Process Optimization and CO₂ Emission Analysis of Coal/Biomass Gasification Integrated with a Chemical Looping Process

Abstract

Biomass gasification is an attractive technology and one of the pathways for producing hydrogen. Due to the variable seasons and low calorific value of biomass, the addition of coal in the gasifier is suggested because coal has a high calorific value and carbon-to-hydrogen ratio. In general, the gaseous product obtained in gasification always contains a high amount of carbon dioxide, therefore, the co-gasification of biomass and coal should integrate with the calcium looping carbon dioxide capture process to provide purified hydrogen. In this work, the model of the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process was developed through an Aspen Plus simulator. The developed model was used to analyze the performance of this process. The sensitivity analysis demonstrated that increasing the gasification temperature, steam-to-feed (S/F) ratio, calcium oxide-to-feed (CaO/F) ratio, and regenerator temperature could improve hydrogen production. Next, further optimization was performed to identify the optimal operating condition that maximizes hydrogen production. The results showed that the optimal operating temperature of the gasifier is 700 °C with an S/F mass ratio of 2 and coal to biomass (C/B) mass ratio of 0.75:0.25. However, the carbonator and regenerator temperatures should be 450 °C and 950 °C, respectively, with a CaO/F mass ratio of 3. Under these operating conditions, the maximum H₂ content and H₂ yield can be provided as 99.59%vol. (dry basis) and 92.38 g hydrogen/kg biomass feeding. The other results revealed that the energy efficiency and carbon capture efficiency of this process are 42.86% and 99.99%, respectively, and that the specific emission of released CO₂ is 80.77 g CO₂/MJ.

1. Introduction

Nowadays, conventional power generation based on fossil fuels encounters several problems, including the depletion of fossil fuel resources and CO₂ emissions as a greenhouse gas. To solve these problems, many researchers have tried to develop alternative renewable energy sources to replace fossil fuels. Hydrogen in particular is often perceived as an energy carrier for the future since it is a clean, efficient, renewable, and non-polluting energy source with a high energy density (33.3 kW h/kg) [1]. This makes it a promising source of renewable energy that could address the global energy crisis while also meeting environmental concerns [2]. Nowadays, hydrogen can be used as fuel for mobile and stationary applications through fuel cells or internal engines without carbon emissions [3], however, it should be derived from renewable resources, e.g., biomass [4], ethanol [5,6], and biodiesel [7], to support a sustainable energy system.

Biomass, as an agricultural residue, is an alternative renewable energy source since it is abundant, readily available, and inexpensive. The main advantage of biomass is CO₂ neutrality, which is a balance between CO₂ emission from the production process and CO₂ adsorption by the plant. In general, biomass is widely used for energy and heat production through the combustion process [8]. In addition, the conversion of biomass into fuels, chemicals, or gaseous products has received significant attention since the energy storage in this product form is easy to store, transport, and handle.

The selection of methods to produce a chemical product from biomass depends on the desired product. This research focuses on hydrogen production from biomass and thus, the gasification process is recommended. Although the product obtained from biomass gasification is synthesis gas or syngas, hydrogen can be further produced through various purification processes. At present, the investigation of the gasification process has received more attention in both the experiment and process simulation. Syngas production through the gasification process is affected by both the operating conditions of the gasifier and gasifying agent types. Arteaga-Pérez et al. [9] studied the gasification of sugarcane bagasse in a bubbling fluidized bed gasifier through simulation with the Aspen Plus simulator. They reported that under the condition of 750 °C with an equivalent ratio of 0.3, the total exergy efficiency and energy efficiency are the highest. The gasifying agent fed to react with the biomass has an effect on both the composition and heating values of the syngas. Fernandez-Lopez et al. [10] investigated the gasification of animal wastes in a dual fluidized bed gasifier by using the Aspen Plus simulator. At different ratios, two types of gasifying agents, i.e., steam and CO₂, were studied. The result indicated that the use of steam as a gasifying agent can produce a higher H₂/CO ratio than that of CO₂. Shen et al. [11] studied the CO₂ gasification of woody biomass in a lab-scale auto-thermal gasifier. Their result showed that the CO composition increased while CH₄ decreased because of the Boudouard reaction; carbon conversion and cold gas efficiency (CGE) increased with the CO₂ addition.

Although biomass is renewable and available, its amounts may fluctuate depending on agriculture, season, cultivated area, and volume of agricultural production. Therefore, biomass power plants cannot operate in some seasons. In addition, the use of biomass has some obstacles to operation since it has a high moisture content, low bulk density, and low calorific value, which have an impact on hydrogen production [12]. In order to resolve the utilization of biomass, there are many researchers focusing on the co-gasification of biomass and coal [13,14,15]. Not only is coal cheap, easily transportable, and abundant in resources, but coal gasification has a high efficiency in hydrogen production. In addition, coal has a high energy calorific value because the thermodynamic efficiency of coal is higher than biomass. Shahabuddin and Bhattacharya [16] assessed the co-gasification of coal and biomass with a CO₂ agent by using the Aspen Plus process simulator. They reported that the blending of coal and biomass in an equal amount can provide a higher lower heating value (LHV) and CGE than pure coal by 12% and 18%, respectively. Biomass blending of up to 50% favors gasification performance with an LHV of 12 MJ/kg and a CGE of 78%. Howaniec et al. [17] determined the reactivity of coal, biomass, and fuel blends in a laboratory-scale fixed bed reactor which was operated at 700, 800, and 900 °C. Their results showed that the reactivity of fuel blends of coal and biomass had the highest value compared to coal and biomass char reactivity at the same operating conditions. Secer et al. [18] implemented response surface methodology (RSM) to determine the optimal operating conditions of the hydrothermal co-gasification of sorghum at a constant temperature of 500 °C for maximum hydrogen content. Their results showed that the highest total gas and hydrogen volumes could be obtained under conditions where the higher levels of the water volume of the reactor and lower levels of the coal percentage of the coal/biomass mixture were combined.

In general, the syngas product derived from gasification is composed of H₂, CO, CO_2, and CH₄. However, to enhance the H₂ yield in the syngas product, CO and CO₂ must be removed. Calcium looping is the most promising technology for the CO₂ capture process and offers many benefits [19]. Calcium oxide (CaO) is cheap and widely distributed; as a sorbent, it can save operation costs and be environmentally friendly [20,21]. When the syngas obtained from the gasification process is delivered to the calcium looping process, the CO₂ in the syngas will adsorb on the surface of CaO where the carbonation reaction as an exothermic reaction can occur (referred to as carbonator), and thus, calcium carbonate (CaCO₃) can be generated. Since CO₂ is produced from the water gas shift reaction while the CO₂ is removed from the carbonation reaction, it is possible to perform the separation of CO₂ simultaneously during the water gas shift reaction in a single unit. When CO₂ is produced from the water gas shift reaction, it will be adsorbed into CaO through the carbonation reaction. Thus, the CO₂ removal causes a shift in the equilibrium of the water gas shift reaction to the product side. The occurrence of the two following reactions: the water gas shift and carbonation reactions in a single reactor, which is referred to as the sorption-enhanced water-gas shift (SE-WGS) reaction, can reduce not only CO₂ but also CO. After the carbonation reaction is complete, CaCO₃ is sent to the regenerator reactor where CaCO₃ can be desorbed to CO₂ and CaO. Since the reaction occurring in the regenerator (called the calcination reaction) is an endothermic reaction, a high external heat source is required. A number of works have focused on the SE-WGS reaction with wider types of sorbents [21,22,23]; for example, Wang et al. [21] revealed that Ni-doped CaO (Ni-CaO) could promote a SE-WGS reaction.

Most of the previous publications focusing on gasification integrated with the calcium looping process have performed a sensitivity analysis of operating conditions. However, optimization should be performed to determine the global optimal operating condition of gasification and calcium looping that can provide the maximum H₂ product. In this research, the statistical method is introduced to discover the important parameters that influence process efficiency. In addition to performance analysis and optimization, an environmental assessment in terms of carbon emission should be considered to ensure that the proposed system has no negative effect on the environment.

This research aims to study hydrogen production from the co-gasification of biomass and coal integrated with the calcium looping process through the Aspen Plus simulator version 10. Firstly, the effect of operating conditions in gasification (i.e., gasification temperature, steam to feed ratio, and coal to biomass mass ratio) and the calcium looping process (i.e., carbonator temperature, regeneration temperature, and CaO to feed mass ratio) on hydrogen production is investigated. Next, the optimization of the gasification and calcium looping process is performed by using Design Expert 11 where the maximum H₂ production is the main objective function. Finally, the energy analysis and carbon dioxide emission-to-energy consumption of the co-gasification of biomass and coal integrated with the calcium looping process is determined.

2. Process Flow Diagram Description

Figure 1 presents a schematic diagram of the integrated co-gasification with calcium looping process designed in the Aspen Plus simulator. The black line represents the material stream, whereas the blue and red lines demonstrate the water or steam and energy streams. In this research, the mixture of pellet pine wood and lignite coal is selected as the raw material for hydrogen production. The process is divided into two sections that include (1) the gasification process and (2) the purification of H₂ from raw syngas through the calcium looping process. Firstly, biomass (BIOMASS) and coal (COAL) are separately fed into a dryer (DRY), which is modeled by the Sep model to remove moisture from the biomass and coal. Next, the dried biomass (BIO-1) and dried coal (BIO-2) are separately supplied to the decomposition reactor (DECOM) which is represented by the RYield reactor model. In the Aspen Plus simulation, both the biomass and coal are defined as non-conventional components. They must be decomposed into conventional components which include the conventional elements: C, H, N, O, S, etc. The components of biomass and coal are defined based on proximate and ultimate data, as shown in

Table 1. Proximate analysis, ultimate analysis (wet basis), lower heating values, and CO₂ emission factors of pellet pine wood, lignite coal, and rice husk.

	Fuel Type
	Pellet Pine Wood	Lignite Coal	Rice Husk
Proximate analysis (wt.%, wet basis)
Fixed Carbon	14.85	33.60	14.99
Volatile matter	74.94	28.82	55.54
Moisture	10.00	31.20	9.95
Ash	0.18	6.38	19.52
Ultimate analysis (wt.%, dry basis)
C	50.50	47.83	39.44
H	5.90	6.67	3.22
N	0.30	0.52	0.08
O	43.00	37.95	37.74
S	0.20	0.65	0.01
LHV (kJ/g)	17.30	19.29	13.52
CO2-emission factors (kg/MJ)	0	0.0946	0

. Next, the outlet product from each decomposition reactor (BIO-3 for biomass and BIO-4 for coal) is mixed and delivered to a gasifier (GASIFIER) corresponding with steam (STEAM). The gasifier, which is modeled by the RGibbs reactor model, can calculate the gas composition at the chemical equilibrium through the minimization of the Gibbs free energy method. The type of gasifier reactor used in this study is a fluidized bed gasifier. Although the RGibbs reactor model does not require the specification of chemical reactions, the possible chemical reactions occurring in the gasifier have been listed in Equations (1)–(7).

Combustion reaction

C + O₂ → CO₂    −394 kJ/mol

(1)

H₂ + 0.5O₂ → H₂O   −242 kJ/mol

(2)

Boudouard reaction

C + CO₂ ↔ 2CO    +172 kJ/mol

(3)

Water gas reaction

C + H₂O ↔ H₂ + CO   +131 kJ/mol

(4)

Water gas shift reaction

CO + H₂O ↔ H₂ + CO₂  −41 kJ/mol

(5)

Methane formation

C + 2H₂ ↔ CH₄    −74 kJ/mol

(6)

Steam methane reforming

CH₄ + H₂O ↔ CO + 3H₂  +206 kJ/mol

(7)

The gas product obtained from gasification (SYNGAS) is further supplied to a cyclone (SEP-ASH) to separate carbon and ash (ASH) from the syngas. Then, the produced syngas (RAWSYNGA) can be provided.

Next, the produced syngas, consisting of H₂, H₂O, CO, CO₂, and CH₄, is cooled through a cooler (COOL) before feeding into the calcium looping process (ADSORBER) which is modeled by the RGibbs reactor model. In this reactor, CO is further reduced through a water gas shift reaction (Equation (5)). CO₂ is produced from the water gas shift reaction and the gasification process is captured by CaO (MIXCAO) as a sorbent through the carbonation reaction (Equation (8)). When CO₂ is removed from the syngas, the H₂ yield in the syngas product is higher. Next, the purified H₂ is separated from the CaCO₃ and other contaminants through a gas-solid separator (SEP-1). Then, the CaCO₃ is sent to the regenerator reactor (DESORP) which is modeled by the RGibbs reactor model. When the CaCO₃ desorbs CO₂ it is converted back into CaO through a calcination reaction (Equation (9)). Some regenerated CaO is deactivated from the regenerator and accumulated in the system. Therefore, it is necessary to split out CaO, recycle it to mix with makeup CaO, and then send it to the carbonator reactor.

Carbonation and calcination reaction

CaO + CO₂ → CaCO₃    −178 kJ/mol

(8)

CaCO₃ → CaO + CO₂    +178 kJ/mol

(9)

3. Methodology

3.1. Parametric Analysis

The investigation of hydrogen production through co-gasification integrated with the calcium looping process is performed through the Aspen Plus simulator version 10. The following assumptions are used in the simulation: (1) the process is operated under steady state and isothermal conditions, (2) the pressure drop and heat loss are negligible, (3) the chemical reactions that occurred in the process reached chemical equilibrium, (4) the gasifier does not produce tar under a high-temperature operation, and (5) charcoal is assumed to be carbon. Although tar formation has not been considered in this work, it should be noted that tar can be represented with a complex mixture of aromatic compounds; for example, toluene, phenol, or naphthalene [24]. In this study, the Peng–Robinson thermodynamic model is selected to predict the gas composition obtained from co-gasification integrated with the calcium looping process. The inlet composition of biomass, coal, steam, and CaO can be specified based on the coal to biomass (C/B), steam to feed (S/F), and CaO to feed (CaO/F) mass ratios which are expressed as Equations (10)–(12).

$$\frac{\mathrm{C}}{\mathrm{B}}=\frac{{\dot{m}}_{\mathrm{coal}}}{{\dot{m}}_{\mathrm{biomass}}}$$

(10)

$$\frac{\mathrm{S}}{\mathrm{F}}=\frac{{\dot{m}}_{\mathrm{steam}}}{{\dot{m}}_{\mathrm{biomass}}+{\dot{m}}_{\mathrm{coal}}}$$

(11)

$$\frac{\mathrm{CaO}}{\mathrm{F}}=\frac{{\dot{m}}_{\mathrm{CaO}}}{{\dot{m}}_{\mathrm{biomass}}+{\dot{m}}_{\mathrm{coal}}}$$

(12)

where ${\dot{m}}_i$ represents the mass flow rate of component i.

Table 2. Input parameters of each stream and unit operation used in the simulation [13,14].

Parameters	Value
Atmospheric condition (atm)	1
Biomass flow rate (kg/h)	1000
Coal flow rate (kg/h)	1000
Coal: Biomass	1:1
Steam flow rate (kg/h)	2000
Steam temperature (°C)	800
Calcium oxide (kg/h)	2000
Gasifier temperature (°C)	700
Carbonator temperature (°C)	450
Regenerator temperature (°C)	900

presents the input parameters of each stream and unit operation. The calculations of the gas composition obtained from the gasifier, adsorber, and regenerator are based on the method of minimizing Gibbs’s free energy. In order to investigate the H₂ production from an integrated system, the gas product composition (% dry basis) at the outlet of the gasification reactor (SYNGAS stream) and CO₂ mole flow rate at the exit of the carbonator reactor (OUTADSOR stream) are determined with a wider range of gasification temperature, C/B mass ratio, S/F mass ratio, CaO/F mass ratio, regenerator, and carbonator temperature.

3.2. Experimental Design and Optimization

Factorial designs are widely used in experiments involving several factors where it is necessary to study the combined effect of the factors on a response. The 2^k factorial design is instrumental in the early stages of experimental work when there are many factors to be investigated [25]. In this study, the 2^k factorial design analysis is used to investigate the main effects and interactions of the parameters related to H₂ mole fraction for the gasification process and CO₂ capture efficiency for the calcium looping process. Two levels of the following five varying parameters: the gasification temperature (A), S/F mass ratio (B), C/B mass ratio (C), carbonator temperature (D), and CaO/F mass ratio (E) are investigated.

In this study, the parameters and their levels are selected from the result obtained in Section 5.1. A response surface method (RSM) analysis is an efficient experimental strategy to determine optimal conditions for a multivariable system. The process variable and function of each factor are represented by

$$y={\beta }_0+{\beta }_1{X}_1+{\beta }_2{X}_2+\epsilon$$

(13)

where y is the response variable, X₁ and X₂ are dependent variables, β₀, β₁, and β₂ are regression coefficients evaluated with the least square method, and $\epsilon$ is the error observed in response variable y [25].

In this study, the optimization of operating conditions in an integrated system to maximize H₂ mole fraction and CO₂ capture efficiency is performed through Design Expert V11.

3.3. Energy Efficiency Analysis

The energy efficiency of hydrogen production through the co-gasification process is used to measure the thermal efficiency of the integrated process and can be calculated from the below equation.

$${\eta }_{{\mathrm{H}}_2}(\%)=\frac{{\dot{m}}_{{\mathrm{H}}_2}LH{V}_{{\mathrm{H}}_2}}{{\dot{m}}_{\mathrm{biomass}}LH{V}_{\mathrm{biomass}}+{\dot{m}}_{\mathrm{coal}}LH{V}_{\mathrm{coal}}+{\dot{m}}_{\mathrm{steam}}{h}_{\mathrm{steam}}}\times 100$$

(14)

where ${\dot{m}}_{{\mathrm{H}}_2}$, ${\dot{m}}_{\mathrm{biomass}}$, ${\dot{m}}_{\mathrm{coal}}$ and ${\dot{m}}_{\mathrm{steam}}$ stand for the mass flow rate of hydrogen, the biomass, coal, and steam, respectively. $LH{V}_{{\mathrm{H}}_2}$, $LH{V}_{\mathrm{biomass}}$, and $LH{V}_{\mathrm{coal}}$ are the lower heating values of hydrogen, the biomass, and coal, respectively. It is noted that $h_{\mathrm{steam}}$ refers to the specific enthalpy of steam at 800 °C = 3663.84 kJ/kg.

3.4. Environmental Impact Analysis

In order to study the environmental impact of the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process, the carbon dioxide emission is evaluated. In this process, CO₂ and other greenhouse gas are considered in terms of the CO₂ equivalent based on their relative global warming potential (GWP). In this study, the CO₂ equivalent calculated from accepted US EPA and IPCC standards can be found in the Aspen Plus simulator. In addition, the carbon dioxide to energy output, which refers to the carbon dioxide emission to net energy, is evaluated by using the following Equation (15) [26].

$${\epsilon }_{\mathrm{em}}=\frac{{\dot{m}}_{{\mathrm{CO}}_2,\mathrm{em}}}{W_{\mathrm{net}}}=\frac{{\dot{m}}_{{\mathrm{CO}}_2,\mathrm{em}}}{{\dot{m}}_{{\mathrm{H}}_2}LH{V}_{{\mathrm{H}}_2}}$$

(15)

where ${\epsilon }_{\mathrm{em}}$ is the specific emission of released CO₂ to the atmosphere, $W_{\mathrm{net}}$ is the net power output from the process, and ${\dot{m}}_{{\mathrm{CO}}_2,\mathrm{em}}$ stands for the total mass flow rate of CO₂ equivalent emission to the atmosphere. According to Figure 1, ${\dot{m}}_{{\mathrm{CO}}_2,\mathrm{em}}$ is based on LTCO2, LTCAOSPL, and DRYH2 streams.

4. Model Validation

In this study, the simulated gasification results of the biomass and coal are validated with experimental data to ensure that the proposed model can provide predictable results. Loha et al. [27] studied the conversion of rice husk into syngas. Rice husk with a proximate and ultimate analysis, as listed in Table 1, was fed at 1 kg/h into a fluidized bed gasifier at 750 °C and 1.05 bar. The comparison between the simulated results and experimental data, as seen in Figure 2, indicates that the model underpredicts the experimental values for CO and CH₄ and overpredicts the value of H₂ and CO_2, but the trend of changing the compositions with the steam-to-biomass ratio agrees with the experimental data. Minutillo et al. [28] experimented with a downdraft gasifier at a temperature of 900 °C and an air-to-pellet pine wood ratio of 1.96. The result, as shown in Figure 2, indicated that the model is in good agreement with the experimental data. However, the amount of CH₄ obtained from the experiment is higher than that from the simulation. This is due to the fact that the calculation is performed based on the equilibrium while the experimental test may not be progressed to chemical equilibrium. Nevertheless, the obtained results are consistent with the literature [13]. Finally, the simulation result is validated for the coal gasification process with experimental data extracted from Mota et al. [29]. In their experiment, the bubbling fluidized bed reactor was operated at a temperature of 750 °C with an oxygen carrier-to-carbon ratio of 1. Figure 2 presents the model prediction of the simulation result and experimental result. It is found that the syngas compositions obtained from the model agree with experimental data.

For the CO₂ capture process, the simulated results are compared with the experimental results of Atsonios et al. [30]. In their experiment, the CO₂ adsorption and desorption were operated at temperatures of 600–700 °C in a 10 kW dual fluidized bed (DFB) reactor while that of the regenerator temperature is above 850 °C. The comparison between the simulated results and experimental results, as shown in

Table 3. Comparison of the CO₂ mole fraction exited from the carbonator reactor and CO₂ capture efficiency from the model prediction and experimental data of Atsonios et al. [30].

	This Study	Atsonios et al. [30]
Parameters
Wet gas composition (mole fraction) CO2, out	0.00456	0.018
CO2 capture efficiency	88.06	85.57

, reveals that the model prediction is in good agreement with experimental data from Atsonios et al. [30]. The CO₂ mole fraction value predicted and the CO₂ capture efficiency predicted are matched with the experiments.

5. Result and Discussion

The results of this study can be divided into four parts. First, the sensitivity analysis of the operating parameters in the gasifier (i.e., gasifier temperature and S/F mass ratio) and adsorber (i.e., CaO/F mass ratio, carbonator temperature, and regenerator temperature) on hydrogen production are investigated. In the second part, the DOE method is applied to identify the optimal operating condition of both processes that can provide the maximum H₂ amount; the response parameters considered are the H₂ mole fraction. Next, the energy analysis is performed to determine the useful energy obtained from this process. Lastly, the environmental assessment is studied to predict the carbon dioxide emission.

5.1. Sensitivity Analysis

5.1.1. Effect of Gasification Temperature on H₂ Production

Figure 3 shows the effect of gasification temperature on compositions of H₂, CO, CO_2, and CH₄ (dry basis). The gasification temperature is varied in a range of 500–1000 °C, whereas the S/F mass ratio of 1 and pressure of 1 atm are fixed as a constant value. It is noted that the mass flow rates of biomass and coal are equal to 1000 kg/h. As shown in Figure 3, the mole fraction of H₂ increases when the temperature increases from 500 °C to 700 °C. However, the mole fraction of H₂ will be stable at a gasifier temperature above 700 °C because the chemical equilibrium is limited at this temperature. Unlike H₂, the mole fraction of CO₂ and CH₄ decrease with increasing gasification temperature. This is because the Boudouard (Equation (3)), water gas (Equation (4)), and steam methane reforming (Equation (7)) as the endothermic reaction can shift forward to the product side when the gasifier is operated at a high temperature. It is noted that the mole of CH₄ at 700 °C is equal to zero since CH₄ is completely consumed at this temperature. Thus, CO, CO₂, and CH₄ can convert to more H₂ under a high-temperature operation of a gasifier. From the simulation result, it is found that the optimal gasifier temperature is 700 °C.

5.1.2. Effect of S/F Mass Ratio on H₂ Production

Figure 4 demonstrates the effect of the S/F mass ratio on the production of H₂, CO, CO₂, and CH₄ (dry basis). The gasifier is operated at a temperature of 700 °C and pressure of 1 atm, whereas the S/F mass ratio increases from 0 to 10. The simulation results indicate that the H₂ mole fraction increases tremendously with the increasing S/F mass ratio in a range of 0–2 and reaches a constant value once the S/F mass ratio is above 2. Increasing the S/F mass ratio means that more steam is fed into the gasifier. This result occurs because more steam feeding into the gasifier can shift reactions (Equations (4) and (7)) forward. In the same manner, higher steam in the process can shift the water gas shift reaction, where CO is converted into H₂ and CO₂. Therefore, Figure 4 shows the reduction in CO and the increase in CO₂ with an increase in the S/F mass ratio. From the simulation result, it can be observed that the CH₄ mole fraction becomes zero at an S/F mass ratio above 1. This is due to the fact that CH₄ is completely consumed at this ratio. In general, water is typically reactive at temperatures greater than 800 °C and thus, the effect of the S/F mass ratio on hydrogen production is further investigated at a temperature above 800 °C. However, the simulation results show that the mole fraction of H₂ is slightly decreased when the gasifier increases. This can be confirmed considering that the chemical equilibrium is limited at 700 °C.

5.1.3. Effect of CaO/F Mass Ratio on H₂ Production

In this section, the influence of the CaO/F mass ratio varied from 0 to 6 on H₂ production is studied, as shown in Figure 5. When the carbonator temperature is at 450 °C, increasing the CaO/F mass ratio from 0 to 2 can strongly enhance the H₂ concentration in the gas product since CO₂ is suddenly removed from the syngas. When the CaO/F ratio is higher than 2, the H₂ amount drops slightly and reaches a constant value. This is because a CaO/F value of more than 2 may be excess from the stoichiometric value. Unlike H₂, an increase in the CaO/F mass ratio causes significant decreases in the amount of CO, CO₂, and CH₄. This is because CO₂ is adsorbed on CaO as an adsorbent. Therefore, the H₂ content is higher while the amount of CO₂ drops through the carbonation reaction. When CO₂ is removed from the system, the amount of CO and CH₄ decrease due to the shifting of the water gas shift reaction to the product side. Since a CaO/F mass ratio of more than 2 does not affect H₂ production, the suitable CaO/F mass ratio is 2.

5.1.4. Effect of Carbonator Temperature on H₂ Production

The impact of the carbonator temperature increasing from 400 °C to 800 °C on H₂ production is also investigated, as illustrated in Figure 6. Because a wide range of carbonator temperatures is investigated, the presence of two plateaus can be observed. The first plateau can be observed when the carbonator operates at a low temperature (400–650 °C). The H₂ mole fraction can be improved while CO₂ is not present since the carbonator reaction as an exothermic reaction is favorable to low-temperature operation. Under this low-temperature range, all of the CO₂ can be removed from the syngas and thus, a stable amount of H₂ can be provided. In contrast, the H₂ mole fraction will drop and the presence of CO and CO₂ can be observed when the carbonator is operated at a high temperature (above 650 °C). Increasing the carbonator temperature can shift the chemical equilibrium backward and thus, CO₂ is desorbed from CaCO₃. In addition, the reversed water gas shift reaction can occur at high carbonator temperatures, which reduces the CO₂ and H₂ amounts. However, it can be observed that the amount of H₂ is almost constant when the temperature is higher than 800 °C (second plateau). This implies that the chemical equilibrium of reactions is limited at this temperature.

5.1.5. Effect of Regenerator Temperature on CO₂ Removal

Figure 7 presents the variation in the CO₂ amount when the regenerator temperature is adjusted from 400 °C to 1400 °C, whereas the CaO/F mass ratio and carbonator temperature are set as 2 and 600 °C, respectively. The simulation result indicates that the regeneration process begins to desorb CO₂ at a temperature of 900–1000 °C. This can be observed from the production of CO₂ at this temperature. However, when the temperature is higher than 950 °C, the CO₂ concentration will be constant. From this study, the optimal regenerator temperature is 950 °C.

5.2. Design of Experimental Procedure

Since the sensitivity analysis can indicate the trend of results and some operating conditions must be carefully selected, the optimal operating conditions in the gasification and calcium looping process are determined by using the DOE method. From the results obtained in Section 5.1, it can be seen that both the gasifier temperature and S/F mass ratio have significant influences on H₂ production. In addition, the proportion of coal in the biomass, or the coal-to-biomass (C/B) mass ratio, is also an important parameter. Therefore, the selected parameters in the co-gasification of biomass and coal consist of (A) the gasifier temperature, (B) the S/F mass ratio, and (C) the C/B mass ratio. In this way, the full factorial design of three factors with two levels is determined; thus, there are eight numbers in the experiment. The response results from the 2³ factorial experimental design analyses with a single replicate are shown in

Table 4. The response results from the 2³ factorial experimental design analysis of the co-gasification of biomass and coal with Aspen Plus simulator version 10.

Run	Gasification Temperature (°C)	S/F Mass Ratio	C/B Mass Ratio	H2 Content (%vol., Dry Basis)
1	700	1	0.5:0.5	61.12
2	700	1	0.75:0.25	61.96
3	700	2	0.5:0.5	64.09
4	750	1	0.75:0.25	61.81
5	750	2	0.75:0.25	64.68
6	750	1	0.5:0.5	61.04
7	700	2	0.75:0.25	65.09
8	750	2	0.5:0.5	64.09

Code variable: −1 (Low) Gasifier temperature = 700 °C, S/F mass ratio = 1, and C/B mass ratio = 0.5:0.5. +1 (High) Gasifier temperature = 750 °C, S/F mass ratio = 2, and C/B mass ratio = 0.75:0.25.

. The ANOVA statistical analysis of the results obtained with a confidence level of 95% or a p-value equal to 0.05 is summarized in

Table 5. Analysis of variance (ANOVA) for the regression model of H₂ content (%vol., dry basis).

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	19.34	2	9.67	494.63	<0.0001	significant
B-S/F	18.05	1	18.05	923.32	<0.0001
C-C/B	1.29	1	1.29	65.95	0.0005
Residual	0.0978	5	0.0196
Cor Total	19.44	7
Std. Dev.	0.1398		R²	0.995
Mean	62.98		Adjusted R²	0.993
C.V. %	0.222		Predicted R²	0.987
			Adeq Precision	44.464

.

The selected process parameters for the calcium looping carbon dioxide capture process are (D) carbonator temperature and (E) CaO/F mass ratio. The full factorial design of two factors with two levels (2²) will provide four numbers in the experiment with a single replicate which are shown in

Table 6. The response results from the 2² factorial experimental design analysis of the carbon dioxide capture process with Aspen Plus V10.

Run	Carbonator Temperature (°C)	CaO/F Mass Ratio	CO2 Capture (%)	H2 Content (%vol., Dry Basis)
1	550	1	79.87	98.47
2	450	3	99.99	99.59
3	550	3	99.65	99.53
4	450	1	78.42	99.01

Code variable: −1 (Low) Carbonator temperature = 450 °C, CaO/F mass ratio = 1. +1 (High) Carbonator temperature = 550 °C, CaO/F mass ratio = 3.

. The ANOVA statistical analysis of the results obtained with a confidence level of 95% is summarized in

Table 7. ANOVA for the regression model of % CO₂ capture in the calcium looping process.

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	427.22	1	427.22	769.02	0.0013	significant
E-CaO/F	427.22	1	427.22	769.02	0.0013
Residual	1.11	2	0.555
Cor Total	3151.42	3
Std. Dev.	0.745		R²	0.997
Mean	89.49		Adjusted R²	0.996
C.V. %	0.832		Predicted R²	0.9896
			Adeq Precision	39.217

.

5.2.1. Effect of Process Parameters on the Gasification Process

From the ANOVA as shown in Table 5, it is found that the p-value for the (B) S/F mass ratio and (C) C/B mass ratio are significant variables because the p-value is less than 0.05. The response surface shown in Figure 8 reveals that increasing the S/F mass ratio from 1 to 2 has a positive effect on the %vol. of H₂. This is because the steam in the feed can shift the chemical equilibrium of the water gas (Equation (4)) and water gas shift (Equation (5)) reactions. In addition, the result shows that increasing the C/B mass ratio can increase the H₂ composition since the biomass has a fixed carbon content less than coal. Therefore, a high biomass content can reduce CO production [31,32].

5.2.2. Effect of Process Parameters on the CO₂ Capture Process

In Table 7, which presents the results from the ANOVA method, it is found that only the CaO/F mass ratio influences the %CO₂ capture. The effect of the CaO/F mass ratio on the %CO₂ capture, as illustrated in Figure 9, indicates that increasing the CaO/F mass ratio from 1 to 3 can significantly improve the %CO₂ capture from ~78% to ~100%.

5.3. The Energy Efficiency of Co-Gasification of Biomass and Coal Integrated with Calcium Looping Carbon Dioxide Capture Process

From the study on the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process in Section 5.1 and Section 5.2, it is found that the optimal operating temperature of the gasifier is at 700 °C with an S/F mass ratio of 2 and C/B ratio of 0.75:0.25 while the carbon dioxide capture process should be operated at a carbonator temperature of 450 °C and a regenerator temperature of 950 °C with a CaO/F mass ratio of 3. Under these operating conditions, a CO₂ capture of 99.99%, a maximum H₂ of 99.59%vol., and a hydrogen yield of 92.38 g hydrogen/kg biomass feeding can occur. Further, an energy analysis is performed, and the heat duty of each unit in the integrated process is shown in

Table 8. The heat duty of each unit in the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process.

Heat Duty of Each Unit (kW)	Values
Gasifier unit	3332.32
Regenerator unit	3207.00
Carbonator unit	−3023.59

. The simulation result reveals that the energy efficiency of H₂ production through the co-gasification process is 42%.

5.4. The Environmental Impact Assessment

In this section, the environmental impact in terms of the CO₂ equivalent emission and specific carbon dioxide emission are investigated. It should be noted that the carbon emission produced by supplying the necessary heat to the process was not taken into account in this work. Four cases of the chemical looping process operating at different conditions of carbonator temperature and CaO/F mass ratio are considered. The environmental impact assessment is performed to identify the optimal operating conditions that provide the least amount of CO₂ emission. It is noted that all cases are operated at the gasifier temperature of 700 °C, S/F mass ratio of 2, C/B mass ratio of 0.75:0.25, and regenerator temperature of 950 °C. Differences in the carbonator temperature and CaO/F mass ratio influence not only the CO₂ emissions but also H₂ production, CO₂ capture efficiency, and energy efficiency.

Table 9. Results of the performance analysis and environmental assessment with different conditions of carbonator temperatures and CaO/F mass ratios.

Parameters	Case No.
	1	2	3	4
CaO/F mass ratio (−)	1	1	3	3
Carbonator temperature (°C)	450	550	450	550
Hydrogen production (kg/h)	183.82	182.92	184.75	184.72
Hydrogen content (%vol., dry basis)	99.01	98.47	99.59	99.53
% CO2 capture (%)	78.42	79.87	99.99	99.65
Energy efficiency of hydrogen production (%)	42.64	42.43	42.86	42.85
Total mass flow rate of CO2 equivalent emission (kg/h)	1788.15	1768.59	1808.62	1807.86
Specific emission of released CO2 (g CO2/MJ)	80.26	79.77	80.77	80.75

shows the results of the performance analysis and environmental assessment at different carbonator temperatures and CaO/F mass ratios. The results show that the mass flow rate of CO₂ equivalent emissions from the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process is around 1788–1808 kg/h and that the specific emission of released CO₂ to the atmosphere is 79.77–80.77 g CO₂/MJ. However, it should be noted that this process utilizes a high coal-to-biomass ratio and thus, the CO₂ emission will likely increase compared to conventional biomass-based processes. Although the chemical looping process is operated at different CaO/F mass ratios and carbonation temperatures, the mass flow rate of CO₂ equivalent emissions and specific emission of released CO₂ to the atmosphere are not different. From the environmental point of view, the operation of a chemical looping process at a CaO/F mass ratio of 1 and carbonation temperature of 450 °C is the most suitable in terms of the high amount of hydrogen and high energy efficiency with a low amount of CO₂ emission.

6. Conclusions

This work proposed the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process to produce purified H₂. The performance of the hydrogen production process is studied through the Aspen Plus process simulator. Further, the statistical method is used to determine the optimal operating conditions of the co-gasification and the calcium looping carbon dioxide capture process and the effects of the gasifier temperature, S/F mass ratio, C/B mass ratio, CaO/F mass ratio, carbonator temperature, and regenerator temperature on H₂ production are investigated.

The sensitivity analysis found that the H₂ concentration is improved when the gasifier temperature, S/F mass ratio, CaO/F mass ratio, and regenerator temperature increase. However, the results obtained from the sensitivity analysis cannot indicate the optimal operating condition of this process. Therefore, optimization is further performed through the DOE method. The ANOVA results revealed that the maximum H₂ content of 99.59%vol. can be obtained when co-gasification is operated at a gasifier temperature of 700 °C with an S/F mass ratio of 2 and a C/B mass ratio of 0.75:0.25, while the CO₂ capture process should be operated at a carbonator temperature of 450 °C and a regenerator temperature of 950 °C with a CaO/F mass ratio of 3. The results of the sensitivity analysis and optimization revealed that a yield of 92.38 g hydrogen/kg biomass feeding of hydrogen gas and CO₂ capture efficiency of 99.99% were obtained. The energy analysis indicated that the energy efficiency of the co-gasification process is 42.86% and the specific emission of released CO₂ is 80.77 g CO₂/MJ.