Abstract
Experiments were carried out with a new method for assessing an updraft gasification reactor. An attached side door enabled the investigation of zone development by stopping air supply at specific times, when the thickness of biomass, char, and ash layers were measured. Development in zone thicknesses of biomass, char, and ash with time associated with temperature distribution provided information about the speed of flame propagation inside the reactor. Initially, pyrolysis and volatile combustion occurred, as evidenced by the high mass loss rate and high growth rate of the char layer. Shrinkage in the char layer took place later, and this phenomenon was governed by char glowing, which was relatively slow in mass loss rate. Finally, the fully developed char layer was obtained. The results from four different air mass fluxes under updraft configuration were presented, showing the differences in layer development. Temperature profiles at each time step revealed that the location of peak temperature coincided with the location of ash-char interface for every air mass flux. This effect was due to the high energy release during the oxidation of fixed carbon.
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A New Method for Zone Development Observation for Updraft Rice Husk Gasification
Kasemsil Onthong and Jarruwat Charoensuk *
Experiments were carried out with a new method for assessing an updraft gasification reactor. An attached side door enabled the investigation of zone development by stopping air supply at specific times, when the thickness of biomass, char, and ash layers were measured. Development in zone thicknesses of biomass, char, and ash with time associated with temperature distribution provided information about the speed of flame propagation inside the reactor. Initially, pyrolysis and volatile combustion occurred, as evidenced by the high mass loss rate and high growth rate of the char layer. Shrinkage in the char layer took place later, and this phenomenon was governed by char glowing, which was relatively slow in mass loss rate. Finally, the fully developed char layer was obtained. The results from four different air mass fluxes under updraft configuration were presented, showing the differences in layer development. Temperature profiles at each time step revealed that the location of peak temperature coincided with the location of ash-char interface for every air mass flux. This effect was due to the high energy release during the oxidation of fixed carbon.
Keywords: Biomass; Gasification; Updraft; New method; Pyrolysis; Volatile combustion
Contact information: Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand; *Corresponding author: jarruwat.ch@kmitl.ac.th
INTRODUCTION
Liquid petroleum gas (LPG) consumption in Thailand is divided into four sectors, which are household, transportation, industrial, and petrochemical. The household sector accounted for 33% of total LPG consumption and was the largest in 2018 (EPPO 2018). Almost all of the household consumption is related to cooking with a burner. Burners are small, having a typical output within the range of 2 to 3 kW. It is important to reduce fuel consumption as it helps to reduce cost of living, which is considered one of the four basic human needs.
Biomass is a low cost energy resource in Thailand. It has an average cost of 5.66 x 10-5 baht/joule, while LPG costs approximately 5.11 x 10-4 baht/joule. Despite its low cost, biomass combustion is more complex than combustion of gaseous fuel. It is difficult to operate and not practical for cooking applications because of poor responses and low precision in tuning to meet specific thermal demands. These problems inhibit the use of biomass cooking stove among unskilled users.
There have been several developments and innovations in burners for household cooking using biomass as fuel. Most research focuses on different conditions in the operation of a conventional top-lift updraft (TLUD) design, with a major parameter of thermal efficiency (Ballard-Tremeet and Jawurel 1996; Reed and Larson 1996; Bhattacharya et al. 2002; Belonio et al. 2011; Belonio and Castillo 2012; Tryner et al. 2014; Shuanghui et al. 2015; Suresh et al. 2016; Sutar et al. 2017; Rasoulkhani et al. 2018; Dinesha et al. 2019; Jain and Sheth 2019). The reported thermal efficiency is in the range of 22% to 37%. More detailed investigation has been proposed with a combined gasification and syngas burner system. Various parameters that affect the operating conditions, including thermal efficiency, temperature distribution in the reactor, syngas temperature and composition, calculated cold gas efficiency (CGE), and the fuel size and type should be studied. The reported CGE ranges from 31% to 80%.
Biomass gasification studies are crucial pre-requisites for cooking burner development. There have been several attempts to maximize the efficiency of the complex indirect gasification process, but simple configurations, for example, updraft or downdraft types, are appropriate for small heating applications (Reed and Das 1988; Sheth and Babu 2009; Gai and Dong 2012; Teixeira et al. 2012).
Recent research has examined gasification of biomass under updraft and downdraft conditions, with a focus on the burning rate or smoldering propagation speed with different airflow supply and syngas composition (Reed and Das 1988; Horttanainen et al. 2002; Ryu et al. 2005; Ryu et al. 2007; Sheth and Babu 2009; Madhiyanon et al. 2011; Gai and Dong 2012; Teixeira et al. 2012; Dion et al. 2013; Chao et al. 2014; Mahapatra and Dasappa 2014b; James et al. 2015; Ma et al. 2015; Kim et al. 2016; Rasoulkhani et al. 2018; Xuan-Huynh et al. 2018; Susastriawan and Saptoadi Purnomo 2019). A similar problem exists in solid fuel combustion over a traveling grate, which is widely adopted for industrial boilers, where the burning rate is related to the fuel thickness layer over the grate. Various physico-chemical processes interact with each other during the operation, and the system is dealing with uncertainty, e.g., abrupt changes in thermal demand. Understanding combustion under updraft conditions will enable engineers to optimize the process for specific plant operations.
In various studies on gasification stoves and combined gasification and syngas burner systems, there has been little research on the transient response of the system. This characteristic is very important in the operation and reliability of the system. As automation equipment becomes affordable at the domestic level, there will be more economic feasibility for various applications. Automation could alleviate the operation difficulties of gasification stoves and make them more practical for non-technical operators. To successfully control the reactor, it is necessary to understand the smoldering characteristics of packed bed fuel during gasification, e.g., flame propagation during start up or operation under load variation, etc. Eventually, the operation algorithm could be transferred to a logical statement in an automation system.
In this paper, experiments were conducted with a new assessment method that allows the investigator to observe zone development inside the reactor. This was achieved by stopping air supply at a specific period of time. The information of zone development and temperature distribution evolution will provide insight to smoldering propagation phenomena under updraft configuration.
EXPERIMENTAL
Materials
Rice husk was used as fuel in this work. Its properties and heating values are shown in Table 1 (Madhiyanon et al. 2011).
Table 1. Proximate and Ultimate Analyses of Rice Husk
Methods
Fixed bed reactor
All experiments were conducted using an updraft fixed-bed gasification facility, as illustrated in Fig. 1a. The reactor had a 90-mm internal diameter and total height of 500 mm. The upper end of the reactor was fitted with a 100-mm free board chamber, and the lower end had a 100-mm air manifold. The fuel grate is designed for supplying uniform updraft air flow through the packed fuel bed. The air hole distribution of the grate was verified that it was small enough to prevent the fuel particle or the ash to fall down through the grate. This reactor was equipped with a unique side door with high temperature gasket allowing visual inspection of the evolution from biomass to char and ash at the specific time elapsed after the ignition process, as shown in Fig. 1b. The reactor was well insulated with ceramic fiber to prevent major heat loss. Fuel was fed into the top of the reactor through the container. Air mass flux was supplied from the bottom by air compressor pump. The internal reactor temperature was measured by nine K-type thermocouples integrated with a Yogokawa MW-100 temperature recording system (Yokogawa Electric Corporation, Tokyo, Japan) along the vertical axis of the reactor. The height of the packed bed was measured by a metering rod, which could be inserted from the top of the reactor during gasification.
Experimental procedure
Initially, 200 g of rice husk was placed in the reactor, allowing it to be randomly packed with a 400-mm height above the fuel grate. The ignition process was performed by introducing the air mass flux, in corresponding to each experimental case, to the air manifold port underneath the reactor. The propane torch was inserted at the ignition port just above the fuel grate. The torch then fired into the fuel bed. The grate temperature (T1) was monitored until it reached 100 °C, which allowed the pyrolysis flame to propagate upward from the fuel grate. At this point, the temperature recording system was activated.
Fig. 1. (a) Schematic diagram for updraft gasification; (b) configuration of the packed bed reactor used for updraft gasification
Calculation
Bed movement and the layer shrinkage and expansion were calculated as follows,
(1)
(2)
(3)
(4)
where x1 = 0 is the reference grate location, x2 is the distance between grate and ash and char interface layer, x3 is the distance between grate and char and biomass interface layer, x4 is the distance between grate and top of the biomass level, x5 is the open top of the reactor, and Δt is the differential interval time period. Zone definitions are illustrated in Fig. 2.
Fig. 2. Different zones in the reactor
The repeatability of this experiment was calculated as follows, (Kyu-Lee et al. 2015),
(5)
where SE is the standard error, SD is the standard deviation, n is the number of samples.
The equivalent ratio (ER) was calculated using equation 6 (Basu 2010):
(6)
Stoichiometric air was calculated in correspondence with the mass of rice husk loaded in the reactor (200g).
Fig. 3. Details of test trials
RESULTS AND DISCUSSION
Zone Development
Zone development and bed movement were observed by stopping air supply at specific periods of time. By doing so, the reaction was halted due to lack of oxidizer. The inlet air pipe and flue gas exit port were plugged to prevent any diffusion of air into the reactor. The reactor was allowed to cool to room temperature and opened for investigation. Thickness layers of biomass, char, and ash were measured for a specific time step. To obtain the development of these layers with time, the reactor was emptied and refilled with fresh fuel, and the entire operation procedure was repeated from the beginning and halted for the result of each time step. It was found that the maximum temperature was increased with the increasing equivalent ratio. This was in good agreement with other updraft gasification configuration (Rowland 2010; Manek et al. 2019). Visual observation was used to detect the ash, char, and biomass zones that developed in the reactor. Figure 3 shows details of test trials.
Air Mass Flux of 60 kg/m2∙ h (ER = 0.50)
Figure 4 shows the zone development when applying the air mass flux at 60 kg/m2h. The char zone was developed from the grate up to 75 mm in the first 5 min. The mass loss was also high (Fig. 16), while ash was not present. This result implied that homogeneous combustion of pyrolysis gas in pore cavities of the fuel bed was dominant in this period.
Fig. 4. Zone width at air mass flux 60 of kg/m2hr
Pyrolysis and volatile combustion were coupled by thermal energy because combustion released heat from the bottom layer where fuel was ignited. Pyrolysis occurred nearby at layer above the ignition zone by absorbing the heat via convection and radiation. Finally, the combustion process was spread from the bottom layer until it reached the distance that the oxygen was depleted. This phenomenon was supported by the abrupt char growth layer of 75 mm within the initial period of 5 min. Massive evolution of smoke was observed at this period, which indicated condensation of steam and heavy hydrocarbon produced by pyrolysis into fine droplets on the top of the reactor when the gas was quenched to the atmospheric condition (Tar).
In the period between the time steps of the 5th and the 15th min from time of ignition, in accordance to Fig. 4, the pyrolysis rate was decreased, as observed by small changes in thickness of biomass layers that occurred between these two time steps, with the average rate around 4.06 mm/min. However, the bed temperature increased almost constantly at the rate between 18.46 °C/min. As suggested in Fig. 6, it could be postulated that char combustion was a dominant process during this second period. The mass loss rate was decreased, while the pyrolysis and homogeneous combustion mode switched to the char glowing mode. This observation coincided with a previous study (Ryu et al. 2007) and a single particle analysis of wood combustion (Mahapatra and Dasappa 2014a). Moreover, it was inferred that within the first 5 min from ignition, the pyrolysis zone grew more abruptly than the glowing rate of char combustion that occurred between the 5th and the 15th min from the initial state.
The temperature at the interphase between char and biomass layers at this period was 100 °C to 150 °C. This condition would have enabled the pyrolysis flaming to propagate if there had been residual oxygen in this region. Therefore, slow pyrolysis without combustion occurred at the char-biomass interphase together with char combustion layer at the grate that was related to a slow mass loss rate. Char glowing has a significantly lower burning rate than pyrolysis flaming (Ryu et al. 2007). Notably, there was very little smoke emitted from the top of the reactor, confirming that char glowing was a dominant process.
At the 20th min, the ash layer started to form at the fuel grate. The temperature of the grate (T1) started to drop, and the combustible fuel at the grate started to burn out. The char layer shrank during the 20th to the 30th min because the bottom char layer had turned into ash, which is depicted in Figs. 4 and 5. Moreover, the weight loss rate of 10% (Fig. 16) was higher than the weight loss within the period of the 5th to the 15th min. As the char layer at the grate had burnt out leaving the incombustible ash behind, the combustion zone then shifted toward the upper location. The shrinkage in char layer led to an increase in heat transfer to the char-biomass interface, resulting in a higher rate of pyrolysis. Figure 4 shows an increase in biomass zone shrinkage from approximately 6.83 mm/min to 12.85 mm/min from the 20th min onward.
From the 20th to the 60th min, the zone development layers moved upward at an almost constant rate, indicating the pyrolysis and char burning rates were constant. Figure 4 indicates a decrease in biomass thickness layer at 4.92 mm/min. The char thickness layer was constant with the width around 65 to 75 mm, while the ash zone was growing at the rate of 2 mm/min. These changes made the rate of bed movement at 2.7 mm/min. Weight loss rate was measured at 1.95 g/min.