Applied Energy 112 (2013) 414–420 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Air-blown gasification of woody biomass in a bubbling fluidized bed gasifier Young Doo Kim a, Chang Won Yang a, Beom Jong Kim b, Kwang Su Kim b, Jeung Woo Lee a, Ji Hong Moon c, Won Yang a,b, Tae U Yu a,b, Uen Do Lee a,b,⇑ a b c Department of Green Process and System Engineering, University of Science and Technology, Republic of Korea Energy System R&D Group, Korea Institute of Industrial Technology, Republic of Korea Yonsei University, Seoul, Republic of Korea h i g h l i g h t s  Air-blown gasification of woody biomass was investigated in a pilot-scale BFB gasifier  The performance of the gasifier was investigated as a function of equivalence ratio and fluidization conditions  The average heating value of the product gas was above 4.7 MJ/Nm  Stable operation of the integrated gasification-power generation system was achieved a r t i c l e i n f o Article history: Received 25 September 2012 Received in revised form 13 March 2013 Accepted 25 March 2013 Available online 24 April 2013 Keywords: Biomass gasification Bubbling fluidized bed Air-blown gasifier Syngas Power generation a b s t r a c t Air-blown gasification of woody biomass was investigated in a pilot-scale bubbling fluidized bed gasifier Air was used as the gasifying agent as well as a fluidizing gas Fuel was fed into the top of the gasifier and air was introduced from the bottom through a distributor In order to control the composition of the product gas, the amounts of feedstock and gasifying agent being fed into the gasifier were varied, and the temperature distribution in the gasifier and the composition of the syngas were monitored It was shown that the distribution of the reaction zones in the gasifier could be controlled by the air injection rate, and the composition of the syngas by the equivalence ratio of the reactants Although the obtained syngas had a low caloric value, its heating value is adequate for power generation using a syngas engine Ó 2013 Elsevier Ltd All rights reserved Introduction Increased industrialization and energy consumption has led to stiff international competition to secure energy resources According to recent research (Jacques et al., 2010), the price of oil and gas will be twice that at present, and therefore, the widespread exploitation of renewable energy is essential to avoid energy shortages, as well for minimization of the greenhouse effect [1] Currently, there is much interest in renewable resources in combustible including woody biomass, sewage sludge and wastes, and various method have been tried to convert these renewable resources to ⇑ Corresponding author at: Energy System R&D Group, Korea Institute of Industrial Technology, Republic of Korea Tel.: +82 41 578 8574; fax: +82 41 589 8323 E-mail addresses: 02yd@kitech.re.kr (Y.D Kim), ycw@kitech.re.kr (C.W Yang), dery01@kitech.re.kr (B.J Kim), verycold@kitech.re.kr (K.S Kim), jwlee93@kitech re.kr (J.W Lee), mjh5635@kitech.re.kr (J.H Moon), yangwon@kitech.re.kr (W Yang), ytu@kitech.re.kr (T.U Yu), uendol@kitech.re.kr (U.D Lee) 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.apenergy.2013.03.072 easy-to-use fuels such as gas or liquid fuel more effectively [2–6] Biomass gasification, in which biomass is converted to quality syngas containing hydrogen, carbon monoxide, and methane without any impurities, is of particular interest [7,8] Gasification can be classified by the type of gasifier, which can be Bubbling Fluidized Bed (BFB), Circulating Fluidized Bed (CFB), moving/fixed bed, or entrained flow gasifier and also by whether the reaction occurs under high- or low-pressure conditions [9] It is also classified by the oxidant, namely, air [10] steam [11], air–steam [12–14], oxygen–steam [15–17], or excess oxygen One application of syngas with negligible tar and dust contents and with a high heating value is as the fuel for syngas engines for the direct generation of electrical power [18] Air-blown gasification is a simple method in comparison to steam or oxygen-blown gasification, as the composition of the syngas can easily be controlled through the heating load, and fuel with various physical and chemical characteristics can be used by layering [19] There have been various approaches for investigating the effects of a gasifier operation on product gas 415 Y.D Kim et al / Applied Energy 112 (2013) 414–420 Nomenclature BFB CFB LHV LNG LPG Bubbling Fluidized Bed Circulating Fluidized Bed lower heating value liquefied natural gas liquefied petroleum gas ER FN PAHs RPM compositions and quality Low heating value (LHV) of syngas was studied in terms of equivalence ratio (ER) [14], and the effects of gasifier temperature, steam to biomass ratio and ER on gas composition was also investigated More specifically, the effect of ER on the gas yield and properties of syngas were reported for different biomass fuels such as rubber wood chip and rice husk In this study, air-blown gasification was investigated for the production of fuel for a syngas-engine power generator Using air as the oxidant results in the syngas containing nitrogen and it thus has a low caloric value Product gas compositions and temperature distribution in the gasifier were monitored as a function of equivalence ratio fluidization number polycyclic aromatic hydrocarbons revolutions per minute equivalence ratio and fuel feeding rate And coupling effects of fluidization condition and gasification reaction were investigated In addition, preliminary test of syngas power generation was conducted and it was found that the producer gas is adequate for power generation using a syngas engine Bfb systems Fig shows a schematic diagram of a BFB system and its reaction zones when feedstock is fed from the top and air is introduced from the bottom Influenced by the heat and mass transfer Biomass Syngas Biomass Drying/ Pyrolysis/ Gasification Biomass+Char Product gas Char Gasification/ Combustion Char Air+Productgas Combustion/ Air preheating Fig BFB gasification system with top feeding and reaction zones of the gasifier 416 Y.D Kim et al / Applied Energy 112 (2013) 414–420 between the fuel particles and environment of the gasifier, the biomass undergoes successive reaction process such as drying, pyrolysis, gasification, and combustion and the thermo-chemical conversion of biomass is strongly related to dimensions and shapes of feedstock [20,21] The heat and product gases from the combustion of the biomass are used for gasification, pyrolysis, and drying under an appropriate atmosphere Char, tar and non-condensable gas are representative by-products from gasification Char is a residual solid material from devolatilization or pyrolysis of carbonaceous in biomass Tars are variable mixture of phenols, polycyclic aromatic hydrocarbons (PAHs) and heterocyclic compounds Because it is principally the char after the gasification process that is combusted This reaction step is particularly important in the air-blown gasification since it supplies necessary reaction heat An air-blown gasification system that uses air and biomass has the advantages of enhanced fuel efficiency, the simplicity of the system, and the ability to control the composition of the syngas The vertical temperature profile in the system can be controlled by the biomass feeding rate and air-flow rate and it can lead different syngas composition as well as tar content in the syngas Experimental setup Fig shows schematic diagrams of the experimental setup used in this study At the bottom of the gasifier, there is an additional fuel-combustion chamber to preheat the whole system in the beginning The screw feeder introduces biomass into the gasifier, and the cyclone located behind the gasifier separates off gas and entrained dust, sand, and unburned char 3.1 Gasifier Fig 3a shows the BFB gasifier The inner diameter of the gasifier is 0.4 m and its height is 3.8 m The gasifier is of the BFB type and is designed to process biomass at a rate of 10–80 kg/h Thermocouples of K-type were installed at eleven points to measure the temperature profile inside the gasifier, as shown in Fig They are arranged at regular intervals, except that between the distributor and TC-1 3.2 Feeder and feedstock Fig 3b shows the screw-type feeder, which can continuously feed a uniform amount of biomass to the gasifier The feed rate (in revolutions per minute, RPM) can be controlled, and the amount of biomass supplied by the feeder at a given RPM was determined before the gasification experiment was conducted The proximate and ultimate analysis of the wood pellet is given in Table The biomass consists of volatiles, fixed carbon, and Biomass Syngas Fig Schematic diagram of gasification system and the temperature measurement points; 1: compressed air, 2: LPG, 3: preheat chamber, 4: gasifier, 5: screw feeder, 6: hopper, 7: cyclone, 8: tar trap, 9: gas analyzer 417 Y.D Kim et al / Applied Energy 112 (2013) 414–420 Fig Direct photographs of the experimental setup Table Proximate and ultimate analysis of feedstock Feedstock Table Experimental conditions Wood pellet Proximate analysis (as received wt.%) Moisture Volatiles Fixed carbon Ash 9.8 72.7 16.7 0.8 Ultimate analysis (d.a.f wt.%) C H O N S LHV (MJ/kg) Single particle size Apparent density (kg/m3) Bulk density (kg/m3) 51.02 7.16 41.73 0.09 0.004 18 Ø mm  20 mm 1050 600 Case Startup Biomass feeding rate (kg/h) Air (Nm3/hr) ER 10 52 1.3 34 37 0.27 55 54 0.24 55 43 0.19 25 33 0.32 into the atmosphere The syngas was collected by a port in the stack, and a continuous gas analyzer was used to analyze the syngas for H2, CO, CO2, O2, and CH4 The data from the gas analyzer was monitored in real-time and recorded on a computer For syngas analysis, the tar in the syngas must be removed, and therefore, a tar trap (Fig 3c) device was installed to absorb the tar from the high-temperature syngas 3.5 Syngas engine system ash, and the volatile and fixed-carbon contents were 72.7% and 16.7%, respectively 3.3 Preheat chamber Fig 3d shows the preheat chamber The preheat chamber was located at the bottom of the gasifier in place of a wind-box to supply air to the gasifier The chamber was equipped with an LPG gas burner and the LPG combustion gas was supplied to the gasifier through the distributor during the startup When the gasification condition is achieved, the LPG supply was stopped and the chamber acted as a simple wind-box 3.4 Gas analyzer Fig 3e shows the device to analyze the syngas produced by the gasifier The syngas is ignited in the stack before being released Table shows the specification of syngas engine Four cycle type syngas engine was ready to use syngas from gasifier to generate electric power Fuel of syngas engine is available for selection either LNG or Syngas Syngas from gasifier of atmosphere condition was supplied by ring blower and introduced into the engine while premixing with air The maximum capacity of the syngas engine is about 30 kWe Electricity from syngas engine was spent at an electronic load resistance Experiment 4.1 Experimental method In this study, silica sand was employed as the bed material At the start of the experiment, to heat up the gasifier, the LPG burner at the bottom of the gasifier was used to preheat the air, which, in turn, heated the sand up to the temperature at which spontaneous 418 Y.D Kim et al / Applied Energy 112 (2013) 414–420 Table Syngas engine specification Cycle syngas engine Syngas Turbo charger 20–30 kW 1800 RPM Case Case ER : 0.24 LHV:5.3MJ/Nm ER : 0.19 LHV:5.7MJ/Nm 900 TC-1 TC-2 TC-3 TC-4 TC-5 TC-6 TC-7 TC-8 TC-9 TC-10 TC-11 800 o Temperature ( C) Engine type Fuel Number of cylinder Intake system Power RPM Case ER : 0.27 LHV:4.7MJ/Nm combustion occurs (400 °C) The temperature of the gasifier was monitored by a computer After biomass was input into gasifier, its spontaneous combustion caused the temperature to rapidly increase, and the LPG burner was turned off [22,23] Then, a uniform amount of biomass was fed by the screw feeder such that the equivalence ratio (ER) (see Section 4.2) was maintained at about 1.3 ER ¼ Actual air fuel ratio Stoichiometric air fuel ratio 700 600 500 10 20 30 40 50 60 Time (m) ð1Þ Fig Temperature of the gasifier for each case The combustion conditions were maintained until the bed temperature of the gasifier had reached 800 °C at TC-3 The air inflow rate was reduced to change ER when the bed temperature was sufficiently high The heat from biomass combustion was used to heat the sand and for the endothermic pyrolysis and gasification reactions The experimental conditions were controlled by the feed rates of biomass and air, as shown in Table The biomass feed rate ranged from 25 to 55 kg/h, and air-flow rate from 33 to 54 Nm3/ h The ratio of the biomass and air feed rates is defined as the equivalence ratio (ER), and the four representative experimental conditions of this study were ER = 0.32, 0.27, 0.24, and 0.19 CASE and CASE are the two cases in which the biomass feeding rate is the same (around 55 kg/h) but the air feeding rate is different, and CASE has the lowest ER (0.19) In addition, long-term operation performance was tested over two days (CASE 4) For all cases, the heating value of the syngas satisfies the minimum requirement of the power generation engine while showing the effect of fluidization number and equivalence ratio on the gasification process in the bubbling fluidized bed gasifier Results 5.1 Temperature of the gasifier The temperature profiles in the gasifier are shown in Fig The temperature of the gasifier is mainly controlled by the ER and fuel feed rates of the biomass For fixed feed rates of biomass, the average temperature increases as ER decreases and for fixed ER, it is proportional to the fuel feed rates It is notable that the temperature distribution is significantly affected by ER or fuel feed rates As shown in Fig 4, the difference between the maximum and minimum temperature in the gasifier becomes large when ER decreases or fuel feed rates decreases In a bubbling fluidized bed with top feeding, the gasifier temperature is also highly coupled with fluidization number (FN) which represents fluidization condition of the fluidized bed With higher FN, fluidization occurs more vigorously and it enhances fuel mixing and heat transfer in the reaction zone In order to investigate the details of the thermo-chemical conversion process of the gasifier, vertical temperature distribution Case Case FN=3.7 ER=0.19 FN=3.2 ER=0.27 FN=4.6 ER=0.24 Gasifier height (m) 4.2 Experimental conditions Case Drying/ Pyrolysis/ Gasification Gasification/ Combustion Combustion/ Air preheating 500 600 700 (a) 800500 600 700 800500 (b) 600 700 800 (c) o Temperature ( C) Fig Vertical temperature distribution of the gasifier as a function of FN of each case was plotted in Fig It is notable that varying the feed rates of biomass or air induced changes the temperature profile which leads to the reaction conditions inside the gasifier significantly As discussed in Fig 1, a bubbling fluidized bed with top fuel feeding system has sequential reaction pathway from the top In the view point of fuel, it experiences drying, pyrolysis, gasification and combustion In the view point of oxidizer, it experiences preheating and combustion and it turns into combustion gases and then take part in the gasification process Again, the product gas from gasification, acts as a heat transfer medium during the pyrolysis and drying process It is interesting that changes of reaction zones can be estimated from the vertical temperature profile of the gasifier As depicted in Fig 5, each reaction zone is overlapped to each other but we can estimate a boundary or layer of a specific reaction zone between the overlaps For convenience, we can tell that main gasification zone is coincided with the constant temperature region though some partial oxidation or pyrolysis occurs simultaneously in the same region Fig 5a shows the temperature profile of Case Comparing to Cases and 3, main gasification zone is relatively narrow and difference between the maximum and minimum temperature is 419 Y.D Kim et al / Applied Energy 112 (2013) 414–420 Table Concentration of syngas composition Table Comparison of gasifier performance with previous studies Case Gas composition (dry vol.%) 14.5 16.0 13.8 4.0 48.2 4.7 16.1 16.0 15.0 4.6 51.6 5.3 16.5 16.4 16.1 5.3 54.2 5.7 15.2 17.0 11.8 4.3 48.3 4.7 H2 CO2 O2 CO CH4 Total Lower heating value (LHV) (MJ/Nm3) Case Case ER : 0.27 ER : 0.24 Type Fuel Method Temperature (°C) CO H2 CO2 CH4 H2O H2/CO SEI ISU EPI This study BFB Wood Air 650–815 15.5 12.7 15.9 5.72 Dry 0.8 BFB Corn Air 730 23.9 4.1 12.8 3.1 Dry 0.2 BFB Wood Air 650 17.5 5.8 15.8 4.65 Dry 0.3 BFB Wood pellet Air 750–800 16.1 16.5 16.4 5.3 Dry SEI – Southern Electric International Inc / ISU – Iowa State University / EPI – Energy Product of Idaho Case ER : 0.19 CO2 O2 CO CH4 20 15 10 o 0 10 20 30 40 50 Time (m) 25 20 CO CO2 15 H2 10 CH4 O2 800 Temperature ( C) Gas composition (dry vol.%) Lower heating value (MJ/Nm ) LHV H2 25 Gas composition (dry vol.%) 30 TC-1 TC-2 TC-3 TC-6 TC-8 TC-10 700 600 500 Fig Syngas composition and lower heating value as a function of ER 5.2 Syngas composition Table shows the syngas composition as a function of ER, and Fig shows how its composition varies with time as well as ER Time (day) Fig Continuous operation results of BFB systems H2 Electronic Power 16 CO2 18 CO CH4 16 14 12 12 10 8 Electronic Power (kWe) 20 20 Concentration (vol.%) larger than other cases This is because the feed rates of biomass is less than the other cases and this means that combustion heat and hot product gas yield are less than the other cases while the heat loss to the wall or sensible heat loss of the product gas to the fresh feedstock is relatively similar to the other cases It is also notable that the temperature of the very first measuring point (TC-1) of Case is less than the second point (TC-2) which implies that combustion zone is also narrow for Case The reason why is less fuel feed rates and lower FN Since the amount of fuel is not enough to reach the very bottom of the gasifier, the main combustion reaction occurs within narrower regions comparing to other cases In addition, leveling-off of temperature in the fluidized bed is less since FN is less than the other cases For Cases and increase of fuel feed rates as well as air flow rate increases gasification zone The area of gasification zone (i.e constant temperature region) increases as FN increases This is because of increase of bed expansion, increase of solid entrainment, and total mass flow of the product gas Note that the temperature maximum occurs in the very first measuring point which implies that the main combustion zone is lower than Case and fuel can reach to the very bottom of the gasifier The enhanced mixing by increased FN can also contribute to the fuel transportation The decrease of drying and pyrolysis zone is also another interesting result For the same fuel feed rates (Cases and 3), the drying and pyrolysis zone is highly affected by ER which results in the heat and mass flow of the product gas which becomes heating medium for the drying and pyrolysis process 4 0 10 20 30 40 Time (min) Fig Preliminary test result of syngas power generation (electric power output and simultaneous syngas composition) The concentration of syngas tended to increase as ER went from 0.27 to 0.19 The hydrogen concentration, which is important for combustion of CO in syngas engines, increased from 14.5% to 16.5%, carbon monoxide increased from 13.8% to 16.1%, and CH4 increased from 4.0% to 5.3% The total volume of the product gas decreased as ER was reduced The caloric value for each ER is 420 Y.D Kim et al / Applied Energy 112 (2013) 414–420 shown in Table 4, and it can be seen that in all cases it exceeded 4.7 MJ/Nm3, which means the syngas produced here is suitable to be used as fuel for syngas-engines Table shows a comparison of our result and the results of previous research, and demonstrates that the performance of our gasifier is good in comparison to other systems It is notable that hydrogen content of our study is higher than the previous results, which is good for syngas engine operation It can be explained by the long residence time of the reactant and product gas due to our gasifier configuration and top fuel feeding system Water–gas shift reaction (i.e CO + H2O À >H2 + CO2) also contributes to the increase of the hydrogen content because CO of this study is less than the other results and CO2 is more than the other results (Table 5) Fig shows long-term operation results of BFB system The experimental condition and average syngas compositions are presented in Table (Case 4) and Table respectively As shown in the Fig 7, stable and continuous operation of BFB gasification system has been proved and syngas quality is appropriate for power generation with a syngas engine 5.3 Syngas engine test Fig shows the preliminary test result of integration of biomass gasification-gas cleaning-syngas engine A gas cleaning system composed of a gas cooler, bag filter, scrubber and I.D fan was adopted for removing tar and dust in the product gas The original product gas composition of the gasifier was similar to Case 2, but the final product gas composition can be altered by some air leakage from the bag filter caused by the suction operation of I.D fan However, this can be compensated in the premixing stage of the syngas engine operation As a result, stable and continuous operation was conducted with the integrated system and electricity of 12–14 kWe was generated in the test run [24] Conclusions Air-blown gasification was investigated for the production of syngas in a (BFB) gasifier The feed rates of biomass and air were controlled to change the ER and vary the internal conditions Changes in the biomass and air feed rates affected the product gas composition and temperature profiles in the gasifier Based on the temperature profiles, the dynamics of reaction zones were investigated in terms of ER, fuel feed rates, and FN The composition of the syngas was significantly affected by ER The concentration of hydrogen is relatively higher than previous researches and it results from the configuration of the gasifier: longer free board and top fuel feeding The caloric value of the product gas was above 4.7 MJ/Nm3, and thus satisfactory for use in syngas engines Preliminary test of integrated gasification-power generation was conducted Acknowledgement The authors would like to gratefully acknowledge to institution support program by the Ministry of strategy and Finance of Korea References [1] Robert Jacques, Lennert Moritz Two scenarios for Europe: ‘‘Europe 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