Article pubs.acs.org/EF
Gasification Kinetics of Biomass- and Fossil-Based Fuels: Comparison Study Using Fluidized Bed and Thermogravimetric Analysis Andreas Mueller,† Herman D. Haustein,‡,∥ Philipp Stoesser,§ Thobias Kreitzberg,‡ Reinhold Kneer,‡ and Thomas Kolb*,†,§ †
Engler-Bunte-Institut, Fuel Technology (EBI ceb), Karlsruhe Institute of Technology, Engler-Bunte-Ring 1, 76131 Karlsruhe, Germany ‡ Institute of Heat and Mass Transfer (WSA), RWTH Aachen University, Augustinerbach 6, 52056 Aachen, Germany § Institute for Technical Chemistry, Department Gasification Technology (ITC vgt), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ABSTRACT: In this work, the influence of experimental setup on derived kinetic data for the heterogeneous Boudouard gasification reaction is studied. A thermogravimetric analyzer (TGA) and a small-scale fluidized bed reactor (FBR) are used. The two systems differ basically in their fuel heating rates and fuel bed configurations. The kinetic study was performed in both reactors for the same temperatures and partial pressures of CO2 at ambient pressure using same-batch fuel samples (biomass, brown coal, and petcoke). Kinetic data are reported, and the influence of the thermal history of the fuel particle prior to the char gasification process is discussed. In general, the activation energies derived from both systems are lower for the brown coal fuels as compared to the wood char fuels. This finding may be explained by the high ash content of the brown coal fuels with multiple catalytic components in the ash. Both experimental setups used in this study agree well in their results for the carbon conversion rate and kinetic parameters for fuels with low volatile content, whereas fuels with high volatile content show different results in the two experimental setups. This may be explained by the physical and chemical structure of the fuel particle not being changed significantly during the in situ pyrolysis prior to gasification for the low volatile fuels. The char properties of the high volatile fuels may differ significantly due to system dependent different heating rates and gas atmospheres during the in situ pyrolysis prior to gasification. Hence, the observed reactivity becomes system dependent. In conclusion, the results show that knowledge of the thermal history of the fuel particle prior to the gasification process is most important for the interpretation of kinetic data as well as for the design of experiments for generation of kinetic data.
1. INTRODUCTION Enhanced gasification processes (e.g., high-pressure entrainedflow gasification, EFG) using low-grade biogenic and fossil fuels for the production of synthesis gas, which can then be converted into chemicals or liquid fuels or fed to IGCC units, will play an important role in satisfying the future demand for basic chemicals and power.1 In an entrained-flow gasifier, the conversion of a fuel particle or droplet is characterized by an initial fast heat-up process, followed by devolatilization, pyrolysis, and the subsequent heterogeneous gasification reaction of the remaining char in a CO2/H2O atmosphere in the final fuel conversion zone of the reactor. The heterogeneous gasification reactions are known to be the slowest in the entire process, limiting the overall fuel conversion rate and dictating the minimum required particle residence time.2 The measurement of intrinsic reaction kinetics with laboratory-scale methods is mandatory for mathematical modeling of the technical process and process design. A vast number of studies are available in the literature, which report reaction kinetic data of solid fuel gasification using thermogravimetric analyzers as well as fluidized bed, droptube, fixed bed, or wire mesh reactors. Each reactor setup has its own advantages and limitations. Only a very limited number of studies comparing reactivity data using the same fuel batch in different experimental setups is found (e.g., Megaritis et al.3 and Zeng et al.4). © 2015 American Chemical Society
In a thermogravimetric analyzer (TGA) a batch of particles is deposited on a scale and is weighed as it reacts with a gas. Typical advantages of a TGA are accurate real-time data on the basis of the directly measured mass of the sample, high reproducibility, and well-defined temperature and gas phase conditions.5,6 The major limitations of a TGA for reactivity measurements are the low heating rates of commercially available systems (typically less than 2 K/s) and the fact that the fuel sample forms a fixed bed inside a crucible, which might lead to thermal or mass transfer effects influencing the experimental result. These limitations can be overcome by using a small sample mass and by optimizing gas flow and the geometry of the sample crucible to achieve a monolayer of particles. Recent work by the authors7,8 has demonstrated that a smallscale fluidized bed reactor (FBR) provides several advantages for kinetic studies: homogeneous temperature and gas composition in the bulk, observation of longer time periods up to reaction completion (opposite to drop-tube reactors), and high heating rates. On the other hand, it is not as optically accessible or as suitable as a drop-tube reactor for rapid reactions and does not have the direct measurement ability Received: May 20, 2015 Revised: August 19, 2015 Published: September 1, 2015 6717
DOI: 10.1021/acs.energyfuels.5b01123 Energy Fuels 2015, 29, 6717−6723
Article
Energy & Fuels
with mC,0 as the initial fixed carbon mass (i.e., not including the ash) at the point of CO2 introduction to the TGA and mC(t) as the remaining fixed carbon mass as a function of time. The simplest expression for the reaction rate, r (also called reactivity), determined from the experimental data is the rate of progress of fuel conversion, which can also be expressed as a massbased conversion rate using the initial mass of carbon in the sample:2,14
found in the TGA; rather an indirect gas sampling and composition analysis is employed. Latest studies on co-gasification of biogenic and fossil fuels have been conducted using a thermogravimetric analyzer9,10 and a bubbling fluidized bed reactor11,12 in order to investigate the catalytic effects of alkali metals on coal gasification. The results indicate that alkali metals can act as catalysts for CO2 and steam gasification as well as tar cracking reactions. In the present study, the gasification reaction kinetics of five different solid fuels (petcoke, two brown coals, and two biochars) using a TGA and FBR are employed, and their results are compared in terms of the time dependent conversion progress and global kinetics of the heterogeneous gasification reaction with CO2. Both reactors are operated with the same fuel batches under similar conditions: approximately atmospheric pressure, temperatures (TGA, T = 800−1000 °C; FBR, T = 800−1100 °C) and partial pressures of CO2 (pCO2 = 0.1− 0.75 bar) in N2. The conditions are carefully chosen to ensure the measurement of intrinsic kinetics. Using TGA experiments, the gasification reactions for lignocellulosic chars in CO2 are typically under the chemically controlled regime for temperatures up to 1000 °C as shown in the review paper of Di Blasi.13 The FBR can generate intrinsic data for temperatures up to 1100 °C as recent studies8 have demonstrated. The fuels under investigation differ regarding their chemical composition, physical structure as well as volatile and ash content. By comparison of the two experimental setups, which inherently impose different thermal profiles on the fuel particle, the influence of the thermal history prior to reaction onset on observed reactivity is studied. In addition the basic understanding of the influence of experimental setup and system characteristic parameters (heating rate, fuel bed configuration, and experimental raw data) on derived kinetic data is discussed. From the comparison of the experimental results the importance of volatile content, thermal history, and active ash components on the measured reaction kinetics is shown.
r=
dX 1 dm =− dt mC,0 dt
(2)
All TGA measurements in this study show a roughly linear carbon conversion progress as a function of gasification time in the range from 15% to 85% conversion. In order to calculate characteristic r, the linear slope is evaluated between 10% conversion and 85% conversion according to
r=
X 0.85 − X 0.1 tX = 0.85 − tX = 0.1
(3)
By choosing this range the change of gas composition from inert to CO2 containing atmosphere at the beginning of the gasification segment and growing uncertainties in the mass measurement at high conversion levels are excluded. 2.2. Fluidized Bed Reactor. The present study employs a bed of inert particles, which is fluidized by a gas mixture whose flow rate and composition are controlled. Into this reactor small batches of fuel are introduced, while the gaseous reaction products are continuously analyzed from the exhaust gas. The experimental system (Figure 1) is described previously in detail.8
2. EXPERIMENTAL METHODS 2.1. Thermogravimetric Analysis. The thermogravimetric analyzer used for this comparison study is a commercially available laboratory TGA (TG209 F1 Iris, Netzsch). The fuel sample is placed in a ceramic crucible (inner diameter, ca. 6 mm; height of wall, ca. 1 mm). Inert as well as reactive gas atmospheres containing variable concentrations of CO2 can be introduced into the reactor. The crucible carrying the fuel sample is constantly weighed by a precision balance. The system has a resolution of 0.1 μg and a maximum sample temperature of 1000 °C. As a preliminary work, the system was modified: Crucible geometry, gas flow, and initial sample mass were optimized in order to minimize mass transfer limitations in the crucible and sample bed. The fuel samples (approximately 2 mg) were heated and pyrolyzed under N2 atmosphere with a constant heating rate of 30 K/min up to the selected reaction temperature followed by a 5 min plateau at this temperature. Afterward, the gas composition was changed and the sample was gasified in a N2/CO2 atmosphere until complete conversion. The temperature and mass loss of the sample due to pyrolysis and gasification were measured over time. The remaining mass after the gasification experiment represents the ash content of the sample. To calculate kinetic data of the gasification reaction from the experiments, the carbon conversion degree X is calculated as a function of time in the reactive gas atmosphere (eq 1) using the measured mass over time signal from the TGA:
X(t ) =
Figure 1. Interior of the fluidized bed reactor. The FBR is located inside a controlled electric oven, allowing accurate temperature control up to 1280 °C. The gas (N2 or N2/CO2 mixtures) for fluidizing and reaction heats up as it flows down an annular gap to the gas distributor, at the base of the bed. A porous ceramic plate (sintered silica glass) uniformly distributes the gas flow to fluidize the inert bed of roughly spherical, sand-like alumina (Al2O3; diameter dp = 112 ± 30 μm). For each run a defined batch of pulverized fuel (
