Fuel Particle Conversion of Pulverized Biomass during Pyrolysis in an


The scanning electron microscopic (SEM) images showed that biomass swelled during heating and shrank during initial stage of pyrolysis. Then, char ...
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Fuel Particle Conversion of Pulverized Biomass during Pyrolysis in an Entrained Flow Reactor Kentaro Umeki,†,* Kawnish Kirtania,‡ Luguang Chen,‡ and Sankar Bhattacharya‡ †

Division of Energy Science, Luleå University of Technology, 971 87 Luleå, Sweden Department of Chemical Engineering, Monash University, Clayton VIC 3800, Australia



ABSTRACT: This study addresses the change of char morphology and fuel conversion during pyrolysis in a laminar entrained flow reactor by experiments and particle simulation. Three experimental parameters were examined: reaction temperature (1073 and 1273 K); particle size (125−250, 250−500, and 500−1000 μm); and the length of reaction zone (650 and 1885 mm). The scanning electron microscopic (SEM) images showed that biomass swelled during heating and shrank during initial stage of pyrolysis. Then, char morphology transformed to cenospheres after the plastic stage. The yields of solid residue from the experiments were reasonably predicted by particle simulation. To give a guideline for the design of laminar entrained flow pyrolysis reactors, the required reactor length for complete conversion of biomass was also calculated for the pyrolysis. High reaction temperature, small particles, and slower gas flow were favorable for high fuel conversion.

1. INTRODUCTION The share of lignocellulosic biomass in the primary energy mix has been increasing especially in Europe as it is gradually replacing fossil fuels as a fuel for domestic, district, and industrial heating. In addition, biorefinery processes with gasification and consecutive gas synthesis technologies are under development to replace chemicals and transport fuels from oil.1 One promising gasification technology is entrainedflow gasification because it generates syngas with negligible amount of tar content due to high operating temperature and it is easy to scale-up.2,3 One of the biggest challenges of biomass gasification in entrained-flow gasifiers is to achieve high fuel conversion because the residence time of fuel particles is usually short, that is less than 4 s. At the same time, it is favorable to keep particle sizes as large as possible to minimize the energy penalty for milling fuel particles. Pyrolysis (devolatilization) is the initial step of fuel conversion, which is followed by char gasification and/or combustion depending on the reaction atmosphere. It is significantly affected by thermal processes,4 which means that the dimension of fuel particles affects the conversion time of pyrolysis by changing heat transfer performance. In addition, the reaction condition during pyrolysis determines the yields and property of char, which affects gasification reactivity subsequently.5,6 Therefore, the present study focuses on the effect of reaction parameters on pyrolysis behavior of woody biomass, that is, fuel conversion and morphological changes. Fuel conversion was investigated by the yields of solid residue under a certain reaction condition (e.g., reaction temperature, pyrolysis time, reactor length, etc.). Particle size is one of the most important parameters in entrained-flow gasifiers because it affects the residence time of fuel particles7 as well as the conversion time. Laminar entrained flow reactors (L-EFR) are often utilized to investigate the fuel conversion behaviors because it can keep the reaction conditions uniform and similar to that of entrained flow gasifiers: 100−10 000 K/s of heating rate and particle size © 2012 American Chemical Society

less than 1 mm. Some studies addressed the fuel conversion behavior of biomass pyrolysis by using L-EFR.8−11 These studies observed that particle sizes affect the fuel conversion of biomass by changing both residence time and heat transfer processes. However, the results of fuel conversion are rather qualitative, so quantitative criteria of transition from complete to incomplete conversion for various particle size and reaction conditions are needed. Previous studies8−11 showed that morphology of char formed at high heating rate was cenosphere, which is completely different from that of raw biomass. Cenosphere particles are formed because of the destruction of biomass cell structures by internal gas pressure of volatiles during the plastic stage in devolatilization. The plastic stage during pyrolysis was called the molten phase in some studies.13,14 The plastic state of fuel particles appears because of the existence of short-term intermediate liquid product during biomass pyrolysis.12 However, no study was found to assess the effect of reaction conditions, especially the particle size, on char morphology at L-EFR, whereas some studies investigated it at tubular reactors and wire mesh reactors.13,14 This article presents the effect of particle size on the yield of solid residue and morphological changes during the pyrolysis of biomass in an entrained flow reactor (L-EFR). Experiments and particle simulation were employed for this purpose. The morphology of fuel particles at various reaction conditions was also examined by using a scanning electron microscope (SEM). After the validation of particle simulation by experimental data, the guideline of pyrolysis reactor design to achieve complete conversion for pyrolysis was provided for various reaction conditions by particle simulation. Received: Revised: Accepted: Published: 13973

June 11, 2012 September 30, 2012 October 11, 2012 October 11, 2012 dx.doi.org/10.1021/ie301530j | Ind. Eng. Chem. Res. 2012, 51, 13973−13979

Industrial & Engineering Chemistry Research

Article

boundary condition of the particle center (t ≥ 0) is shown in eq 4.

2. EXPERIMENTAL SECTION 2.1. Sample. Sawdust of Norway spruce (stem wood) grown in Sweden was used as fuel. The sample was ground and sieved to three particle size ranges: 125−250, 250−500, and 500−1000 μm. 2.2. Experimental Procedure. A laminar entrained flow reactor (L-EFR) with an inner diameter of 50 mm and the length of 2 m was used to investigate the yield of permanent gases and solid residue by the pyrolysis of biomass particles at high heating rate. The L-EFR was controlled at certain temperature (1073 and 1273 K) under atmospheric pressure. Around 10 g of biomass was fed by a bowl feeder at feeding rate of 0.1−0.6 g/min entrained with a primary N2 flow rate of 1.0 L/min at a standard state. Secondary N2 was also supplied from the top of the reactor with a flow rate of 4.0 L/min at a standard state. Reynolds number of the gas flow inside the reactor was 50−60. Biomass was pyrolyzed when it fell down through the reactor. Two different water-cooled sample injectors were used to change the length of reaction zone inside the reactor to 650 and 1885 mm. Residence time of fuel particles were calculated numerically by considering density change during pyrolysis (for more detail, section 3.1 and Figure 5). At the bottom of the reactor, solid residue was collected by a flask and a thimble filter quenched by dry ice. The mass of collected solid residue was measured to determine its yield. The morphology of the solid residues was examined by a scanning electron microscope (SEM). The detail of the experimental methods by the L-EFR can be found elsewhere.15

λ

∂r

=0

(4)

The boundary condition on the surface (t ≥ 0) was given by eq 5. λ

∂Ts = h(Tg − Ts) + σε(Tg4 − Ts4) ∂r

(5)

The density, specific heat, and conductivities was calculated by the following equations. ρ = ρb + ρc

(6)

Cp = (ρb Cp,b + ρc Cp,c)/ρ

(7)

λ = (ρb λb + ρc λc)/ρ

(8)

Heat transfer coefficient was calculated by Nusselt number (Nu = hDp/λ) that was given by eq 9. 1/3 Nu = 2 + 0.6Re0.5 p Pr p

(9)

3.3. Pyrolysis Kinetics. The total mass loss during pyrolysis of biomass at high heating rate can be expressed by a single first-order reaction model. The mass conservation of biomass and char can be expressed by eqs 10−12.

3. PARTICLE MODEL 3.1. Particle Motion. In this study, the reaction conditions inside the L-EFR were under the laminar flow regime according to the regimes presented in ref 16. To calculate the slip velocity (difference between solid and gas flow speed) as well as residence time of particle, particle velocity and position was calculated along the axial direction by eq 1. π 3 du p π Dp ρp = C Dρg Dp2|ug − u p|(ug − u p) 6 dt 8 π + Dp3(ρp − ρg )g (1) 6 Here, Schiller and Naumann’s expression (applicable for Rep