Modeling and model performance evaluation of sewage sludge gasification in fluidized-bed gasifiers using Aspen Plus

Figures & data

Table 1. Reactions considered in the model.

Reaction	Reaction type	Reaction number
Homogeneous reactions
H2 + 0.5O2 → H2O	Hydrogen combustion	R-1
CH4 + H2O → CO + 3H2	Steam reforming	R-2
CO + H2O → CO2 + H2	Water-gas shift	R-3
0.5N2 + 1.5H2 → NH3	NH3 formation	R-4
Heterogeneous reactions
C + O2 → CO2	Carbon combustion	R-5
CO2 + C → 2CO	Boudouard	R-6
C + 2H2 → CH4	Methanation	R-7
C + H2O → H2 + CO	Water-gas	R-8
H2 + S → H2S	H2S formation	R-9

Table 2. Description of the Aspen Plus reactor blocks considered in the model.

Aspen Plus ID	Block ID	Description
RYIELD	DRIER	Dries the biomass eliminating the moisture
	MASSPROC	Converts nonconventional biomass into conventional components
RSTOIC	NS	Reacts nitrogen and sulfur in the fuel to form NH3 and H2S
	NONEQ	Reacts part of the carbon in the fuel to form tar
RGIBBS	GASIFIER	Minimizes Gibbs free energy to reach chemical equilibrium and calculate outlet composition
	GASIF2	Restricted equilibrium to calculate syngas composition
SEP	WSEP	Separates the moisture from the fuel and sends it to the gasifier
	ELEMSEP	Separates the reacted NH3 and H2S as well as the unreacted carbon and sends them to the final outlet
	B2	Separates tar from the rest of the reaction stream
	ASHSEP	Separates ashes from the outlet of the gasifier
	SYNSEP	Separates syngas from moisture and solids
HEATER	HEATER	Heats the stream to the same temperature as that of the outlet of the gasifier by means of calculator block
	COOLER	Simulates the gas cooling to the required clean-up temperature of 298 K
MIXER	MIX	Mixes the different outlets of the blocks to reproduce a single product stream

Table 3. Model validation: Comparison of experimental (De Andrés, Narros, and Rodríguez 2011) and model results.

Parameter	Experimental	Model	% Error
Syngas composition (% v/v)
H2	10.41	10.41	−0.01
CO	7.97	8.03	0.72
CO2	14.10	14.08	−0.19
CH4	3.01	3.24	7.45
N2	60.81	58.18	−4.33
LHVgas (MJ/Nm^3)	3.33	3.72	11.95
CGE (%)	37.25	38.90	4.43

Figure 1. Comprehensive Aspen Plus flow sheet for the fluidized-bed gasification process.

Figure 2. Effect of the gasification temperature on (a, b, c, d) syngas composition, (e) gas production (Y_gas), and (f) carbon conversion (X_C).

Figure 3. Effect of the equivalence ratio (ER) on (a, b, c, d) syngas composition, (e) gas production (Y_gas), and (f) carbon conversion (X_C).

Figure 4. Comparison of modeled and experimental results with air + steam as gasifying agent (S/B = 0.5): (a, b, c, d) syngas composition; (e) gas production (Y_gas); (f) carbon conversion (X_C).

Figure 5. Modeled and experimental results under different temperatures: (a) lower heating value (LHV_gas); (b) cold gas efficiency (CGE).

Figure 6. Modeled and experimental results under different equivalence ratios: (a) lower heating value (LHV_gas); (b) cold gas efficiency (CGE).

De Andrés, J. M., A. Narros, and M. E. Rodríguez. 2011. Air-steam gasification of sewage sludge in a bubbling bed reactor: Effect of alumina as a primary catalyst. Fuel Process. Technol. 92:433–440. doi:10.1016/j.fuproc.2010.10.006.

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