## 1 Introduction  
Oil palm (_Elaeis guinensis_) is a well-known economic crop throughout the world [1]. In Thailand, oil palm generates an income similar to rice (_Oryza sativa_ L.) and para rubber (_Hewea brasiliensis_) [2, 3]. In 2018, the oil palm plantation area in Thailand was 0.94 million hectares, producing 15.5 million tons of palm oil. [4]. Recently, Thailand has become the world's third-largest producer and exporter of palm oil [4, 5], primarily due to the increased usage of biodiesel internationally. This usage of biodiesel is driven by increasing petroleum price, and so crude palm oil (CPO) consumption has also increased [5, 6, 7].  
The yield of CPO is about 17-20% of the fresh fruit bunch (FFB) [8]. In the CPO process, decanter cake or palm oil sewage sludge (POSS) is the semi-solid residue obtained after the dehydration of palm oil mill effluent (POME). During the oil extraction and cleaning process, the report of [PERSON] et al. [9] showed that an average value of 42 kg POSS per ton of FFB is generated by palm oil mills. Meanwhile, the report of the Office of Agricultural Economics [4] showed that palm oil production is about 15.5 million tons per year, and the production of POSS is forecasted to be 651,000 tons.  
Reviews of Renewable Energy Source (RES) produced from the sewage sludge (SS) of industry wastewater treatment plants were discussed as follows. The study of [PERSON] et al. [10] shows that SS should be considered a good RES due to the fact that energy generation from SS produces substantially lower air pollution emissions than energy produced from fossil fuel combustion. For POSS, the reports of [PERSON] et al. [9] and [PERSON] et al. [11] show that POSS comprises high mineral, fat, and protein content. Thus, it is used in biogas production, soil cover materials, and fertilizer in oil palm plantation areas. The report of [PERSON] [12] shows that POSS has a good potential as a future energy source, because it has recently been used as renewable energy source for power generation and as furnace combustion material.  
Smoke particles and polycyclic aromatic hydrocarbons (PAHs) are produced by the incomplete combustion of carbonaceous materials [13; 14; 15; 16]. The physicochemical properties of smoke particles and PAH profiles from the combustion of various kinds of raw materials such as rubber wood [17; 18; 19; 20; 21], bamboo wood [22; 23; 24], and rice husk [25; 26; 27] were investigated. The results of the smoke particles and PAHs emissions were found to depend on the moisture content, fuel type, amount of excess air, and the biomass blending ratio [17; 18; 28]. However, there is a lack of understanding of important parameters that significantly influence the emission of smoke particles and are associated with PAHs from POSS combustion under different combustion conditions. Until now, there has been no research on the smoke particle parameters, PAHs, and the prediction of the potential human health risks by analyzing the total Benzo[a]pyrene Toxic Equivalence (_BaPTE_) values from POSS bio-char combustion.  
Thus, this article focuses on the characteristics of smoke particles, PAHs, and the prediction of the total _BaPTE_ values in the emissions from POSS bio-char combustion. The characteristics include size distribution, concentration, and PAH components. The effects of the POSS bio-char burning period, clean air speeds, and moisture content on the smoke particles and PAHs emission are elaborated upon. This study should be useful for helping to solve energy decreases, environmental problems, and potential future human health risks.