the tar filter unit, packed with 1 kg of biochar (the amount ratio of palm kernels to biochar of 1:1), for the adsorption of gaseous tar. Figure 1 illustrates the gasification process diagram used in this study.

Composition and Heating Value Analysis
Compositions of biomass (kernel shell) and synthesized biochar from durian and mangosteen peels in this study were analyzed according to ASTMD3172-3175 standard. Thermo Quest model Flash EA 1112 was used to analyze elemental compositions of both biomass and biochar. The heating value of the kernel shell was analyzed using a Bomb calorimeter model AC 500.

Morphology
Morphological properties of biomass, biochar, and catalyst were examined by Scanning Electron Microscope (SEM, Model Merlin compact, Zeiss). The specimens were mounted on aluminum stubs, sputter-coated with gold, and