Quality Enhancement of Producer Gas From Cassava Rhizome Using High Temperature Air-Steam Downdraft Gasification

High Temperature Air-Steam Gasification (HTAG) was performed on a bench-scale downdraft gasifier. A cassava rhizome was used as feedstock for the gasification. Air and steam were utilized as the gasifying agents. The objectives in this study were to study the potential of HTAG technology applied with a downdraft gasifier to improve producer gas quality in terms of Higher Heating Value (HHV) and lower tar content. The results were compared with conventional air-steam gasification (without preheating). The results were that the HHV of the producer gas from the HTAG process at 900°C improved by as much as 5.1 MJ/Nm 3 (at S/B ratio = 0.3), while the air-steam gasification, HTAG at 300°C and HTAG at 600°C couldonly obtain a HHV of 3.8 MJ/Nm 3 (at S/B = 0.4), 4.2 MJ/Nm 3 (at S/B = 0.1) and4.8 MJ/Nm 3 (at S/B = 0.2), respectively. In addition, tar content in the producer gas of the HTAG process at 900°C had the lowest value (95 mg/m 3 ) which could be used as fuel in an internal combustion engine. While the minimum tar content of the air-steam gasification was 320 mg/m 3 . In the HTAG process at 900°C, the maximum Cold Gas Efficiency (CGE) was 65%, which was slightly lower than the CGE of air-steam gasification (69%). However, in the HTAG process at 300, 600 and 900°C the maximum Hot Gas Efficiency (HGE) increased 33% (from 72 to 96%), 25 (from 72 to 90%) and 7 (from 72 to 77%), respectively; when compared with air-steam gasification.


INTRODUCTION
The cassava rhizome is located between the stalk and root of the cassava plant (Fig. 1). It becomes a residue left from the cassava plant when it is burnt in preparation for next seasons planting. Burning the crop residue from a field or from part of a field can cause smoke and impact the environment; moreover, there is no benefit from the potential use of agriculture wastes.
Researchers have reported that cassava production in Thailand is about 25 million tons per year (Atong et al., 2011) and about 8-10 million tons are made up of the rhizome. Therefore, the potential conversion of the cassava rhizome into renewable energy could be substantial. Biomass gasification is a series of thermo-chemical processes which can convert solid biomass into combustible gases called syn gas or producer gas. The main compositions of producer gas are H 2 , CO, CH 4 and CO 2 . High quality producer gas can be used as fuel in the Internal Combustion Engine (ICE) for producing electricity. High Temperature Air-steam Gasification (HTAG) technology has previously been developed (Lucas et al., 2004;Tsuji et al., 2002), this technology was discovered that using air and mixtures of air and steam preheated to high temperature (300-1000°C) as gasifying agent for biomass gasification process can increase the quality of producer gas. This is because sensible heat from preheating can be utilized to replace partial heat which is produced by biomass combustion and can support the endothermic gasification reaction.

Fig. 1. Cassava rhizome
As a result, the air quantity for combustion decreases extremely, the nitrogen content in the product gas decreases and concentration of combustible gas in the product gas increases. Thus, high heating value and gas yield of the product gas are improved. HTAG technology has been studied with various types of gasifiers, such as the updraft gasifier (Lucas et al., 2004;Umeki et al., 2010), the entrain flow gasifier (Kobayashi et al., 2009) and the peddle bed gasifier (Pian and Yoshikawa, 2001); however, there has not been any study of HTAG integrated with the downdraft gasifier. The downdraft gasifier has shown an advantage in producing syn gas, low tar content and is easy to design and construct when compared to fluidized bed gasification systems. Moreover, producer gas from the downdraft gasifier has higher temperatures which can be used as recovery heat to preheat gasifying agents (Lv et al., 2007). The objective of this study is to enhance producer gas quality from the cassava rhizome in terms of HHV and lower tar content for ICE application by using HTAG technology. In this study, the downdraft gasifier will be used as a reactor. The results of the HTAG process will be compared to the conventional air-steam gasification process (without preheating). The expectations of this study are to find the potential of HTAG technology when applied with the downdraft gasifier to improve the gas quality and the gasification efficiency from the cassava rhizome.

Feedstock Materials
The feedstock used in this experiment was the cassava rhizome ( Fig. 2). Its bulk density was measured and found to be 281 kg/m 3 . The proximate and ultimate analysis of the feed stock is shown in Table 1.

Experimental Set Up
The gasification system consists of five main units: downdraft gasifier, preheater, steam generator, air blower and cyclone separator. The schematic diagram of the high temperature air-steam gasification system is shown in Fig. 3.
The downdraft gasifier reactor was cylindrical in shape with a 40 cm diameter and a height of 150 cm. Six nozzles were installed in the combustion zone 20 cm away from the gate (Fig. 4). The diameter of the drying zone and the reduction zone were 30 and 20 cm, respectively. At the bottom of the reduction zone, a gate was installed to support the charcoal from the pyrolysis process. Steam at atmospheric pressure was generated by the steam generator (Fig. 3). The steam generator was made of stainless steel of cylindrical shape and the heater was installed at the bottom of the steam generator. The flow rate of the steam which was controlled by peristaltic pump in the ranges 1, 2, 4 and 6 kgh −1 were studied. A blower was used to provide air for gasification process.

AJAS
Air flow rate was controlled by valve and rotameter and varied at 100, 200 and 300 lpm, respectively. In this study, the regenerative preheater used a ceramic ball for heat storage, this material transferred heat to the gasifying agent (air mixed with steam). Many researchers (Lucas et al., 2004;Pian and Yoshikawa, 2001) have applied this preheater concept as well because the temperature of the gasifying agent can be increased to 1200°C (depending on the heat storage properties). The preheating temperature can be raised by adjusting the quantity of LPG gas and air through the burner. In this study, gasifying agent was preheated in the range of temperature 300, 600 and 900°C, respectively.

Measurements System
At the steady state condition, producer gas was sampled (by the gas collector) and the gas composition was analyzed. The main gas composition (CO 2 , H 2 , CO and CH 4 ) was detected by gas chromatogram (Shimadzu GC-2014) provided with a Thermal Conductivity Detector (TDC), connected with unibeads C column. Argon gas was chosen as the carrier gas in all analyses. Cold trapping was used as a tar measurements unit which consisted of six glass bottles soaked in an ice bath (-15°C). Initially, sampling gas flow into the bottle after that the contaminated tar in the sampling gas was condensed. Then the tar content was measured and compared with the flow rate of sampling gas in mg/m 3 units. The temperature profiles along the gasifier vertical axis were measured by thermocouple (K-type, Chromel-Alumel) as illustrated in Fig. 4. The temperature of the drying zone (T1) was measured at 60 cm, pyrolysis zone (T2) at 40 cm, combustion zone (T3) at 25 cm and reduction zone (T4) at 10 cm above the gate. In the cases of T5 and T6 thermocouples were located at the inlet and outlet of the gasifier, respectively. All thermocouples were connected with a data logger for recording the temperature profiles and represented the real-time database. When the process entered the steady state condition (observed by a nearly constant temperature), the tests began and experimental data was recorded. All experimental data recorded usedaverage value (three measurements spaced 5 min apart).

Experimental Procedure
The experiment was divided into two parts, the High Temperature Air-Steam Gasification (HTAG) and airsteam gasification (without preheating) compared to the HTAG process. The first part of the experiment began by preheating the temperature of the gasifying agent to a desired level (300, 600 and 900°C). When the temperature of the gasifying agent was stable, the cassava rhizome charcoal generated from this experiment was fed into the downdraft gasifier to the upper combustion zone level (to avoid tar formation inside the gasifier). Then, the cassava rhizome was fed into the gasifier until fully loaded; afterwards, the gasifier was ignited at the ignition port, which was installed at the combustion zone. Air flow rate, steam flow rate and gasifying agent temperature were operational parameters and were tested in the ranges of 100-300 lpm, 0-6 kgh −1 and 300-900°C, respectively. Gas sampling, tar sampling and temperature measurements were recorded when the process was in a steady state condition (after 40-50 min). Finally, by-products from the experiment were weighed to calculate the feeding rate (kg/h). After the HTAG experiment was completed, the air-steam gasification was performed.

Investigating Variables Definition
The steam to biomass ratio (S/B) was calculated by

Reduction Temperature
Gasification is a thermo-chemical process where the temperature plays an important role. The comparison result in the reduction temperatures between the HTAG process and the air-steam gasification (without preheating) were investigated in this section. The preheated temperatures of the HTAG process were examined at 300, 600 and900°C, respectively. In addition, the S/B ratio was changed from 0 to 0.8 while keeping the air supply at 300 lpm. The results are presented in Fig. 5.
The results were that temperature changes were similar in all cases, namely, the temperature decreased continuously when the S/B ratio increased. Air-steam gasification and the HTAG process (at 300, 600 and 900°C) have the reduction temperature in the range of 567-742°C (S/B ratio in the range 0-0.6), 623-796°C (S/B ratio in the range 0-0.7), 703-871°C (S/B ratio in the range 0-0.5) and 744-902°C (S/B ratio in the range 0-0.4), respectively. The HTAG process is a process where the gasifying agent (air and steam) was preheated to a high temperature before being fed into the reactor. The sensible heat from preheating improves the thermal energy to the system besides the heat produced from the combustion of oxygen and feedstock only. Therefore, the reduction temperature of the HTAG process is higher than in air-steam gasification which did not have preheating.

Gas Composition and Higher Heating Value (HHV)
CO, H 2 and CH 4 are the main combustible gas components in the producer gas and the concentration of those gases have a direct effect on the HHV of the producer gas (estimated by Equation 2). However, it was found that CO and H 2 changed significantly, while CH 4 changed very little. In this study, therefore, CO and H 2 were used as variables to compare the HTAG process with the air-steam gasification process. The experiment used 300 lpm of air supply flow while the S/B ratio varied. Figure 6a and b illustrate the changing concentration of CO and H 2 , while Fig. 7 represents the variation of HHV of the producer gas in each process.
From the experiment results, it can be observed that the H 2 concentration (Fig. 6a) obtained from the HTAG process at 900°C has a significantly higher volume than the air-steam gasification process in every S/B ratio, while the concentration of H 2 obtained from the HTAG process at 300 and 600°C are in close proximity with the air-steam gasification process (S/B ratio range 0.3-0.5). It was found that the CO concentration (Fig. 6b), at every preheating temperature condition in the HTAG process had a higher volume than in the air-steam gasification process in every S/B ratio. As shown in Fig. 7, The HHV of the producer gas from HTAG process at 900°C was improved by as much as 5.1 MJ/Nm 3 (at S/B ratio = 0.3), while the air-steam gasification, the HTAG at 300 and the HTAG at 600°C processes couldonly reach a HHV of 3.8 MJ/Nm 3 (at S/B = 0.4), 4.2 MJ/Nm 3 (at S/B = 0.1) and 4.8 MJ/Nm 3 (at S/B = 0.2), respectively. Typically, the pyrolysis process is likely to occur at high temperature; therefore, the HTAG process (where the sensible heat from preheating can help to increase the process temperature) has higher volatile product than the airsteam gasification process.This resulted in improved concentrations of combustible gas (CO, H 2 and CH 4 ) in the producer gas and the gas flow rate was improved (Lucas et al., 2004;Ponzio et al., 2006). Furthermore, high temperatures can support the endothermic gasification and steam reforming, which resulted in enhanced CO and H 2 concentrations. Therefore, the HHV of producer gas from HTAG process was higher than in the air-steam gasification process.

Tar Content
In the producer gas during the gasification process, there is an undesirable complex mixture involving heavy hydrocarbons called tar. Tar elimination by thermal cracking (Bui et al., 1994) or by using a catalyst (Ketcong et al., 2009) are widely used methods. Tar can condense at low temperatures, which can lead to clogged gas pipelines and it can adhere on the filters of engines. The variation in tar content obtained from the air-steam gasification process and the HTAG process can be represented in Fig. 8.
As shown in Fig. 8, the changing of tar content in all cases was similar in every condition, namely, the tar content increased continuously with the increasing S/B ratio and the lowest tar content was at the no steam injected condition (S/B = 0) or air gasification. For the HTAG process at 300°C, the lowest tar content was equal to 277 mg/m 3 , which was lower than the lowest tar content obtained in the air-steam gasification process (320 mg/m 3 ). In the case of HTAG at 600°C and HTAG at 900°C, the lowest tar content was equal to 170 mg/m 3 and 95 mg/m 3 , respectively. From these results, it can be observed that the lowest tar content from the HTAG process at 900°C (95 mg/m 3 ) is lower than the recommended value (100 mg/m 3 ) (Bhattacharya et al., 2001); for use in an Internal Combustion Engine (ICE). The tar content reduction in the HTAG process compared to the air-steam gasification process can be explained by the tar cracking well at higher temperatures (Devi et al., 2003;Li et al., 2009), wherethe gasifier temperature of the HTAG process ismuchhigher than in the air-steam gasification process. Therefore, tar in the HTAG process had more decomposition resulting in decreased tar content.

Cold Gas Efficiency(CGE) and Hot Gas Efficiency (HGE)
CGE and HGE are important factors to analyze in the economic energy efficiency of the gasifier. CGE is based on the chemical energy from the producer gas and all energy used in the process (calculated by Equation  3). In the case of the HTAG process, the energy used in the process is the summation of the energy from feedstock, energy from steam generation and the energy from the preheated gasifying agent. On the other hand, the HGE is a parameter that considers the benefits of sensible heat energy from the producer gas (calculated by Equation 4). The comparison CGE and HGE of the air-steam gasification and HTAG processes can be shown in Fig. 9.

ACKNOWLEDGEMENT
I would like to acknowledge Khon Kaen University and Rajabhat Mahasarakham University for providing the Grant to support this project.