A Phenomenological Model of a Downdraft Biomass Gasifier Flexible to the Feedstock Composition and the Reactor Design
Abstract
:1. Introduction
2. Materials and Methods
2.1. One-Dimensional Phenomenological Approach
- Moisture (M) evaporation by a 1st order kinetic equation;
- Biomass/residual material (W) devolatilization described by one-step global reaction;
- Char (C) gasification reactions;
- Combustion of volatile species.
- HS = cpS∙(TS − 298.15);
- HG = cpG∙(TG − 298.15);
- QSW = (4∙hSW/Dreact)∙(TS − Twall);
- QGW = (4∙hGW/Dreact)∙(TG − Twall);
- QSG = hSG∙ASG∙(TS − TG);
- ASG = 6∙(1 − ε)/dp.
2.2. Adopted Kinetic Rates
2.3. Experimental Characterization of Biomass Gasification System
- The biomass/residual material composition, flow rate and chemical features, the latter reported in Table 4 in terms of proximate analysis (expressed in dry basis, d.b.) defined using a LECO CHN-628 analyzer according to the ASTM D5373 procedure, ultimate analysis (expressed in dry-ash free basis, daf) using a TGA 701 LECO thermo-gravimetric analyzer according to the ASTM D5142 procedure, and calorific values—each measurement was performed five times. Table 4 reports the calculated mean values;
- Low-frequency pressure sensors and k-type thermocouples were mounted along the reactor axis, respectively, for the measurement of the gaseous pressure and temperature to be used for the validation of the model results. Table 5 reports the gasifier characteristics, together with the details about the position of each thermocouple and pressure sensor applied.
3. Results
3.1. Model Validation and Results
3.2. Parametric Analysis of the Influence of the Gasification ER
3.3. Parametric Analysis of the Influence of the Bed Porosity
3.4. Parametric Analysis of the Biomass Moisture Content
4. Conclusions
- Higher ERs lead to syngas characterized by a lower heating values and to higher temperatures achieved in the reduction zone;
- A greater biomass packaging in the reactor determines a general shift of reactivity towards the exit section: the greatest influence can be noticed on the pressure drop along the reactor axis. The delay in the combustion and gasification reactions also causes a reduction in CH4 and H2;
- An increase in the biomass moisture content delays the evolution of the temperature profile, as greater energy is spent during the evaporation process. This also occurs at the expense of the H2 and CH4 produced in the subsequent phases, resulting in a syngas of poorer quality. The pressure evolution is instead influenced in a negligible way.
Supplementary Materials
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Nomenclature
Latin symbol | Quantity and SI Unit |
1D | Mono Dimensional |
, , , , , , , , | Stoichiometric coefficient gas phase for N2, H2O, CO2, CO, H2, CH4, H2S, CmHn |
A | Ash |
AH2O | Pre-exponential factor of the drying reaction— |
Specific surface area of the particle— | |
C | Char |
CHP | Combined Heat and Power |
CH4 | Methane |
CO | Carbon monoxide |
CO2 | Carbon dioxide |
CmHn | Tar |
— | |
— | |
Di | Species Diffusivity Coefficient— |
Dreact | Reactor Diameter— |
Particle diameter— | |
Inner core diameter— | |
ER | Equivalence Ratio |
Moisture | |
MC | Molecular weight Char— |
Mj | — |
N2 | Nitrogen |
GHG | Green House Gases |
H2 | Hydrogen |
H2O | Vapour water |
H2S | Hydrogen sulfide |
— | |
— | |
Coefficient of solid-gas heat exchange— | |
Coefficient of gas-wall heat exchange— | |
Coefficient of solid-wall heat exchange— | |
Kinetic constant for homogeneous reactions gas phase— | |
— | |
— | |
— | |
Kinetic constant of the water gas shift reaction— | |
K0 | Pre-exponential factor of the devolatilization reaction— |
Equilibrium constant of the water gas shift reaction | |
LHV | Lower Heating Value |
ODE | Ordinary Differential Equations |
O2 | Oxygen |
P | Total pressure— |
Gas-wall heat exchanged— | |
Solid-gas heat exchanged— | |
Solid-wall heat exchanged— | |
R | Universal gas constant— |
Reaction rate devolatilization— | |
Reaction rate drying— | |
Reaction rate homogeneous reactions gas phase— | |
Reaction rate heterogeneous reactions involving carbon— | |
SP | Shell progressive |
TG | Gas temperature— |
TS | Solid temperature — |
TWall | Wall temperature — |
UG | Gas velocity— |
US | Solid velocity— |
Wdaf | Biomass/residual material (dry-ash free basis) |
Greek symbol | Quantity and SI Unit |
α1, β1, γ1, δ1, ε1 | Stoichiometric coefficient for raw material |
Stoichiometric coefficient gas phase for C | |
Heat of reaction water vapour— | |
Heat of reaction for homogeneous reactions gas phase— | |
Heat of reaction for heterogeneous reactions involving carbon— | |
ΔV | Differential volume— |
Δz | Height along the gasifier— |
ε | Degree of vacuum of the bed |
Correction factor for solid-gas heat exchange coefficient | |
μG | Fluid viscosity— |
ηi | Devolatilization yields |
Stoichiometric coefficient gas phase f for j = O2, N2, H2O, CO2, CO, H2, CmHn | |
ξ | Fraction of the particle radius occupied by unreacted char |
ρi | Solid partial bed densities for i = WdafSS, M, C and A— |
ρj | Gas phase partial densities for j = O2, N2, H2O, CO2, CO, H2, CH4, H2S, CmHn— |
Subscripts and superscripts | Quantity and SI Unit |
ash | Referred to ash |
C | Referred to char |
dev | Referred to devolatilization |
diff | Referred to diffusion |
dry | Referred to drying |
G | Referred to gas |
G1,2,3,4 | Referred to gas homogeneous reactions gas phase |
Gl | |
Gj | |
P | Referred to particle |
R | Referred to reaction |
S | Referred to solid |
Si | |
Sp1,2,3,4 | Referred to heterogeneous reactions involving carbon |
GW | Referred to gas-wall |
SG | Referred to solid-gas |
SW | Referred to solid-wall |
W | Referred to wall |
WG | Referred to water gas shift reaction |
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Reaction | Reaction Name |
---|---|
Biomasswet → Biomassdry + H2Ovapour | Drying |
Biomassdry → Char + Volatiles (Gas + Tar) | Devolatilization |
Heterogeneous Reactions | |
C + ER∙O2(g) → 2(1 − ER)∙CO + (2∙ER − 1) CO2 | Oxidation |
C + CO2(g) → 2CO | Boudouard Reaction |
C + H2O(g) → CO + H2 | (Water/gas heterogeneous reaction) |
C + 2H2(g) → CH4 | (Methane formation reaction) |
Homogeneous Reactions | |
CO + H2O(g) ↔ CO2 + H2 | (Water/gas shift reaction) |
Tar + O2 → CO + H2O | |
Tar + H2O →CO + H2O | (Tar reforming) |
Tar → CO + CO2 + CH4 + H2 | (Tar cracking) |
CH4 + 2O2 → CO + 2H2O | |
CH4 + H2O ↔ CO + 3H2 | (Methane reforming reaction) |
CO + 0.5O2 → CO2 | |
H2 + 0.5O2 → H2O |
Drying |
Biomass wet → Biomass dry + H2O vapor [23] |
Rdry (kg/(s∙m3bed)) = ρH2O∙5.56 × 106∙exp(−8.79 × 104/(8.31∙TS)) |
Devolatilization |
Cα1Hβ1Oγ1Nδ1Sε1 → α2∙C + a∙H2 + b∙CH4 + c∙CO + CmHn + d∙H2O + e∙CO2 + f∙H2S + g∙N2 [33] |
Rdev (kg/(s∙m3bed)) = ρW∙2 × 104∙exp(−8467/TS) |
Homogeneous reactions gas phase |
G1: CO + ½ O2 → CO2 [23] |
RG1 (kmol/(s∙m3)) = ε∙kG1∙[CO]∙[H2O]0.5∙[O2]0.5 with kG1 = 1.3 × 1014·exp(−62,700/TG) |
G2: H2 + ½ O2 → H2O [23] |
RG2 (kmol/(s∙m3)) = ε∙kG2∙[H2]∙[O2] with kG2 = 8.83 × 1011·exp(−12,005/TG) |
G3: CH4 + 2O2 → CO2 + 2 H2O [23] |
RG3 (kmol/(s∙m3)) = ε∙kG3∙[CH4]∙[O2] with kG3 = 2.552 × 1017·exp(−11,196/TG) |
G4: CmHn + (m/2 + n/4) O2 → m CO2 + n/2 H2O [38] |
RG4 (kmol/(s∙m3)) = ε∙kG4∙[CmHn]0.5∙[O2] with kG4 = 1891·TG·exp(−12,200/TG) |
Water–gas shift reaction |
CO + H2O → H2 + CO2 [13,23] |
RWG (kmol/(s∙m3)) = ε∙kWG∙{[CO]∙[H2O] − [CO2]∙[H2]/KWGeq} |
with kWG = 2.78 × 103·exp(−1513/TG) KWGeq = 0.0265·exp(−3966/TG) |
Heterogeneous reactions involving carbon |
(P1): C + ½ O2 → CO [31] |
(P2): C + CO2 → 2 CO [23] |
(P3): C + H2O → H2 + CO [31] |
(P4): C + 2 H2 → CH4 [23] |
RSP (kgC/(s∙m3bed)) = {ρGl/[(1/kdiff,Gl) + (1/kash,Gl)∙(1/ξ−1) + (1/kRp,Gl∙ξ2)]}∙[MC/(υRp,Gl∙MGl)]∙ASG |
with Gl = O2, CO2, H2O, H2. The coefficients ki are listed in Table S2 in the Supplementary Materials. |
Biomass | As Delivered | After Briquetting | Average Dimensions | Average Density [kg/m3] | Porosity ε |
---|---|---|---|---|---|
Woodchip Case 1 | - | 10–30 mm | 860 | 0.5 | |
Woodchip Case 2 | - | 10–30 mm | 810 | 0.5 | |
Hydro-char | Length: 40–80 mm. Diameter: 35 mm. | 1250 | 0.55 | ||
Olive Pomace | Length: 40–80 mm. Diameter: 35 mm. | 808 | 0.5 |
Parameter | Woodchip Case 1 | Woodchip Case 2 | Hydro-Char | Olive Pomace |
---|---|---|---|---|
(a) Proximate analysis | ||||
Initial Moisture | 11.2% | 15.9% | 12.4% | 8.4% |
Ash (db) | 0.56% | 0.36% | 17.36% | 3.72% |
Volatile matter (db) | 82.89% | 78.64% | 59.13% | 74.34% |
Fixed carbon (db) | 16.67% | 21% | 23.51% | 21.94% |
(b) Ultimate analysis | ||||
Carbon (daf) | 45.5% | 44.4% | 65.2% | 50.2% |
Hydrogen (daf) | 5.6% | 5.2% | 6.3% | 5.9% |
Nitrogen (daf) | 0.0% | 0.2% | 1.3% | 0.8% |
Oxygen (daf) | 48.9% | 50.0% | 27.2% | 43.1% |
(c) Heating value | ||||
High calorific value kJ/kg (daf) | 17,068 | 17,026 | 26,815 | 19,654 |
Lower calorific value kJ/kg (daf) | 15,710 | 16,854 | 25,457 | 18,296 |
Characteristic | Value |
---|---|
Maximum Reactor Diameter. | 0.21 m |
Minimum Reactor Diameter | 0.10 m |
Gasifier Length | 1 m |
Equivalence ratio | 0.3 |
1st Thermocouple position | 0.06 m |
2nd Thermocouple position | 0.78 m |
3rd Thermocouple position | 0.98 m |
1st Pressure Sensor position | 0.06 m |
2nd Pressure Sensor position | 0.98 m |
Species | Woodchip Case 1 | Woodchip Case 2 | Hydro-Char | Olive Pomace |
---|---|---|---|---|
H2% | 1.21 | 1.65 | 1.53 | 1.58 |
N2% | 61.79 | 43.2 | 54.64 | 45.0 |
CO% | 20.55 | 26.87 | 21.45 | 11.24 |
CH4% | 1.17 | 1.09 | 1.52 | 3.1 |
CO2% | 15.29 | 26.52 | 17.73 | 39.08 |
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Costa, M.; La Villetta, M.; Piazzullo, D.; Cirillo, D. A Phenomenological Model of a Downdraft Biomass Gasifier Flexible to the Feedstock Composition and the Reactor Design. Energies 2021, 14, 4226. https://doi.org/10.3390/en14144226
Costa M, La Villetta M, Piazzullo D, Cirillo D. A Phenomenological Model of a Downdraft Biomass Gasifier Flexible to the Feedstock Composition and the Reactor Design. Energies. 2021; 14(14):4226. https://doi.org/10.3390/en14144226
Chicago/Turabian StyleCosta, Michela, Maurizio La Villetta, Daniele Piazzullo, and Domenico Cirillo. 2021. "A Phenomenological Model of a Downdraft Biomass Gasifier Flexible to the Feedstock Composition and the Reactor Design" Energies 14, no. 14: 4226. https://doi.org/10.3390/en14144226