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Modeling of two-dimensional natural downward smoldering of peat Jiuling Yang, Haixiang Chen, and Naian Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02293 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016
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Title page
Modeling of two-dimensional natural downward smoldering of peat Jiuling Yang, Haixiang Chen*, Naian Liu State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
*
Corresponding author: Haixiang Chen, Tel.: +86-551-63600275; fax: +86-551-63601669. E-mail address:
[email protected] 1
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Modeling of two-dimensional natural downward smoldering of peat Jiuling Yang, Haixiang Chen*, Naian Liu State Key Laboratory of Fire Science, University of Science and Technology of China
Abstract Smoldering combustion of natural organic layers like peatlands leads to the largest fires on Earth. The multi-dimensional smoldering structure of peatlands is observed in the field but the mechanism is poorly understood. In this work, a 2-D cylindrical axi-symmetric model with considering volume shrinkage is developed for the natural downward smoldering of peat with different inorganic contents. Numerical results show that the model captures the two-dimensional multi-subfront structure of peat smoldering process. For the peat with lower inorganic content, the predicted bi-dimensionality of wall-attachment is more obvious, the mass loss rate and the spread rate are higher, and the smoldering front is thinner. For the peat with higher inorganic content, smoldering is more sensitive to lateral heat loss. Both the inorganic content and the lateral heat loss have negative effects on the volume shrinkage, peak temperature and in-depth spread rate. Additionally, two groups of experiments using peat samples with added inorganic contents of 0% and 66.7% were conducted in an un-insulated furnace. Comparison of the numerical and experimental data shows that the model predicts the smoldering characteristics very well at lower inorganic content and qualitatively well at higher inorganic content. The predicted effects of the inorganic content and lateral heat loss on peat smoldering process provide a theoretical basis for fire-fighting measures of smoldering peatlands with different levels of inorganics. Key words: Peat; Smoldering; Shrinkage; Heat loss; Inorganic content
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Nomenclature Letters A c D g ht ,hl hcv hm h0 ∆h Hm IC k K m M MC MI, ML n OM P Q q r ∆r R T u v W0/2 X y z ∆z
Greek symbols
reactant species specific heat capacity (J/kg K) diffusivity (m2/s) gravity (m/s2) convective heat transfer coefficient (W/m2 K) volumetric heat transfer coefficient (W/m3 K) mass transfer coefficient (kg/m2 s) initial height of calculation domain (m) moving displacement (m) transient height of the mth column (m) inorganic content thermal conductivity (W/m K) permeability (m2) mass (kg) molecular mass (g/mol) moisture content maximum node number in the calculation domain reaction order organic matter pressure (Pa) heat of reaction (J/g) heat flux (W/m2) radial distance (m) radial grid size (m) universal gas constant (J/mol K) Temperature (oC) horizontal velocity (m/s) vertical velocity (m/s) width of calculation domain (m) volume fraction mass fraction (mi/m0) vertical axis (m) longitudinal grid size (m)
ε
µ ρ
emissivity non-dimensional reaction rate (1/s) mass fraction coefficient dynamic viscosity (kg/m s) bulk density (kg/m3)
ρ ′′
surface density (kg/m2)
σ
ϕ
Stefan-Boltzmann constant (W/m2 K4) porosity
Subscripts 0,∞ cond conv eff fg g gp i
Initial, ambient conductive heat transfer convective heat transfer effective coefficient gas formation gas phase gas products condensed-phase species i
ω& ′′′
υ
j k mix n oc rad s sh t wp wv superscripts (-) (o) (')
gaseous species j heterogeneous reaction k gas mixture natural organic base radiation solid phase shrinkage total wet peat water vapour weighted or averaged next time step transient status
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1. Introduction Peat is a substance formed by forest litter deposition which has undergone complex biological, physical and chemical processes for thousands of years. It is a porous material containing abundant organics that can sustain smoldering combustion. Smoldering peat fires occur with some frequency during the dry seasons in, for example, Indonesia, Canada, Russia, UK and USA. Smoldering combustion propagates slowly through the organic layers of the ground and has the potential to release sequestered carbon deep into the soil.1-3 Pollutants in smoldering haze also substantially increase the risk of cardiopulmonary diseases.4 Smoldering peat fires can propagate downward and laterally, consuming the organic matter.1 As observed in the field, most smoldering peat fires spread slowly at the in-depth front without notice for a long time. The primary controlling mechanisms are the net fuel load, the oxygen diffusion, the heat loss predominantly by evaporation and heat dissipation from the reaction zone by conduction, convection and a lesser quantity of radiation.5, 6 The moisture reduces the fuel load per total mass and also results in a lower net heat release value because of water evaporation. The inorganic content has double-side effects of reducing the convective heat loss and slowing reactions due to heating up of minerals and suppressing oxygen mass transport towards the smoldering front.7 Laboratory studies have revealed that the moisture and mineral contents, bulk density, together with other parameters, play important roles in smoldering combustion of organic soils, like peat and duff.8-11 However, the smoldering mechanism of peatlands with higher inorganic content is indeed poorly understood. During the natural smoldering combustion, oxygen diffuses downward to the combustion front from air by convection and diffusion, while gas products flow upward through the residue layer. Meanwhile, the residue layer above the reaction front becomes thicker and the volume shrinks1, 3, 6, 12-17
or swells.18 This smoldering scenario has received intensive attentions in the literature, both
experimentally and numerically. The simplified one dimensional (1-D) numerical study with the 4
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drying and combustion fronts assumption concluded that the stable natural downward smoldering was dominated by the mass transport of oxidizer.6 Since peat layer during smoldering is a multi-component system containing moisture, organics, char residue and ash,2 a multi-component scheme with 5-step reaction rather than a single-step7, 15 or 3-step reaction19-21 is required for better understanding of the smoldering mechanism.3 The 1-D numerical model with the multi-component reaction scheme predicted the experimental smoldering thresholds related to the moisture and inorganic contents for ignition.10, 11 A further improved model was used to investigate the in-depth smoldering of peat with heterogeneous profiles of moisture, inorganic content and density that varied with depth.22 The predicted final depth of burn agrees well with the experimental results.23 Up to present, most smoldering models available in the literature are 1-D models
3, 6, 13, 20-22, 24, 25
except for few 2-D7, 19, 26-29 and scarce 3-D models.30, 31 These multi-dimensional models had not taken volume shrinkage into consideration, thus the shrinkage caused by smoldering is poorly understood. Actually, the inner peat smolders faster while the outer lower due to the lateral heat loss.2 The volume of inner peat also shrinks faster than that of outer peat, which cannot be captured by 1-D models. In this work, a 2-D model with the improved 5-step kinetic scheme2, 3 is developed for capturing the structure of natural downward smoldering of peat. The effect of added inorganic content (ICa, the ratio of the inorganics mass to the initial mass (m0) of the whole peat sample) is also examined on the two-dimensional smoldering structure, volume shrinkage and spread rate. Meanwhile, the effect of lateral heat loss is investigated to reveal the bi-dimensionality of smoldering structure. This is the first time that a 2-D numerical model considering the shrinkage profile for the natural downward smoldering of peat is developed. The smoldering experiments were also conducted to validate this numerical model. The predicted results of the model provide a theoretical guidance for the suppression of smoldering peat fires. 5
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2. Numerical model 2.1. Chemical kinetics The chemical kinetic scheme of peat smoldering includes water evaporation (dr), peat pyrolysis (pp), peat oxidation (po), β-char oxidation (βo), and α-char oxidation (αo): 2, 3 Peat·υw,dr H2O → Peat +υw,dr H2O
(dr)
Peat →υs1 α – Char +υg1 Gas
(pp)
Peat +υO2,po O2 →υs2 β – Char +υg2 Gas
(po)
β – Char +υO2,βo O2 →υs3 Ash +υg3 Gas
(βo)
α – Char +υO2,αo O2 →υs4 Ash +υg4 Gas
(αo)
The non-dimensional reaction rate is expressed by the Arrhenius law:
ω& k′′′ = Z k e− E
k
/ RT
f ( yk,A ) g ( yO2 )
(1)
where Zk and Ek are the pre-exponential factor (1/s) and the activation energy (kJ/mol) for reaction k. f (yk,A) and g(yO2) are the reaction model for the reactant A and oxygen, respectively.
f ( yk,A ) = (
m A nk ) msA,0
(2)
(1 + yO ) nO2 ,k − 1 2 g ( yO2 ) = 1
(3)
where nO2,k=1 is assumed for oxidation reactions.2 For the non-oxidation reactions, g(yO2) equals 1. Genetic algorithm (GA) is applied to obtain the kinetic parameters of the reactions from the experimental data of Thermogravimetric (TG) measurements (STA 449F3) both in nitrogen and air atmospheres at four heating rates (10, 20, 30 and 40 oC/min). The peat used in this work was purchased from German HAWITA Gruppe, which strictly follows the Regulation of the German Institute for Quality Assurance and Labeling (RAL). The results of elemental analysis are C (36.62 %), O (40.87 %), H (4.83 %), N (0.78 %) (by Elementar vario EL cube, Germany) and S 6
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(0.87 %) (by Elementar Vario EL III, Germany).32 The best kinetic parameters, obtained by Genetic algorithm,2 are listed in Table 1. Table 1 2.2. Two-dimensional governing equations In the geometry of computational domain (Fig. 1), the cylindrical axi-symmetric coordinate system is applied to solve the energy, momentum and mass transport equations. The radial coordinate originates at the center of the cylinder bottom, i.e. the symmetric axis locates at r=0. So the radial coordinate of right lateral side of the cylinder along the r-axis is r= W0/2 as shown in Fig. 1. The vertical z-axis originates from the sample bottom center and thus the position of upper boundary is z=h0. The upper boundary of samples is exposed to air and heated by a radiative panel in order to initiate smoldering. Figure 1 The conservation equations of the numerical model are: Eq (4) condensed-phase mass, Eqs (5-9) condensed-phase species, Eq (10) condensed-phase energy, Eq (11) gas-phase mass, Eq (12) gas species mass, Eq (13) gas-phase energy, Eq (14) gas momentum and pressure. On the right of Eq (9), the yield rate of ash comes from two parts: the intrinsic content (the first two terms) and the added inorganic content (ICa) (the third term).2 It is assumed that the added inorganic minerals are converted to ash by a physical transformation process after smoldering. Here yi is the mass fraction of condensed reactants, which is normalized by the initial total mass, i.e. yi=mi/m0. Subscripts of w, p, α, β and a represent water, peat, α-char, β-char and ash, respectively. Generally, α-char and β-char have different structures, compositions and reactivites.2 Subscript “j” represents gas species, including water vapor (wv), oxygen (O2), nitrogen (N2) and gas products (gp). ∂ρ = − ρ 0ω& ′fg′′ ∂t
(4)
∂y w ′′′ = −ω& dr ∂t
(5) 7
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∂y p
′′ − ω& ′po ′′ = −ω& ′pp
∂t
(6)
∂yα ′′ − ω& αo ′′′ = v s1 ω& ′pp ∂t
∂yβ
(7)
′′ − ω& ′βo ′′ = vs2ω& ′po
∂t
(8)
∂ya ′′ + vs4ω& αo ′′′ + = vs3ω& ′βo ∂t
( ρc )
∂ ((1 − ∑ yi ) ICa ) i
(9)
∂t
∂Ts ∂ ∂T 1 ∂ ∂T = (k s s ) + (rk s s ) + hcv (Tg − Ts ) + ∑ ρ 0ω& k′′′Qk ∂r ∂t ∂z ∂z r ∂r k
∂ 1 ∂ ∂ ( ρ g φ) + ( rρ g φu ) + ( ρ g φv ) = ρ 0 ω& ′′fg′ r ∂r ∂t ∂z
∂( ρg φy j ) ∂t φρg cg u=−
+
∂Tg ∂t
∂Tg ∂r
+ φρg cg v
∂Tg ∂z
=
(10) (11)
∂y ∂y 1 ∂ ∂ ∂ 1 ∂ ′′ j (rρg φuy j ) + ( ρg φvy j ) = ( ρg Dφ j ) + (rρg Dφ j ) + ρ0ω& ′fg, r ∂r ∂z ∂z ∂z r ∂r ∂r
+ φρg cg u
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∂Tg 1 ∂ ∂Tg ∂ (φkg )+ (rϕk g ) + hcv (Ts − Tg ) ∂z ∂z r ∂r ∂r
ρg K ∂P K ∂P , v=− ( − ρg g ) , P = RTg , M mix = 1/( ∑ y j / M j ) µ ∂r µ ∂z M mix j
(12)
(13)
(14)
The following initial conditions in Eq (15) show that initial pressure and temperature are assumed to be atmospheric and 27 oC.
yw,0 = 0.03, y p,0 = 1 − yw,0 − ICa , yα ,0 = yβ ,0 = ya ,0 = 0, yO2 ,0 = 0.23, y N 2 ,0 = 0.77 ywv,0 = y gp,0 = 0, Ts,0 = Tg,0 = 27 o C, P0 = 1.013 × 105 Pa , u0 = v0 = 0
(15)
The 2-D control volume system for discretizing the governing equations is depicted in Fig. 2. For a random node P(m,n) in the interior (Fig. 2(1)), four neighboring nodes are N(m,n-1), S(m,n+1), W(m-1,n) and E(m+1,n). More details about the discrete equations for interior cells are presented in Appendix A. Figure 2 Peat smoldering is initiated by a radiative panel (heat flux is 20 kW/m2) for a given duration (15 min). The symmetric axis (r=0) is assumed to be impermeable for gas mass transfer. For all the 8
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nodes at the upper surface (Fig. 2(2)), both the convective and radiant heat losses are considered. Thus the thermal boundary condition at the upper surface of peat sample is expressed by Eq. (16). Analogy of the heat exchange process,33 the mass transfer across the upper surface by diffusion is given in Eq. (17). Heat loss through convection (Fig. 2(3)) is imposed on the right lateral side (r=W0/2) (Eq. (18)), while no heat flux (Fig. 2(4)) flows through the bottom side at z=0 (Eq. (19)). Both lateral and bottom sides are impermeable to gas mass transfer (Eqs. (20) - (21)). At the outlet (z=h0), the pressure is assumed to be atmospheric. The main parameters used in the model are listed in Table 2. It is noted that each parameter prevails the whole cell due to the assumption of homogeneity of fuel bed, which is relaxed in a transient 2-D model for the thermal degradation of a square cellulosic particle.29 −k
∂Ts ∂z
− φρg D
− k eff
−k
= εqig (0
