Measurement of beech wood pyrolysis kinetics in a fluidized bed reactor

Detailed experimental data for pyrolysis kinetics of pulverized solid fuels at high heating rates for temperatures below 1000 K is essential for the validation of reaction mechanisms in complex pyrolysis network models. The present study demonstrates the capability of a fluidized bed reactor setup to provide experimental data in the temperature range from 523 to 973 K carried out with pulverized beech wood. A comparison with simple 1and 2-step models has shown plausible results. Introduction For the simulation of pulverized fuel boilers, modeling of devolatilization behavior plays an important role to describe ignition, volatile combustion and pollutant formation. With increasing computational power, typically used single first order models are replaced with more complex models e.g. network models like Chemical Percolation Devolatilization (CPD), Flashchain or Functional-Group, Depolymerization, Vaporization, Cross-linking Model (FG-DVC). Necessary validation data for those models are typically provided by drop tube reactors (DTR) at high temperatures and high particle heating rates. A fluidized bed reactor (FBR) is capable to extend the validation data set to lower temperatures in combination with high particle heating rates. Furthermore, a continuous analysis of the reaction products like in the FBR allows for more data points than the analysis at discrete time steps in the DTR. The higher number of data points is helpful to prove the plausibility of possible reaction mechanisms in network models. The present study demonstrates the capability of an FBR to determine pyrolysis kinetics in the temperature region from 523 to 973 K. Experimental Setup For determination of pyrolysis kinetic data, a small-scale fluidized bed reactor with a diameter of d = 55 mm is used. Reactor temperature is controlled with an electrical heated furnace around the reactor (see Figure 1). The main part of the reactor consists of two axial Al2O3 pipes, mounted in a stainless steel reactor head. The annular gap between the pipes serves as heat up zone for the fluidizing gas JOINT MEETING THE GERMAN AND ITALIAN SECTIONS OF THE COMBUSTION INSTITUTE SORRENTO, ITALY – 2018 flow (150–300 slph N2 depending on temperature), while the inner part encloses the fluidized bed consisting of Al2O3 particles (d50 = 116 μm). Figure 1. Scheme of the fluidized bed reactor (dimensions in mm) Fuel samples (15–50 mg depending on temperature) are supplied to the reactor in batches. Particle transport from the fuel lock to the bed is forced with a gas flush from (10 ml N2) to achieve fast heating rates of approximately 10 4 K/s [1]. Product gases are captured just above the bed and then transported to an FTIR spectrometer. The sampling line is heated to 453 K to avoid condensation of H2O and tar components. The ceramic packing reduces dead volume, while the distributor in front of the gas sampling line allows for higher volume flows without particle entrainment. Both increase the gas exchange rate, which was identified to be the limiting factor in the determination of reaction rates of solid fuel particles [2]. Time dependent gas concentrations ci(t) are measured with an Agilent Cary 670 FTIR spectrometer, which analyzes in the wavenumber range from 600 to 6500 cm -1 , a spectral resolution of 4 cm -1 and a sampling frequency of 10 Hz. More detail on construction, operation and error analysis is given elsewhere [1–3]. JOINT MEETING THE GERMAN AND ITALIAN SECTIONS OF THE COMBUSTION INSTITUTE SORRENTO, ITALY – 2018 For the present investigation pulverized and pre-dried beech wood particles from a sieving mesh size of 125–160 μm are used. Corresponding results from proximate and ultimate analysis are given in Table 1. Table 1. Fuel properties based on ultimate and proximate analysis for investigated beech wood samples (mesh size: 125–160 μm) Ultimate analysis C (daf) H (daf) N (daf) S (daf) O (daf) 48.83 % 5.64 % 0.23 % 0.02 % 45.24 % Proximate analysis Moisture (raw) Ash (dry) Volatiles (daf) 7.28 % 0.36 % 85.34 % Evaluation and Modeling In the first step of the evaluation procedure for determining pyrolysis kinetic parameters, the experimentally derived rate of released gases dYGas,exp/dt is calculated according to equation (1) from the measured FTIR gas concentrations ci of 21 different gas species (including CO, CO2, CxHyOz, SO2 and NOx). Concentration of H2O was ignored to avoid interferences with underlying drying processes. dYGas,exp dt = ρN2 ⋅ ? ̇? N2 mF ⋅ MN2 ⋅ ∑ ci ⋅ Mi n i=1