Numerical Simulation of Partial Combustion for Biomass Tar Elimination in Two-Stage Gasifier

Shanhui Zhao, Yi Su, Wenguang Wu, Yunliang Zhang, Yun Wang, Yonghao Luo

Institute of Thermal Energy Engineering, Shanghai Jiao Tong University, Shanghai, China.

DOI: 10.4236/jsbs.2013.31012                

Abstract

Biomass tar is an obstacle in biomass gasification. Partial combustion is a potential method for tar elimination. To better study the tar conversion conditions and design reasonable partial combustion reactor, 2D/3D throat models are establish to calculate the tar reduction during partial combustion using numerical method. Different number of nozzles, injection directions and injection velocities were investigated. SIMPLE algorithm was used in this calculation. The results indicated that the best performance of partial combustion was obtained when ER (equivalent ratio) = 0.34. A performance of 3 nozzles, perpendicular injection and 20 m/s injection velocity could reach lowest tar content of 3.09 wt%.

Keywords

Biomass Tar; Partial Combustion; Numerical; ER

1. Introduction

Biomass gasification is a potential way for the usage of biomass energy. Tar contained in the fuel gas is the main obstacle for the usage of biomass widely. There are quite a lot of methods to reduce tar content [1]. Considering the economy and environment-friendly, Partial oxidative/ (combustion) is a attracting way , for it’s no need for expensive mechanical equipment and can lead to a very high tar reduction. Partial combustion is kind of combustion process that ER is no more than 1. Some scholars have conducted a series of research on this area. Brandt et al. [2] did some work on a 100-kW two-stage gasifier, and the tar content after char bed could be as low as 10 - 40 mg/kg dry wood. X. Guo et al. [3] did research on air-steam gasification of biomass micron fuel (BMF) in a cyclone gasifier. The experimental results showed that the gasification performance was best with ER being 0.37 and S/B being 0.31. Two-stage downdraft gasifier is a kind of low-tar method for biomass conversion. The throat in the gasifier is a crucial part for tar elimination.

The design of throat and choice of reaction condition is crucial. Numerical calculation is a good way to solve this problem, for it’s cheap and high efficient [4]. But there are still very few researches on this area especially on the designing of the throat for partial combustion of biomass tar. This paper gives a detail simulation that will optimize the operation of gasification and the designing of gasifier. A 2D laminar flow reaction was calculated and compared to experiment results to test and verify our model. 3D model of throat combustion area was established to optimize partial oxidation for tar destruction.

2. The Physics and Mathematic Model

Partial combustion is a complex process that contains several parts: fluid flow, mass transfer, heat transfer and chemical reaction. These four parts couple with each other in a particular reactor and it’s nearly impossible to obtain its analytic solution. Numerical calculation is a considerable method to solve it. To use numerical calculation, we first need to establish the physic and mathematic model.

2.1. Flow and Heat Transfer

Fluid flow equations:

(GE-1)

(GE-2)

Heat transfer equation:

(GE-3)

Turbulence k-e equation:

(GE-4)

(GE-5)

Special transport reaction:

(GE-6)

(GE-7)

2.2. Chemical Mechanism

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

Table 1 is the kinetic parameters used in this model. All this parameters are from references.

(17)

Arrhenius reaction rate was calculated from Equation (17).

2.3. Boundary Condition and Solutions

Commercial CFD software FLUENT was used to stimulate this model. Standard model was chosen to calculate turbulent flow. SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm couples velocity and pressure. EDC model can couple turbulent flow and chemical reaction very precisely. Total mesh grid is

Table 1. Kinetic constants used in this model.

about 30,000 and mesh dependence test has been conducted. Calculation was done on a 3.0 GHz computer for 10 hours.

Verification mechanism experiments were conducted on the Laminar flow reactor in Biomass Research Center in SJTU as introduced in our formal works [15].

Figure 1 shows the boundary of the reactor. Boundary conditions: As phenol is the main pyrolysis products of wood, we choose phenol as the tar. Composition and flow rate of the inlet gases: C₆H₆O (0.45478), CO (0.0303), CO₂ (0.14078), CH₄ (0.0063), H₂ (0.00037), H₂O (0.36747); Inlet temperature: 500˚C, Inlet turbulence intensity: 10%, Inlet turbulence length: 0.035 m, Air inlet temperature: 27˚C, Wall temperature: 900˚C.

3. Results and Analysis

3.1. Mechanism Verification Model

This part was to test and verify our model in a Pipe flow reactor. Compared to the experiments in different conditions (ER = 0, 0.029, 0.1, 0.153, 0.2, 0.278, 0.34, 0.4), we can have a clear idea whether this model is reasonable and optimize the ER for our next research.

Figure 2 shows that the temperature increases with the increase of ER. Partial oxidation is an exothermal process. The more oxygen, the more sufficient the reaction will be. But more oxygen will decrease the calorific value of product gas. At the same time, temperature distribution may have an influence on the soot production because high temperature without oxygen may lead PAHs converted into soot.

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	S. Anis and Z. A. Zainal, “Tar Reduction in Biomass Producer Gas via Mechanical, Catalytic and Thermal Methods: A Review,” Renewable and Sustainable Energy Reviews, Vol. 15, No. 5, 2011, pp. 2355-2377. doi:10.1016/j.rser.2011.02.018
[2]	P. Brandt, E. Larsen and U. Henriksen, “High Tar Reduction in a Two-Stage Gasifier,” Energy & Fuels, Vol. 14, No. 4, 2011, pp. 816-819.
[3]	X. J. Guo, B. Xiao, X. L. Zhang, S. Y. Luo and M. Y. He, “Experimental Study on Air-Stream Gasification of Biomass Micron Fuel (BMF) in a Cyclone Gasifier,” Bioresource Technology, Vol. 100, No. 2, 2009, pp. 1003-1006. doi:10.1016/j.biortech.2008.07.007
[4]	L. Gerun, M. Paraschiv, R. Vijeu, J. Bellettre, M. Tazerout, B. G?bel and U. Henriksen, “Numerical Investigation of the Partial Oxidation in a Two-Stage Downdraft Gasifier,” Fuel, Vol. 87, No. 7, 2008, pp. 1383-1393. doi:10.1016/j.fuel.2007.07.009
[5]	D. Shin and S. Choi, “The Combustion of Simulated Waste Particles in a Fixed Bed,” Combustion and Flame, Vol. 121, No. 1, 2000, pp. 167-180. doi:10.1016/S0010-2180(99)00124-8
[6]	W. P. Jones and R. P. Lindstedt, “Global Reaction Shcemes for Hydrocarbon Combustion,” Combustion and Flame, Vol. 73, No. 3, 1988, pp. 233-249. doi:10.1016/0010-2180(88)90021-1
[7]	K. M. Bryden and K. W. Ragland, “Numerical Modeling of a Deep, Fixed Bed Combustor,” Energy & Fuels, Vol. 10, No. 2, 1996, pp. 269-275.
[8]	C. K. Westbrook and F. L. Dryer, “Chemical Kinetic Modeling of Hydrocarbon Combustion,” Progress in Energy and Combustion Science, Vol. 10, No. 1, 1984, pp. 1-57. doi:10.1016/0360-1285(84)90118-7
[9]	C. D. Blasi, “Dynamic Behaviour of Stratified Downdraft Gasifiers,” Chemical Engineering Science, Vol. 55, No. 15, 2000, pp. 2931-2944. doi:10.1016/S0009-2509(99)00562-X
[10]	J. Macak and J. Malecha, “Mathematical Model for the Gasification of Coal under Pressure,” Industrial & Engineering Chemistry Process Design and Development, Vol. 17, No. 1, 1978, pp. 92-98.
[11]	A. Jess, “Reaktionskinetische Untersuchungen zur thermischen Zersetzung von Modellkohlenwasserstoffen,” Erdol Erdgas Kohle, Vol. 111, 1995, pp. 479-484.
[12]	P. Morf, P. Hasler and T. Nussbaumer, “Mechanisms and Kinetics of Homogeneous Secondary Reactions of Tar from Continuous Pyrolysis of Wood Chips,” Fuel, Vol. 81, No. 7, 2002, pp. 843-853. doi:10.1016/S0016-2361(01)00216-2
[13]	J. Andreas, “Mechanisms and Kinetics of Thermal Reactions of Aromatic Hydrocarbons from Pyrolysis of Solid Fuels,” Fuel, Vol. 75, No. 12, 1996, pp. 1441-1448. doi:10.1016/0016-2361(96)00136-6
[14]	X. Li, J. Xu, F. Wang, J. Gao, L. Zhou and G. Yang, “Direct Oxidation of Toluene to Benzoic Acid with Molecular Oxygen over Manganese Oxides,” Catalysis Letters, Vol. 108, No. 3, 2006, pp. 137-140. doi:10.1007/s10562-006-0034-x
[15]	Y. Su, Y. Luo, Y. Chen, W. Wu and Y. Zhang, “Experimental and Numerical Investigation of Tar Destruction under Partial Oxidation Environment,” Fuel Processing Technology, Vol. 92, No. 8, 2011, pp. 1513-1524.