Gasification Characteristics of Hydrothermal Carbonized Biomass in

Article pubs.acs.org/EF

Gasification Characteristics of Hydrothermal Carbonized Biomass in an Updraft Pilot-Scale Gasifier Duleeka Sandamali Gunarathne,*,† Andreas Mueller,‡ Sabine Fleck,§ Thomas Kolb,‡ Jan Karol Chmielewski,† Weihong Yang,† and Wlodzimierz Blasiak† †

Division of Energy and Furnace Technology, Department of Material Science and Engineering, Royal Institute of Technology, Brinellvägen 23, 100-44 Stockholm, Sweden ‡ Engler-Bunte-Institute, Division of Fuel Chemistry and Technology (EBI-ceb), Karlsruhe Institute of Technology (KIT), Engler-Bunte-Ring 1, 76131 Karlsruhe, Germany § Karlsruhe Institute of Technology (KIT), Institute for Technical Chemistry (ITC), Department Gasification Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ABSTRACT: Biocoal pellets were gasified in an updraft high-temperature agent gasification (HTAG) unit with preheated air at 900 °C to study the performance of the air gasification of hydrothermal carbonized biomass. In comparison to raw biomass, hydrothermal carbonization increased the carbon content from 46 to 66% and decreased the oxygen content from 38 to 16%. As a result, the heating value of biomass on a dry basis was increased from 19 to 29 MJ/kg after hydrothermal carbonization. Thermogravimetric analysis (TGA) of biocoal featured early decomposition of hemicellulose and a shoulder attached to the cellulose peak corresponding to lignin decomposition. Char gasification demonstrated a peak near conversion of 0.2. Syngas with 7.9 MJ Nm−3 lower heating value (LHV) was obtained from gasification experiments performed in the pilot-scale gasifier. The maximum cold gas efficiency was 80% at the lowest equivalence ratio (ER) and also resulted in high-purity syngas. The LHV and cold gas efficiency were higher than that of the previously studied unpretreated biomass pellets. The fuel conversion positively correlated with the fuel residence time in the bed, and almost 99% conversion could be achieved for a residence time of 2 h. The superficial velocity (or hearth load) and specific gasification rate were higher than the reported values of updraft gasifiers because of the high-temperature operation and specific fuel used.

1. INTRODUCTION Because of the increasing concern for global warming and targets to reduce greenhouse gas (GHG) emissions, renewable energy, particularly biomass, currently plays a major role and most likely will continue to play a major role in energy production in the future. Gasification is an important energy conversion technology that converts solid fuel to gaseous fuel, which can be classified as clean fuel. When solid biomass is processed into syngas, one faces several challenges that reduce the effectiveness of energy conversion, including a high moisture content, low energy content, low bulk density, and lack of fuel uniformity. Drying and pelletization have been the most common and widely used upgrading technologies in recent decades. Furthermore, thermochemical pretreatment technologies are emerging, which ultimately generate much more effective solid fuels for energy conversion technologies, such as gasification. Hydrothermal carbonization is one such technology that serves as an ideal pretreatment method for wet biomass. This technology has been used for not only improving solid fuels but also producing soil amendments. At elevated temperatures near 200−250 °C at or above the saturation pressure, the process is carried out in a medium of water with a residence time varying between 3 and 8 h.1 The main mechanism assumes the cleavage of the ester and ether bonds between sugar molecules via the addition of water.2 This artificial coalification process is sometimes referred to as wet torrefaction and produces hydrophobic fuel “biocoal”. This process facilitates mechanical © 2014 American Chemical Society

dewatering, which reduces the energy requirement for thermal drying. An improved grindability of biocoal is another important mechanical property for fuel processing. Increasing the C/O ratio results in a high heating value and energy density, which are advantageous in logistics. Hydrothermal carbonization has been applied to various feedstocks, such as lignocellulosic biomass,3,4 agricultural waste,5−8 sewage sludge,9−11 grass,12 biologically treated residues,13,14 microalgae,15 etc. While most studies have focused on ultimately fermenting these feedstocks to ethanol, some studies focused on combustion or co-combustion5,8,10,11,16−18 and gasification.1,19 Both of the studies published on gasification focused on entrained flow gasification, and studies on the fixed-bed gasification of hydrothermal carbonized biomass pellets have not been found to date. With regard to the thermal degradation behavior of biocoal, thermogravimetric analysis (TGA) in an inert atmosphere (pyrolysis conditions) was previously studied for hydrothermal carbonized low-rank coal, biomass, and agricultural and biologically treated residues.18,20−22 However, TGAs of biocoal in a CO2 atmosphere to facilitate char gasification have been lacking. The objective of this research was to study the performance of the air gasification of hydrothermal carbonized biomass pellets (biocoal) in a high-temperature agent updraft gasifier. Received: November 28, 2013 Revised: February 20, 2014 Published: February 20, 2014 1992

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

The Van Krevelen plot shows that dehydration is dominant and that decarboxylation also plays a role in hydrothermal carbonization, which results in more condensed material near the coal region. Another important parameter of a fuel is the slagging behavior. Table 2 shows the mineral composition of biocoal to identify the slagging behavior. The ash slagging behavior of biocoal was identified using the molar Si/(Ca + Mg) ratio, which is successfully used for fuels that contain less P to identify a relationship with the ash sintering temperature.25 For biocoal, the Si/(Ca + Mg) ratio of 0.2 was found to result in an ash sintering temperature approximately between 1300 and 1400 °C according to the above study. Therefore, low equivalence ratio (ER) values should be used in biocoal gasification to minimize the ash slagging problem. 2.2. TGA Experiment. The decomposition characteristics of the fuel and its gasification reactivity were evaluated in a laboratory thermogravimetric analyzer (TG 209 F1 Iris, Netsch, Germany) at atmospheric pressure. A schematic of the system is presented in Figure 3. The fuel sample was placed in a ceramic crucible that was surrounded by a high-temperature oven and connected to a balance system. Inert or oxidizing gas atmospheres containing variable concentrations of CO2 can be used inside the reactor. The balance has a resolution of 0.1 μg, and the system can be operated up to sample temperatures of 1000 °C. Typically, sample masses of 2 mg were used in the experiments to avoid transfer limitations. However, this mass requirement is a limitation of TGA that will not demonstrate decomposition behavior similar to that of a gasifier because we cannot avoid transfer limitations in the gasifier. Furthermore, the use of a small sample risks that the actual composition of biomass is not accurately represented. Therefore, the sample was prepared such that its uniformity was ensured. Ground biocoal pellets were pyrolyzed under a N2 atmosphere in a thermogravimetric analyzer with a constant heating rate of 30 °C/min from 30 to 1000 °C, followed by a 15 min holding time at 1000 °C. This period was used to analyze the pyrolysis behavior and remove the volatile matter from the sample. Prior to the gasification behavior being analyzed, the sample was cooled to 900 °C at a rate of 30 °C/ min, followed by a 15 min holding time at 900 °C. The sample was then gasified in a N2/CO2 atmosphere at 900 °C with 10 vol % CO2 until the fuel was completely converted. The temperature and mass loss of the sample because of pyrolysis and gasification were measured as a function of time. The constant heating rate period in the devolatilization stage was further studied using the first derivative of the thermogravimetry (TG) curve (DTG), which can be defined as follows:

First, a TGA study was performed for hydrothermal carbonized biomass, which included the pyrolysis behavior and char gasification. Subsequently, a pilot updraft gasifier with highly preheated air was used to evaluate the effect of the gasification performance of biocoal in terms of gas composition, energy content, cold gas efficiency, and conversion. Finally, the study focused on major design parameters, such as the gas superficial velocity and specific gasification rate for the gasification of biocoal.

2. EXPERIMENTAL SECTION 2.1. Feedstock Materials. Biocoal that was produced via the hydrothermal carbonization of wet biomass was used as the biomass for this study (see Figure 1).

Figure 1. Biocoal. The feedstock for producing biocoal consisted of spent grains from a brewery. The biocoal was produced at the HTC demonstration plant at AVA-CO2 Forschung Gmbh in Karlsruhe, Germany, using the following process conditions: a temperature of 210−215 °C and a residence time of around 4 h. The solid yield was around 67% of dry input. The composition and ash content of raw biomass and biocoal were measured at an external laboratory and are presented in Table 1.

DTG = −

Table 1. Fuel Characteristics parameter

raw biomass (brewery residue)

biocoal

moisture (%) ash (wt %, dry) C (wt %, dry) H (wt %, dry) O (wt %, dry) N (wt %, dry) heating value (MJ/kg, dry)

82.2 4.6 45.6 6.8 38.5 4.2 19.2

10.6 6.7 66.3 7 16.1 3.7 28.9

dα dT

(1)

where the residual mass ratio m α= m0

(2)

The isothermal (900 °C) char gasification period was studied by defining the char gasification rate (r) as follows: r=

dx dt

(3)

where char conversion The raw biomass contains 80% moisture, while biocoal possesses only 10%. The carbon content increased from 46 to 66%, and the oxygen content decreased from 38 to 16%. The H content remained almost constant, and the N content decreased. The heating value on a dry basis improved from 19 to 29 MJ/kg because of a significant increase in the carbon content and a significant decrease in the oxygen content of biocoal. The change in the atomic ratios is demonstrated on a Van Krevelen diagram (Figure 2), along with the ratios for common wood23 and coal24 species from the literature. Dehydration and decarboxylation lines are also plotted.

x=

(m0 − m) (m0 − mf )

(4)

2.3. Gasification Experiment in a High-Temperature Agent Gasification (HTAG) Updraft Gasifier. 2.3.1. Gasifier System. The experiments were carried out in a HTAG unit in an updraft configuration (Figures 4 and 5). HTAG is a method in which a preheated air is used as the gasification agent. This preheated air oxidizer supplies additional energy to the gasification process, which enhances the thermal 1993

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

Figure 2. Van Krevelen diagram with raw biomass, biocoal, common biomass and coal types. updraft gasifier, a fuel feeding system, and a syngas post-combustion unit. Biomass pellets stored in the feed tank are transported to the gasifier via a screw conveyor. Preheated air from the preheater is introduced to the gasifier at the side of the bottom section below the perforated grate made from Kanthal steel, which is suitable for hightemperature operation. The flow of hot gases and biomass is countercurrent. The syngas flows upward, leaves the gasifier at the side of the top section, and is burned out at the combustion chamber. The small particles that remain after the reaction can pass through the grate and are collected below. 2.3.2. Experimental Procedure and Measurements. The feeder was precalibrated with the biocoal pellets used in the experiment. Once the desired air temperature was reached using a preheater (approximately 900 °C in this case), the feed rate was set by adjusting the frequency of the feeder. Three runs were carried out with varying feed rates and/or airflow rates to change the ER as follows in Table 3.

Table 2. Mineral Composition of Biocoal component

composition (wt %, dry)

SiO2 Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 TiO2

0.0783 0.0089 0.29 0.0107 0.103 0.0496 0.0109 0.0053 0.0176 0.0004

decomposition of the biomass feedstock and increases the product gas yield, gas composition, heating value, and cold gas efficiency while reducing the tar content.27 The unit consists of a feed gas preheater, an

Figure 3. Schematic diagram of the laboratory thermogravimetric analyzer.26 1994

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

Figure 4. HTAG system.

Figure 5. Cross-section of the gasifier with temperature measuring points. compositions. The average values of gas compositions and temperatures within this time interval were analyzed. The temperatures inside the gasifier were measured with eight typeS thermocouples located along the reactor height and recorded by a data acquisition system connected to a personal computer (PC). The pressure was recorded at four different locations along the gasifier using digital manometers, and a negative pressure was ensured at the top for safety reasons. The dry syngas compositions were monitored

Table 3. Parameter Variation run

biocoal (kg/h)

air (m3/h)

ER

1 2 3

40 50 37

70 92 70

0.2 0.21 0.22

For each run, a time interval of approximately 15−20 min was selected for the analysis based on the stable temperatures and gas 1995

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

Figure 6. Mass loss curve obtained with TGA. with gas chromatography (GC) every 3 min. Samples to the GC were supplied through water traps to avoid tar and dust. Because of the negative pressure at the top of the gasifier, air infiltration to the system through the biomass feeding line cannot be avoided. Therefore, a correction procedure was used for N2 in the syngas to obtain a realistic composition to allow for a comparison. First, a carbon balance was applied to the input and N2-free gas stream at the outlet assuming negligible carbon content in the ash to calculate the volumetric flow rate of the N2-free gas stream. The known amount of N2 contained in air and fuel was then directly added to the N2-free gas stream to obtain the total gas flow. The following definitions were used in the data analysis. The equivalence ratio is defined as the molar ratio of actual O2 moles supplied to the system and the number of O2 moles needed for stoichiometric combustion. ER =

The constant heating rate period in the devolatilization stage was further studied using the first derivative of the TG curve (DTG), as given in Figure 7.

NO2,actual NO2,stoi

(5)

The cold gas efficiency is the ratio of the chemical energy content in the cold product gas to the chemical energy in the fuel. η=

LHVgĠ LHVf F ̇

(6)

Figure 7. DTG plot of biocoal at a heating rate of 30 °C/min.

Because of the hydrothermal carbonization pretreatment, a portion of the hemicellulose in the biomass is decomposed and released into the solution, while the remaining hemicellulose is depolymerized. This depolymerized hemicellulose clearly results in an early stage peak from 128 to 265 °C, with a maximum decomposition rate of 0.26%/°C near 207 °C attached to the major peak. The major peak, which corresponds to cellulose decomposition, ranges from 265 to 419 °C and features a maximum decomposition rate of 0.35%/°C near 374 °C. The cellulose peak follows a definite shoulder above 419 °C, which can be attributed to lignin decomposition, with a maximum decomposition rate of 0.19%/°C. Hydrothermal carbonization reportedly results in the release of lignin into the solution, followed by its redeposition on the surface. Several authors validated this fact with scanning electron microscopy (SEM) images.20−22,28 A detailed study for this type of lignin

3. RESULTS AND DISCUSSION 3.1. Characterization by TGA. The decomposition behavior of biocoal was studied on the basis of the thermogravimetric mass loss curve presented in Figure 6. According to Figure 6, approximately 80% of the mass was lost in the first stage, which was carried out under an inert atmosphere (pyrolysis stage). After the completion of this mass loss, the introduction of CO2 yields another mass loss region, as shown in the figure. This mass loss, which is approximately 16%, can be attributed to the gasification of residual C with CO2. Nearly 4% of the mass remained, which can be identified as ash. Notably, the ash content differs between the ASTM method (see Table 1) and TGA method because of several factors, such as the heating rate and oxidizing atmosphere, which is typically O2 for the ASTM method and CO2 for the TGA performed here. 1996

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

migration for aqueous high-temperature pretreatments can be found elsewhere.29 This study hypothesizes that lignin may have undergone a phase transition. The distinctive feature of biocoal, i.e., the shoulder beyond the cellulose peak, may be due to the presence of these lignin globules on the surface. A similar observation was reported for biocoal based on wheat straw and dried digestate, and the thermal decomposition of the separated globules showed a peak near 400 °C.21 Furthermore, the breakdown of β-O-4 linkages and the Cα−Cβ bonds of lignin were reported because of hydrothermal carbonization.22 Because of the breakdown of lignin bonds and presence of lignin on the surface, the early thermal degradation of lignin can be expected. The considerable maximum degradation rate may be due to the enhancement of the lignin content as a result of the loss of hemicellulose. Beyond the aforementioned shoulder, a gradual decomposition region with small peaks can also be observed, which is related to further lignin decomposition and may be attributed to the lignin that was less depolymerized and had not migrated to the solution and redeposited. The presence of lignin globules on the surface may affect the cellulose decomposition by covering melted lignin on the cellulose surface. Therefore, the peak cellulose decomposition rate can decrease, while the cellulose thermal stability can increase. A decrease in the peak cellulose decomposition rate and increase in the cellulose thermal stability because of hydrothermal carbonization have been reported.18,21,22 To compare the characteristics of the DTG plot obtained in the present study to the literature data, Table 4 summarizes the temperatures that show peak decompositions for different studies.

Figure 8. Variation of the char gasification rate with conversion.

3.2. Gasification in Updraft HTAG. 3.2.1. Temperature Profile and Syngas Composition. The temperature profiles are demonstrated in Figure 9. The ER and bed height are included in the legend. Because the average temperatures were taken for the plot, the standard deviation was plotted with error bars. Figure 9 shows the trends of the temperature variation along the height of the gasifier for changing ER values. The temperature profiles feature two regions: a significant temperature drop in the bed zone and small temperature variation in the gas phase. Even though the experiment was carried out at low ER values, the combustion zone temperature increased with ER even reaching 1400 °C. The high temperature caused ash slagging, which resulted in discontinuous operation. Controlling the temperature inside the gasifier to completely avoid ash slagging is difficult because of the high carbon content in biocoal. Therefore, the use of an air and steam mixture as the gasification agent should be encouraged for biocoal gasification, which enables temperature control and, hence, a smooth operation in non-slagging mode. Furthermore, significant CO2 production and high temperatures with high ER result in the domination of the endothermic Boudouard reaction in the gasification zone and an associated temperature decrease. This decrease was most pronounced for the highest ER value. The large difference in the temperature profile for the highest ER value is not only the result of the ER value, but the higher bed also likely influenced this effect. The slight temperature increase for the low ER values immediately after this temperature drop may be due to the slightly exothermic water−gas shift reaction that occurs in the gas phase. The run with the highest ER value shows low temperatures during the gas phase. Thus, the water−gas shift reaction does not play a significant role. The gas exit temperature is minimum in this case. Figure 10 shows the compositions and characteristic ratios of syngas after gasification with highly preheated air. The ER and bed height are included in the legend. Because the average compositions were used in the plot, the standard deviation was plotted with error bars. CxHy is the sum of the higher hydrocarbons, where x ranges from 2 to 6. Figure 10 shows the gas composition variation for different ER values. Even a slight variation in the ER value significantly

Table 4. Summary of Literature Data on TGA of Biocoal study present study reference study20 reference study21

source of biocoal spent grains sunflower stem straw and digestate

DTG peak 1 /high intensity in DTG plot ( °C)

DTG peak 2 ( °C)

DTG peak 3 /shoulder (°C)

207

374

419

≈200

≈380

≈420

≈200

≈350

≈420

Table 4 clearly shows that the temperature ranges of peak decomposition were comparable for all biocoal samples. The isothermal (900 °C) char gasification period was further studied using the plot of the char gasification rate with conversion, as given in Figure 8. Figure 8 shows that the char gasification rate increases up to a char conversion of approximately 0.2, with a maximum of 0.06 min−1, and then begins to decrease. At the final stage of conversion, the gasification rate rapidly decreases and ultimately reaches zero at full conversion. The literature that was the basis for the random pore model explains that the initial increase in the rate is due to growth of the reaction surface from the initial pores and the later drop in the reaction rate is due to the intersection of growing surfaces.30 Hydrothermal carbonization reportedly does not significantly improve the surface area.20,31,32 Furthermore, the volatile matter content of biocoal also decreases because of removal of hemicellulose during pretreatment, which decreases the number of available pores during pyrolysis. The initially poor porosity of char may result in the peak in the plot of the char gasification rate. 1997

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

Figure 9. Temperature profile.

Figure 10. Gas compositions.

Figure 11. Gas characteristic ratios.

affected the gas composition for the highest ER case. This effect is likely not solely the result of the ER; the high bed height also likely influenced the temperature profile. While the H2, CO2, and hydrocarbon contents decrease with the ER value, the CO content increases. The significantly high CO content for an ER value of 0.22 can be attributed to the higher bed temperature and bed height, which facilitate the endothermic Boudouard reaction, as discussed earlier. The reduced CH4 content may be due to the limitation of the exothermic CH4 formation at high ER values, which resulted in a high temperature in the bed area. Figure 11 shows the behavior of the gas characteristic ratios for different ER values. As a result of the Boudouard reaction, the CO/CO2 ratio is significantly increased for high ER values. High H2/CO ratios for low ER values imply that a slightly exothermic water−gas shift reaction is dominant. This implication is further evidenced by the temperature profiles of these cases, which show a slight temperature rise in the gas phase. At high temperatures, the formation of C2H4 and C2H2 is possible because of the decomposition of C2H6. This cracking of C2 hydrocarbons qualitatively correlated with the tar cracking because of the high temperatures, which demonstrated the equal trends of the C2H6/(C2H4 + C2H2) ratio and total tar.33 This relationship was further validated by some other researchers.34−36 The syngas purity in this study was also

analyzed by comparing the C2H6/(C2H4 + C2H2) ratio, which can be directly measured by GC and is given in Figure 12.

Figure 12. Syngas purity based on the C2H6/(C2H4 + C2H2) ratio and gas exit temperature.

Even though the ER is increasing, the C2H6/(C2H4 + C2H2) ratio is increasing because of the decreasing gas exit temperature. Most of the tar cracking reactions can be expected at the top of the reactor, and the temperature drop at the top significantly affects the tar content of gas. The drop in the temperature at the top for the highest ER value was mainly due to the domination of the Boudouard reaction in the bed area, 1998

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

which ultimately also affected the temperature profile in the gas phase. For other cases, the temperature at the gas phase was further increased because of the slightly exothermic water−gas shift reaction, as shown in Figure 9. 3.2.2. Lower Heating Value (LHV) and Cold Gas Efficiency. Figure 13 shows the LHVg and cold gas efficiency of the tested cases.

r=

X tr

(10)

Figures 14 and 15 graphically represent the interference of these three parameters.

Figure 14. Influence of the residence time on the overall fuel conversion. Figure 13. Variation of LHVg and cold gas efficiency with ER.

Figure 13 indicates that the LHVg and cold gas efficiency negatively correlate with the ER because of the low hydrocarbon and H2 contents, even though the CO content was high. The highest LHVg of 7.9 MJ Nm−3 and cold gas efficiency of 80% were observed for biocoal gasification. In our previous studies of the high-temperature air gasification of unpretreated biomass pellets at the same range of operating conditions, we observed LHVg values near 6 MJ Nm−3 and a cold gas efficiency of 76%, which are considerably lower than the values observed for biocoal. This difference is likely due to the higher carbon content, which results in a high heating value of biocoal. Furthermore, the energy efficiency of the AVA-CO2 biocoal production process is reportedly greater than 90%.37 Thus, the efficiency, including the total system efficiency, would be in the same range as that for unpretreated biomass while still resulting in a higher energy value of gas. 3.2.3. Residence Time and Fuel Conversion. To minimize the unreacted residue after gasification, the fuel feed rate should be sufficiently higher than the gasification rate to maintain a fuel bed that results in a long enough residence time for the biomass to react. In general, the overall fuel conversion can be given by the following:

Figure 15. Influence of the overall fuel conversion on the conversion rate.

In all cases, the fuel conversion is equal to or higher than 90%. A conversion rate of almost 99% can be achieved if a residence time of approximately 2 h is maintained. Overall conversion near 90% at 1000 °C operation is reported for biocoal gasification in a lab-scale entrained flow reactor.19 According to Figure 14, the overall fuel conversion positively correlates with the residence time of fuel in the bed (bed height), irrespective of the ER value. Furthermore, a small change in the residence time (bed height) resulted in large differences in conversion at low conversions. However, when the conversion is almost complete, this change becomes less prominent. Figure 15, which shows the variation in the conversion rate as a function of conversion, clearly demonstrates this trend. This behavior is similar to the results obtained with TGA, which are shown in Figure 8 for the conversion of fixed carbon under a CO2 atmosphere. Thus, the decrease in the conversion rate may be due to the intersection of growing reaction surfaces discussed earlier in section 3.1, which is further validated here. The carbon conversion should clearly be lower than the overall conversion because overall conversion accounts not only for carbon but also for volatiles, which are readily converted. However, TGA seems to provide a good basic understanding of the conversion rate of the fuel.

Fċ (7) Ḟ The fuel consumption rate can be calculated by the following: Fċ = F ̇ − rbhρ A (8) X=

b

where rbh is the rate of change of the bed height. Furthermore, the residence time of the fuel in the gasifier bed is given by the following: tr =

Ahb Fċ /ρb

(9)

The fuel conversion rate can also be calculated by the following: 1999

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

3.2.4. Design Parameters of Biocoal Gasification. The specific gasification rate is a major design parameter in gasification. It is defined as a measure of the fuel consumption rate per unit cross-sectional area of the gasifier.

rsg =

Fċ A

parameters obtained from various gasification studies of biocoal. Table 6. Summary of Literature Data on Biocoal Gasification

(11)

study

The superficial gas velocity is also an important measure that affects heat transfer as well as the char and tar production rates. The superficial gas velocity is defined as the gas generation rate per unit cross-sectional area of the gasifier, as given below. The units are similar to that of velocity (m/s). VS =

Ġ A

present study reference study1 (ASPEN model) reference study19

Another variation of the superficial gas velocity is the hearth load, which is defined in more practical units for the crosssectional area (cm2) and gas flow (m3/h). According to the unit conversion, the relationship between these two parameters then becomes Table 5 shows these three design parameters for each run. Table 5. Main Design Parameters ER

rsg (kg m−2 h−1)

VS (m/s)

LH (m3 cm−2 h−1)

1 2 3

0.2 0.21 0.22

275.5 319.8 259.4

0.23 0.3 0.22

0.08 0.11 0.08

6.9−7.9 (LHV) 9.874 (HHV) -

overall conversion (%)

70−80

>90

82.8

-

-

≈90

4. CONCLUSION The changes in the important fuel properties because of hydrothermal carbonization were identified. Because of the increase in the carbon content from 46 to 66% and the decrease in the oxygen content from 38 to 16%, the heating value on a dry basis increased from 19 to 29 MJ/kg. Low atomic ratios permit co-gasification with coal. The decomposition characteristics of biocoal were analyzed with TGA at a constant heating rate in a N2 atmosphere. An additional peak related to the hemicellulose decomposition was observed, and a comparatively low cellulose decomposition rate was also observed. The lignin decomposition was characterized by a shoulder attached to the cellulose peak, which was attributed to the decomposition of lignin globules redeposited on the surface after pretreatment. Introducing CO2 resulted in another mass loss region attributed to char gasification. The char gasification rate demonstrated a peak with a maximum of 0.06 min−1 at a char conversion near 0.2. When the gasification experiments were performed in a pilotscale gasifier and air preheated to 900 °C was used as the gasifying medium, the H2, CO2, and hydrocarbon contents decreased with the ER value and the CO content increased. A CO/CO2 ratio near 5 was observed for the highest ER value. The syngas purity, which was analyzed by comparing the C2H6/ (C2H4 + C2H2) ratio, was maximized when the ER value was minimized because of the higher gas exit temperature. The LHV and cold gas efficiency were also maximized at 7.9 MJ Nm−3 and 80%, respectively, when the ER was minimized. The fuel conversion positively correlated with the fuel residence time in the bed, and almost 99% conversion could be achieved for a residence time of 2 h. Similar to results obtained with TGA, the conversion rate decreased as the conversion progressed. The superficial velocity (or hearth load) and specific gasification rate were higher than the reported values of updraft gasifiers because of the high-temperature operation and specific fuel used. Possible interactions between these design parameters will be studied in detail in the future.

(13)

run

fixed-bed updraft entrained flow entrained flow

cold gas efficiency (%)

Table 6 clearly shows that the results obtained with the present study are comparable to other gasification studies of biocoal.

(12)

L H = 0.36VS

gasifier type

heating value (MJ Nm−3)

The specific gasification rate shows the same trend as the conversion rate discussed in section 3.2.3, which is inversely related to the conversion and, hence, the residence time or bed height. Therefore, the specific gasification rate differed because of bed height changes, even for small changes of ER. The intensity of the difference depends upon the conversion, as discussed earlier. At high conversion values, the change in the conversion rate is less pronounced and vice versa. The specific gasification rate positively influences the superficial gas velocity or hearth load. In comparison to the literature data for updraft gasifiers, the superficial velocity and specific gasification rates obtained in the present study are higher than those reported for long stick wood gasification in updraft gasifiers, which ranged from 0.16 to 0.2 m/s and from 170 to 190 kg m−2 h−1, respectively.38 This difference can be due to two reasons: the hightemperature operation and specific fuel type used. Because of the high-temperature operation, tar cracking is expected to result in more gas. Furthermore, the compact reactor can accommodate a high fuel load because of the coal type nature of biocoal. These two attributes facilitate the high superficial gas velocity and specific gasification rate in this study. Because detailed studies of this type of design parameter are lacking, especially for updraft gasifiers, the interactions of the above parameters are interesting to study. We forward our focus on this regard and will analyze these possible interactions in detail in the future. 3.2.5. Comparison to Similar Work. To compare the characteristic biocoal gasification in the present study to the literature data, Table 6 summarizes the major performance

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +46-8-790-8402. Fax: +46-8-207-681. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2000

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

■

Article

(10) Parshetti, G. K.; Liu, Z.; Jain, A.; Srinivasan, M. P.; Balasubramanian, R. Hydrothermal carbonization of sewage sludge for energy production with coal. Fuel 2013, 111, 201−210. (11) He, C.; Giannis, A.; Wang, J. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel characteristics and combustion behavior. Appl. Energy 2013, 111, 257− 266. (12) Cybulska, I.; Lei, H.; Julson, J. Hydrothermal pretreatment and enzymatic hydrolysis of prairie cord grass. Energy Fuels 2010, 24, 718− 727. (13) Mumme, J.; Eckervogt, L.; Pielert, J.; Diakité, M.; Rupp, F.; Kern, J. Hydrothermal carbonization of anaerobically digested maize silage. Bioresour. Technol. 2011, 102, 9255−9260. (14) Heilmann, S. M.; Jader, L. R.; Sadowsky, M. J.; Schendel, F. J.; von Keitz, M. G.; Valentas, K. J. Hydrothermal carbonization of distiller’s grains. Biomass Bioenergy 2011, 35, 2526−2533. (15) Heilmann, S. M.; Davis, H. T.; Jader, L. R.; Lefebvre, P. A.; Sadowsky, M. J.; Schendel, F. J.; von Keitz, M. G.; Valentas, K. J. Hydrothermal carbonization of microalgae. Biomass Bioenergy 2010, 34, 875−882. (16) Liu, Z.; Quek, A.; Hoekman, S. K.; Balasubramanian, R. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 2013, 103, 943−949. (17) Liu, Z.; Quek, A.; Hoekman, S. K.; Srinivasan, M. P.; Balasubramanian, R. Thermogravimetric investigation of hydrochar− lignite co-combustion. Bioresour. Technol. 2012, 123, 646−652. (18) Yadav, V.; Baruah, B. P.; Khare, P. Comparative study of thermal properties of bio-coal from aromatic spent with low rank subbituminous coals. Bioresour. Technol. 2013, 137, 376−385. (19) Tremel, A.; Stemann, J.; Herrmann, M.; Erlach, B.; Spliethoff, H. Entrained flow gasification of biocoal from hydrothermal carbonization. Fuel 2012, 102, 396−403. (20) Román, S.; Nabais, J. M. V.; Laginhas, C.; Ledesma, B.; González, J. F. Hydrothermal carbonization as an effective way of densifying the energy content of biomass. Fuel Process. Technol. 2012, 103, 78−83. (21) Funke, A.; Reebs, F.; Kruse, A. Experimental comparison of hydrothermal and vapothermal carbonization. Fuel Process. Technol. 2013, 115, 261−269. (22) Xiao, L.; Sun, Z.; Shi, Z.; Xu, F.; Sun, R. Impact of hot compressed water pretreatment on the structural changes of woody biomass for bioethanol production. BioResources 2011, 6 (2), 1576− 1598. (23) Grammelis, P. Solid Biofuels for Energy: A Lower Greenhouse Gas Alternative; Springer: New York, 2010. (24) Neavel, R. C. Coal structure and coal science: Overview and recommendations. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1979, 24 (1), 73−82. (25) Sommersacher, P.; Brunner, T.; Obernberger, I. Fuel indexes: A novel method for the evaluation of relevant combustion properties of new biomass fuels. Energy Fuels 2012, 26, 380−390. (26) Netsch GmbH. User Manual TG 209 F1 Iris; Netsch GmbH: Germany. (27) Lucas, C.; Szewczyk, D.; Blasiak, W.; Mochida, S. Hightemperature air and steam gasification of densified biofuels. Biomass Bioenergy 2004, 27, 563−575. (28) Kristensen, J. B.; Thygesen, L. G.; Felby, C.; Jørgensen, H.; Elder, T. Cell-wall structural changes in wheat straw pretreated for bioethanol production. Biotechnol. Biofuels 2008, 1 (5), 1754−6834. (29) Selig, M. J.; Viamajala, S.; Decker, S. R.; Tucker, M. P.; Himmel, M. E.; Vinzant, T. B. Deposition of lignin droplets produced during dilute acid pretreatment of maize stems retards enzymatic hydrolysis of cellulose. Biotechnol. Prog. 2007, 23, 1333−1339. (30) Bhatia, S. K.; Perlmutter, D. D. A random pore model for fluidsolid reactions: I. Isothermal, kinetic control. J. AIChE 1980, 26 (3), 379−386. (31) Sevilla, M.; Maciá-Agulló, J. A.; Fuertes, A. B. Hydrothermal carbonization of biomass as a route for the sequestration of CO2:

ACKNOWLEDGMENTS This research was supported by the European Institute of Innovation and Technology (EIT) and Knowledge and Innovation Community (KIC) InnoEnergy under the project xGaTe. The authors acknowledge AVA-CO2, which provided the biocoal sample for the experimental work. Duleeka Sandamali Gunarathne acknowledges the financial support from the European Commission. This publication reflects the views of only the authors, and the European Commission cannot be held responsible for any use that may be made of the information contained therein.

■

NOMENCLATURE A = cross-sectional area of the gasifier (m2) ER = equivalence ratio Ḟc = fuel consumption rate (kg/h) Ḟ = fuel feed rate (kg/h) Ġ = dry gas flow rate (Nm3 h−1) hb = bed height (m) LH = hearth load (m3 cm−2 h−1) LHVf = lower heating value of fuel (MJ/kg) LHVg = lower heating value of gas (MJ Nm−3) m = mass (g) m0 = initial mass (g) mf = final mass (g) NO2,actual = actual O2 moles per 1 mol of fuel (mol) NO2,stoi = stoichiometric O2 moles per 1 mol of fuel (mol) r = conversion rate (min−1) rbh = rate of change of the bed height (m/h) rsg = specific gasification rate (kg m−2 h−1) tr = residence time of fuel in the bed (min) VS = superficial velocity (m/s) X = conversion η = cold gas efficiency (%)

■

REFERENCES

(1) Erlach, B.; Harder, B.; Tsatsaronis, G. Combined hydrothermal carbonization and gasification of biomass with carbon capture. Energy 2012, 45, 329−338. (2) Bobleter, O. Hydrothermal degradation of polymers derived from plants. Prog. Polym. Sci. 1994, 19, 797−841. (3) Hoekman, S. K.; Broch, A.; Robbins, C. Hydrothermal carbonization (HTC) of lignocellulosic biomass. Energy Fuels 2011, 25, 1802−1810. (4) Cuvilas, C. A.; Yang, W. Spruce pretreatment for thermal application: Water, alkaline, and diluted acid hydrolysis. Energy Fuels 2012, 26, 6426−6431. (5) Parshetti, G. K.; Hoekman, S. K.; Balasubramanian, R. Chemical, structural and combustion characteristics of carbonaceous products obtained by hydrothermal carbonization of palm empty fruit bunches. Bioresour. Technol. 2013, 135, 683−689. (6) Jamari, S. S.; Howse, J. R. The effect of the hydrothermal carbonization process on palm oil empty fruit bunch. Biomass Bioenergy 2012, 47, 82−90. (7) Oliveira, I.; Blöhse, D.; Ramke, H. Hydrothermal carbonization of agricultural residues. Bioresour. Technol. 2013, 142, 138−146. (8) Liu, Z.; Balasubramanian, R. Hydrothermal carbonization of waste biomass for energy generation. Procedia Environ. Sci. 2012, 16, 159−166. (9) Escala, M.; Zumbühl, T.; Koller, Ch.; Junge, R.; Krebs, R. Hydrothermal carbonization as an energy-efficient alternative to established drying technologies for sewage sludge: A feasibility study on a laboratory scale. Energy Fuels 2013, 27, 454−460. 2001

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002

Energy & Fuels

Article

Chemical and structural properties of the carbonized products. Biomass Bioenergy 2011, 35, 3152−3159. (32) Sevilla, M.; Fuertes, A. B. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 2009, 47, 2281−2289. (33) Brage, C.; Yu, Q.; Sjöström, K. Characteristics of evolution of tar from wood pyrolysis in a fixed bed reactor. Fuel 1996, 75, 213−219. (34) Ponzio, A.; Kalisz, S.; Blasiak, W. Effect of operating conditions on tar and gas composition in high temperature air/steam gasification (HTAG) of plastic containing waste. Fuel Process. Technol. 2006, 87, 223−233. (35) Donaj, P.; Yang, W.; Blasiak, W. Conversion of industrially processed biomass waste into value-added products using high temperature agents. Proceedings of the International Conference on Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC); Jacksonville, FL, May 10−13, 2011. (36) Gunarathne, D. S.; Chmielewski, J. K.; Yang, W. High temperature air steam gasification of steam exploded biomass. Proceedings of Finnish−Swedish Flame Days; Jyväskylä, Finland, April 17−18, 2013. (37) AVA-CO2. http://www.ava-co2.com (accessed Feb 7, 2014). (38) Saravanakumar, A.; Haridasan, T. M.; Reed, T. B.; Bai, R. K. Experimental investigation and modeling study of long stick wood gasification in a top lit updraft fixed bed gasifier. Fuel 2007, 86, 2846− 2856.

2002

dx.doi.org/10.1021/ef402342e | Energy Fuels 2014, 28, 1992−2002