New Process Combining Fe-Based Chemical Looping and Biomass Pyrolysis for Cogeneration of Hydrogen, Biochar, Bio-Oil and Electricity with In-Suit CO₂ Separation

Abstract

Fe-based chemical looping gasification is a clean biomass technology, which has the advantage of reducing CO₂ emissions and the potential of self-sustaining operation without supplemental heating. A novel process combining Fe-based chemical looping and biomass pyrolysis was proposed and simulated using Aspen Plus. The biomass was first subjected to pyrolysis to coproduce biochar, bio-oil and pyrolysis gas; the pyrolysis gas was subjected to an Fe looping process to obtain high-purity hydrogen and carbon dioxide. The influences of the pyrolysis reactor operating temperature and fuel reactor operation temperature, and the steam reactor and air reactor on the process performance are researched. The results showed that, under the operating condition of the established process, 23.07 kg/h of bio-oil, 24.18 kg/h of biochar, 3.35 kg/h of hydrogen and a net electricity of 3 kW can be generated from 100 kg/h of rice straw, and the outlet CO₂ concentration of the fuel reactor was as high as 80%. Moreover, the whole exergy efficiency and total exergy loss of the proposed process was 58.98% and 221 kW, respectively. Additionally, compared to biomass direct chemical looping hydrogen generation technology, the new process in this paper, using biomass pyrolysis gas as a reactant in the chemical looping hydrogen generation process, can enhance the efficiency of hydrogen generation.

1. Introduction

Hydrogen is regarded as one of the most promising energy sources due to its high energy density and cleanness [1,2]. Nevertheless, the H₂ molecule does not exist in nature, and most of the hydrogen exists in water [3,4]. Nowadays, natural gas steam reforming [5] is the most extensive hydrogen production method, which accounts for 48% of the world’s hydrogen production [6,7]. Nevertheless, natural gas steam reforming needs H₂ separation and purification as well as the production of a large amount of carbon dioxide (CO2) [8]. Accordingly, it is necessary to develop alternative and renewable sources to produce H₂ [9].

Biomass is considered to be a promising renewable source with carbon neutrality to produce H₂. From the view of thermodynamics, steam gasification of biomass to generate 53–55 vol% of H₂ should be the most popular method for H₂ production [6,10], while only 8–10 vol% of H₂ can be generated by air gasification of biomass [11]. Nevertheless, steam gasification of biomass requires heat because it is an endothermic reaction, whereas air gasification of biomass is an exothermic process due to the partial combustion of biomass with oxygen. Therefore, the mixing of steam with air in biomass hydrogen production has been extensively developed; however, its H₂ content is only 25–30 vol% [12]. Thus, an H₂ purification and CO₂ separation system are also needed.

As shown in Figure 1, biomass direct chemical looping is a novel H₂ generation method; that is, the solid biomass is directly mixed with the oxygen carrier. High-purity H₂ and sequestration-ready CO₂ can be generated by biomass direct chemical looping without purification technology, which can theoretically reduce energy consumption [13].

However, there are some disadvantages in biomass direct chemical looping for hydrogen generation technology. Firstly, the reaction rate of solid biomass with solid oxygen carriers is low due to the low efficiency of solid–solid contact [14]. Secondly, it is hard to separate oxygen carriers from biomass ash and/or biochar [15]. Third, bio-oil has certain value: directing bio-oil to reduce oxygen carrying will reduce the economy of the chemical looping system. More importantly, in the fuel reactor, solid biomass, small molecular gas products (CO, CH₄), biochar, bio-oil and oxygen carrier reactions will interact with each other. For example, biochar will hinder the reduction of pyrolysis gas to oxygen carrier, and tar will affect the pore structure of biochar [16,17]. If a chemical looping is coupled to a polygeneration system, biochar, bio-oil, pure hydrogen and sequestration-ready CO₂ can be obtained simultaneously. Additionally, the efficiency of the whole biomass conversion process can be increased.

Therefore, it will be interesting to study the combination of chemical looping and polygeneration concepts for biomass to H₂ systems. Nguyen et al. [18] proposed novel polygeneration concepts to cogenerate H₂, biochar, CH₄, bio-oil and methanol from biomass hydropyrolysis and hydrodeoxygenation integrated with water electrolysis. The results demonstrated that the promising processes are those with H₂ generation, while the risky systems are those with water electrolysis. Situmorang et al. [19] presented a new process of H₂ generation by integrating steam bio-oil reforming as well as a biochar chemical looping hydrogen production system. The results showed that the proposed system can enhance H₂ generation efficiency by more than 50% compared with the biomass direct chemical looping hydrogen generation process.

However, to the best of the authors’ knowledge, the polygeneration system for H₂, biochar, bio-oil and electricity generation and coproduction by biomass pyrolysis coupled with a chemical looping hydrogen generation process using biomass pyrolysis gas as a reactant has not been reported.

In this paper, a new process integrating biomass pyrolysis and chemical looping H₂ generation using pyrolysis gas from the pyrolysis of biomass as the reactant is designed and simulated to produce H₂, bio-oil, biochar and electricity. According to the energy and mass balances of the whole process, the exergy efficiency and exergy loss, and the influences of the pyrolysis reactor operating temperature and operation temperatures at the fuel reactor, steam reactor and air reactor on the process performance are studied. It is expected to achieve a novel process to effectively produce H₂, bio-oil and biochar from biomass.

2. Results and Discussion

2.1. Material and Energy Calculation of the Proposed System

Based on the simulation results, the whole process can produce 23.07 kg/h of bio-oil (

Table 1. Simulation results for the components of the bio-oil.

Components	Value (wt%)
Water	28.11
Hydroxyacetaldehyde	25.32
Hydroxypropanone	14.12
Propionic acid	2.63
Isoeugenol	9.86
Acetic acid	15.37
Phenol	1.52
Syringol	0.75
(5H)-furan-2-one	2.32

), 24.18 kg/h of biochar, 52.75 kg/h of pyrolysis gas, 3.35 kg/h of hydrogen and a net electricity of 3 kW per 100 kg/h of rice straw. A total of 100 kg/h of Fe₂O₃/Al₂O₃ oxygen carrier, which contains 45% excess as heat carrier, is circulated to completely convert pyrolysis gas in the chemical looping hydrogen generation unit.

The steam reactor needed 45 kg/h of water, and about 36.65 kg/h of water was recycled back into the steam reactor, which can be gained from the flash drum. Thus, about 8.35 kg/h of make-up water is needed from outside.

Table 2. Stream and electricity results of the proposed process.

Stream	Component	Mass Flow (kg/h)	P (MPa)	T (°C)
1	Rice straw	100	0.1	25
2	Crushed rice straw	100	0.1	25
3	Biochar, bio-oil and pyrolysis gas	100	0.1	600
4	Biochar	24.18	0.1	600
5	Bio-oil and pyrolysis gas	75.82	0.1	600
6	Bio-oil	23.07	0.1	600
7	Pyrolysis gas	52.75	1	600
8	Fe/FeO	90	1	900
9	Air	120	0.1	25
10	Compressed air	120	1.2	200
11	Compressed air	80	1.2	200
12	Compressed air	40	1.2	200
13	Fe3O4	95	1	850
14	Depleted air	75	1.1	850
15	Depleted air	115	0.1	350
16	Steam	45	1	250
17	Depleted air	115	0.1	50
18	Steam + H2	40	1	850
19	Hot water + H2	40	0.1	400
20	Cooled water + H2	40	0.1	60
21	H2	3.35	0.1	25
22	Recycled water	36.65	0.1	25
23	Make-up water	8.35	0.1	25
24	Total water	45	1	25
25	Hot waste gas	62.75	1	850
26	Cooled waste gas	62.75	0.1	350
27	Waste gas	62.75	0.1	50
28	Fe2O3	100	1	850

demonstrates the stream results of the proposed system shown in Figure 2 and

Table 3. Electricity results of the proposed process.

Equipment	Value (kW)
Crusher	−25
Pyrolysis reactor	−20
Pump	−1
Compressor-1	−8
Compressor-2	−7
Expander-1	15
Expander-2	29
Expander-3	20
Net electricity	3

shows the electricity results of the proposed system.

According to Equations (4)–(6), the efficiency of H₂ generation, efficiency of bio-oil generation and efficiency of biochar generation are 6.35%, 23.07% and 24.18%, respectively. The efficiency of hydrogen generation from coal can reach 15% [20]; in contrast, that for biomass can only approach about 5% [14]. Nevertheless, although the efficiency of hydrogen generation for pyrolysis gas is lower than coal, using pyrolysis gas as a reductant in the chemical looping hydrogen generation process can enhance the efficiency of hydrogen generation from biomass.

2.2. Exergy Flowchart

The exergy flowsheet of the proposed system is demonstrated in Figure 2. It can be seen that the overall exergy efficiency of the proposed process is as high as 58.98%. The exergy loss of the whole system is 221 kW. Additionally, the chemical reactions’ irreversibility is the main reason for the losses of exergy for the pyrolysis reactor, fuel reactor, steam reactor and air reactor. The compression and expansion processes’ irreversibility is the primary reason for the exergy losses of compressor-1, compressor-2, the pump, expander-1, expander-2 and expander-3. Heat loss is the main reason for the exergy loss of the heat recovery steam generator, separator, flash drum-1 and flash drum-2.

In order to improve the proposed system’s exergy efficiency, the fuel reactor, air reactor, steam reactor, heat recovery steam generator and compressor-1 are preferred to be improved. Improving the chemical reaction efficiency is necessary to reduce the loss of exergy for the fuel reactor, air reactor and steam reactor such as improving the performance of the oxygen carrier and enhancing the mass and heat transfer efficiency in the above reactor. Enhancing the mechanical efficiency of the compressor can increase exergy efficiency. Additionally, the heat recovery steam generator’s exergy efficiency can be increased by improving the efficiency of heat utilization.

2.3. Effect of Pyrolysis Temperature

The influence of the pyrolysis reactor operating temperature on the hydrogen production yield, bio-oil production yield, biochar production yield and electricity production are shown in Figure 3. The temperatures of the fuel reactor, steam reactor and air reactor are kept at 900 °C, 700 °C and 980 °C, respectively. As the pyrolysis reactor operating temperature increases from 400 °C to 600 °C, the bio-oil production yield and biochar production yield decline from 40.5 kg/h and 35.9 kg/h to 15.12 kg/h and 15.8 kg/h, respectively. The reason is that bio-oil and biochar are converted to pyrolysis gas owing to the primary secondary cracking reactions [10,21].

Nevertheless, the H₂ production yield and electricity production are enhanced from 2.6 kg/h and −5 kW to 4.1 kg/h and 10 kW, respectively. The reason is that elevating the amount of pyrolysis gas means more Fe₂O₃ is converted into FeO and Fe, which react with steam to produce hydrogen. At the same time, more related gas (waste gas from the fuel reactor, H₂/steam from the steam reactor) is generated, which affects the work of the expanders.

2.4. Effect of Fuel Reactor Operation Temperature

For the chemical looping process, the fuel reactor operation temperature is usually from 750 °C to 950 °C to convert Fe₂O₃ into FeO/Fe. The influence of the fuel reactor operation temperature on the hydrogen production yield and electricity production is demonstrated in Figure 4. The temperatures of the pyrolysis reactor, steam reactor and air reactor are maintained at 500 °C, 700 °C and 980 °C, respectively.

As the fuel reactor operating temperature increases from 700 °C to 1000 °C, the H₂ production yield and electricity production increase from 2.12 kg/h and 6 kW to 4.43 kg/h and 10 kW, respectively. The pyrolysis gas reaction with Fe₂O₃ is an endothermic process, which means more FeO is produced with the increase in the fuel reactor temperature; thus, more H₂ is produced in the steam reactor. Additionally, more waste gas (CO₂ and steam) is produced in the fuel reactor, which results in the increased electricity production of expander-1. However, a higher fuel reactor operation temperature could lead to insufficient air reactor heat [22].

Table 4. Gas products and oxygen carrier results of the proposed process.

Item	Fuel Reactor	Steam Reactor	Air Reactor
	kmol/h
H2	0.0415994	0.0149086	0
CO	0.284967	0	0
CH4	2.29202 × 10−6	0	0
CO2	1.15597	0	0
Fe	0	0	0
FeO	1.25241	1.22259	0
Fe2O3	0	0.0149086	0.626205
O2	2.29196 × 10−16	1.00022 × 10−17	0.276666
N2	0	0	2.19061
H2O	0.212528	2.48297	0

shows the outlet gas composition of the fuel reactor at 900 °C. It can be seen that the CO₂ concentration of the dry base is about 80%, indicating that the biomass pyrolysis gas in the fuel reactor has been basically oxidized. Therefore, 900 °C is suitable for the fuel reactor.

2.5. Effect of Steam Reactor Temperature

In the steam reactor, H₂ is generated from steam via the oxidation of FeO/Fe to Fe₃O₄. The influence of the steam reactor operating temperature on the H₂ production yield and electricity production is demonstrated in Figure 5. The temperatures of the pyrolysis reactor, fuel reactor and air reactor are maintained at 500 °C, 900 °C and 980 °C, respectively. As the steam reactor operating temperature increases from 600 °C to 900 °C, the hydrogen production yield decreases from 2.7 kg/h to 4.2 kg/h, whereas the electricity production increases from −3 kW to 13 kW.

Steam with FeO/Fe are exothermic reactions with the equilibrium obtained at low temperatures. The lower the steam reactor operating temperature is, the higher the hydrogen production yield becomes. Nevertheless, the heat is not to support the chemical looping hydrogen generation process auto-thermally at low temperatures. As the steam reactor operating temperature goes up, the conversion of steam decreases, resulting in an increase in expander-2 electricity generation.

2.6. Effect of Air Reactor Operating Temperature

The operating temperature of the air reactor is a crucial parameter for hydrogen and electricity production. The air reactor operating temperature can affect the fuel reactor temperature due to the energy balance for the chemical looping process. On the other hand, the air reactor operating temperature is limited by the oxygen carrier temperature’s endurance. The influence of the air reactor operating temperature on hydrogen and electricity is shown in Figure 6.

The temperatures of the pyrolysis reactor, fuel reactor and steam reactor are kept at 500 °C, 900 °C and 700 °C, respectively. As the air reactor operating temperature increases from 780 °C to 1080 °C, the hydrogen production yield enhances from 2.26 kg/h to 4.13 kg/h, whereas the electricity production drops from 9–4 kW. The reason for the decrease in net electricity is that the air flow declines with the rise in the air reactor operating temperature, leading to the temperature declines of the expander-3 inlet stream.

The increase in the air reactor operation temperature leads to the increase in the Fe₂O₃ temperature, and more FeO is generated in the fuel reactor, which results in the increase in H₂ production in the steam reactor. Nevertheless, the high operating temperature of the reactors can cause the sintering of the oxygen carrier [23]. Therefore, the air reactor operation temperature should be less than 1000 °C.

Table 4 shows the gas composition and oxygen carrier results based on Figure 4, Figure 5 and Figure 6. As expected, under optimal conditions, the dry base CO₂ concentration at the outlet of the fuel reactor is about 80%. All the Fe is oxidized by steam into FeO in the steam reactor, and the product is pure H₂.

3. Methods and Materials

3.1. Materials

China has abundant straw production and the price of straw is low. Therefore, rice straw is selected as the raw material and its properties are listed in

Table 5. Properties of rice straw [24].

Proximate analysis (wad/%)
Ash	9.22
Volatile Matter	69.16
Fixed Carbon	14.86
Moisture	6.76
Ultimate analysis (wdaf/%)
C	44.50
O	45.66
H	6.24
N	1.45
S	2.15

.

3.2. Model Description

The new process for hydrogen, bio-oil and biochar from biomass was designed in Aspen Plus V9. To simulate the process, all of the reactions are assumed to be in equilibrium and the system is assumed to be in a steady state. As shown in Figure 7, the cogeneration system primarily consists of two units: (i) biomass pyrolysis and (ii) chemical looping hydrogen generation.

3.2.1. Biomass Pyrolysis Unit

First of all, 100 kg/h of rice straw is transported to the crusher to obtain the required particle size. Particle size of rice straw obtained through the crusher less than 1 mm accounts for 30%. Particle size in the range of 1–2 mm accounts for 40% and that of more than 2 mm accounts for 30%. Then, the crushed rice straw is sent to the pyrolysis reactor. According to the related reference [20], the reaction temperature and pressure of the pyrolysis reactor are set to be 600 °C and 0.1 MPa, respectively. Exiting from the pyrolysis reactor, biochar is separated from the mixture of gas and bio-oil via the separator. The mixture of gas and bio-oil is then transported to the flash drum-1 and the bio-oil is obtained. The pyrolysis gas is sent to the chemical looping hydrogen generation unit.

RK-SOAVE is selected as the property method to achieve the mass and energy balances in the biomass pyrolysis unit. Rice straw, biochar and ash are regarded as nonconventional components and their enthalpy is calculated using HCOALGEN and DCOALIGT.

Bio-oil is a complex mixture of hundreds of organic compounds and the description of pyrolysis modeling for bio-oil is difficult [18]. Therefore, the modeling of bio-oil needs to be simplified. Bio-oil is simulated as a mixture of water, hydroxyacetaldehyde, hydroxypropanone, propionic acid, isoeugenol, acetic acid, phenol, syringol and (5H)-furan-2-one [21] in this paper.

Pyrolysis gas is considered to be a mixture of H₂, CO, CH₄, CO₂ and others (C₂H₆, C₂H₄, etc.). Crusher, SSplit and Flash blocks are used to simulate the crusher, separator and flash drum-1, respectively. RYield, REquil and RGibbs blocks are developed to model the pyrolysis reactor. As shown in

Table 6. The simulation and experiment results of the rice straw pyrolysis [24].

Product		Simulation (wt% of Rice Straw)	Experiment (wt% of Rice Straw)	Relative Error (%) *
Pyrolysis gas	CO	17.01	18.68	8.94
	CO2	29.51	28.84	2.32
	H2	1.53	1.73	11.56
	CH4	3.66	3.83	4.44
	Others	1.04	0.92	13.04
Bio-oil		23.07	21	9.86
Biochar		24.18	25	3.28

* Relative error = (|Experiment–Simulation|/Experiment) × 100%.

, the simulated results of biomass pyrolysis are verified by the work of Zhang [20]. Due to the difference between simulation and actual reaction process, there are relative errors between simulation results and experiment results. The relative errors between the experiment results and simulation results are small, hence the simulation results are rational.

3.2.2. Chemical Looping Hydrogen Generation Unit

The pyrolysis gas from the biomass pyrolysis unit is compressed to 1 MPa by the compressor-1 and sent to the fuel reactor (900 °C, 1 MPa) and reacts with oxygen carrier Fe₂O₃/Al₂O₃ (100 kg, mass ratio 4:6), which is chosen as oxygen carrier due to its advantages of low cost, thermodynamic property and oxygen capacity [6], generating H₂O, CO₂ and Fe/FeO.

After that, Fe/FeO is sent to the steam reactor (700 °C, 1 MPa) and partially oxidized by steam with the production of H₂ and Fe₃O₄. Then, Fe₃O₄ is transported to the air reactor (980 °C, 1 MPa) and completely oxidized to Fe₂O₃ by air. The regenerated oxygen carrier is sent to the fuel reactor and a cycle is completed.

The flue gas is utilized in expanders to generate electricity and introduced to the heat recovery steam generation. The operating parameters of the fuel reactor, steam reactor and air reactor are determined based on previous publications [19,22]. The reactions that take place in the chemical looping hydrogen generation unit can be expressed by Equations (1)–(3).

Fuel reactor C_xH_yO_z (pyrolysis gas) + Fe₂O₃ → CO₂ + H₂O + FeO/Fe

(1)

Steam reactor FeO/Fe + H₂O → Fe₃O₄ + H₂

(2)

Air reactor 4Fe₃O₄ + O₂ → 6Fe₂O₃

(3)

RK-SOAVE is developed for nonpolar or mildly polar mixtures such as hydrocarbons and light gases (e.g., carbon dioxide, hydrogen sulfide and hydrogen). Therefore, RK-SOAVE is selected as the property method in the chemical looping hydrogen generation unit to achieve the mass and energy balances. Fe₂O₃, Al₂O₃, Fe, FeO and Fe₃O₄ are assumed to be solid components. The minimum ΔT for heat exchanger is 10 °C. The isentropic efficiency and mechanical efficiency of the pressure changer (compressor, expander and pump) are 0.80 and 0.90, respectively. RGibbs block is used to model the fuel reactor, steam reactor and air reactor [23]. Flash, Pump and MHeatX blocks are developed to simulate the flash drum-2, pump and heat recovery steam generator, respectively. Compr block is used to model the compressor-1, compressor-2, expander-1, expander-2 and expander-3. The Aspen model and input data for the equipment of the proposed system are given in

Table 7. Aspen model and input data for the equipment of the proposed system.

Equipment	Aspen Model	Design Parameters
Crusher	Crusher	25 °C, 0.1 MPa
Separator	SSplit	600 °C, 0.1 MPa
Flash drum-1	Flash	600 °C, 0.1 MPa
Flash drum-2	Flash	25 °C, 0.1 MPa
Pyrolysis reactor	RYield, REquil and RGibbs	600 °C, 0.1 MPa
Fuel reactor	RGibbs	900 °C, 1 MPa
Steam reactor	RGibbs	700 °C, 1 MPa
Air reactor	RGibbs	980 °C, 1 MPa
Pump	Pump	25 °C, 1 MPa
Heat recovery steam generator	MHeatX
Compressor-1	Compr	1 MPa
Compressor-2	Compr	1.2 MPa
Expander-1, Expander-2 and Expander-3	Compr	0.1 MPa

.

3.3. Evaluation Index of the Proposed System

The performance of the proposed process is assessed by hydrogen production yield, bio-oil production yield, biochar production yield, efficiency of hydrogen generation, the efficiency of bio-oil generation, the efficiency of biochar generation as well as electricity production.

$${\mathrm{H}}_2\hspace{0.17em}\hspace{0.17em}\mathrm{generation}\hspace{0.17em}\hspace{0.17em}\mathrm{efficiency}=\frac{{\mathrm{H}}_2\hspace{0.17em}\hspace{0.17em}\mathrm{generation}\hspace{0.17em}\hspace{0.17em}\mathrm{yield}}{\mathrm{Mass}\hspace{0.17em}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\hspace{0.17em}\mathrm{pyrolysis}\hspace{0.17em}\hspace{0.17em}\mathrm{gas}}$$

(4)

$$\mathrm{Bio}-\mathrm{oil}\hspace{0.17em}\hspace{0.17em}\mathrm{generation}\hspace{0.17em}\hspace{0.17em}\mathrm{efficiency}=\frac{\mathrm{Bio}-\mathrm{oil}\hspace{0.17em}\hspace{0.17em}\mathrm{generation}\hspace{0.17em}\hspace{0.17em}\mathrm{yield}}{\mathrm{Mass}\hspace{0.17em}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\hspace{0.17em}\mathrm{biomass}}$$

(5)

$$\mathrm{Biochar}\hspace{0.17em}\hspace{0.17em}\mathrm{generation}\hspace{0.17em}\hspace{0.17em}\mathrm{efficiency}=\frac{\mathrm{Biochar}\hspace{0.17em}\hspace{0.17em}\mathrm{generation}\hspace{0.17em}\hspace{0.17em}\mathrm{yield}}{\mathrm{Mass}\hspace{0.17em}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\hspace{0.17em}\mathrm{biomass}}$$

(6)

$$\mathrm{Electricity}\hspace{0.17em}\hspace{0.17em}\mathrm{production}={\displaystyle \sum {\mathrm{E}}_{\mathrm{expander}}}-{\displaystyle \sum {\mathrm{E}}_{\mathrm{compressor}}}-{\displaystyle \sum {\mathrm{E}}_{\mathrm{pump}}}$$

(7)

where E_expander, E_pump and E_compressor are the electricity produced by the expander and the electricity consumed by the pump and compressor, respectively.

3.4. Thermodynamic Analysis

Exergy usually consists of kinetic exergy E^KN, potential exergy E^PT, physical exergy E^PH and chemical exergy E^CH [24]. The kinetic exergy and potential exergy can be neglected in a chemical industrial process [25]. Thus, exergy is defined as:

$${\mathrm{E}=\mathrm{E}}_{}^{\mathrm{CH}}{+\mathrm{E}}_{}^{\mathrm{PH}}$$

(8)

The E^CH of gas mixture can be expressed as follows [26]:

$${\mathrm{E}}_{}^{\mathrm{CH}}={\displaystyle \sum {\mathrm{x}}_{\mathrm{i}}}{\mathrm{E}}_{0,\mathrm{i}}^{\mathrm{CH}}{+\mathrm{RT}}_0{\displaystyle \sum {\mathrm{x}}_{\mathrm{i}}}{\mathrm{lnx}}_{\mathrm{i}}$$

(9)

The E^CH of nongaseous mixture can be defined as follows [27]:

$${\mathrm{E}}_{}^{\mathrm{CH}}={\displaystyle \sum {\mathrm{x}}_{\mathrm{i}}}{\mathrm{E}}_{0,\mathrm{i}}^{\mathrm{CH}}$$

(10)

where ${\mathrm{E}}_{0,\mathrm{i}}^{\mathrm{CH}}$ is the component’s standard E^CH, x_i stands for the i component’s molar fraction, T₀ is 298.15 K, R equals 8.314 kJkmol⁻¹K⁻¹ and the subscript 0 stands for 1 atm and 298.15 K (the reference state). The standard chemical exergy values are demonstrated in those references [26,27,28].

The E^PH is expressed as follows [12]:

$${\mathrm{E}}_{}^{\mathrm{PH}}{=(\mathrm{H}-\mathrm{H}}_0{)-\mathrm{T}}_0{(\mathrm{S}-\mathrm{S}}_0)$$

(11)

where H and S stand for the enthalpy and entropy at a given temperature and pressure, H₀ and S₀ are the enthalpy and entropy at 298.15 K and 1 atm. H, H₀, S and S₀ are calculated by Aspen Plus. In addition, the ash exergy and rice straw E^PH are neglected owing to minor contribution [29].

Additionally, the exergy of biomass can be expressed as:

$${\mathrm{E}}_{\mathrm{biomass}}^{}{=\mathsf{\beta }\times \mathrm{LHV}}_{\mathrm{biomass}}$$

(12)

$$\mathsf{\beta }=\frac{1.0414+0.0177[\mathrm{H}/\mathrm{C}]-0.3328[\mathrm{O}/\mathrm{C}]\{1+0.0537[\mathrm{H}/\mathrm{C}]\}}{1-0.4021[\mathrm{O}/\mathrm{C}]}$$

(13)

$${\mathrm{LHV}}_{\mathrm{biomass}}=0.0041868(1+0.15\left[\mathrm{O}\right]\left)\right(7837.667\left[\mathrm{C}\right]+33888.889\left[\mathrm{H}\right]-\left[\mathrm{O}\right]/8)$$

(14)

where C, H and O are carbon, hydrogen and oxygen elements, and are obtained from the ultimate analysis. In addition, the calculation method of exergy for biochar and bio-oil is listed in reference [29].

The exergy destruction E_D and exergy efficiency ${\mathsf{\eta }}_{\mathrm{ex}}$ can be calculated as follows:

$${\mathrm{E}}_{\mathrm{D}}={\displaystyle \sum {\mathrm{E}}_{\mathrm{in}}}-{\displaystyle \sum {\mathrm{E}}_{\mathrm{out}}}$$

(15)

$${\mathsf{\eta }}_{\mathrm{ex}}={\displaystyle \sum {\mathrm{E}}_{\mathrm{out}}}/{\displaystyle \sum {\mathrm{E}}_{\mathrm{in}}}$$

(16)

where E_in stands for the entering exergy and E_out is the outflow exergy.

4. Conclusions

A new process for biochar, bio-oil, H₂ and power cogeneration from rice straw was developed. The whole process can produce 23.07 kg/h of bio-oil, 24.18 kg/h of biochar, 52.75 kg/h of pyrolysis gas and 3.35 kg/h of H₂ per 100 kg/h of rice straw. The results showed that the process combined biomass pyrolysis with chemical looping hydrogen production, and the hydrogen production efficiency (6.35%) is higher than that of biomass direct chemical looping hydrogen production technology. The proposed process also produces positive net electricity, which is 3 kW per 100 kg/h of rice straw. Additionally, the overall exergy efficiency of the proposed process is 58.98% and the total exergy loss is 221 kW. Furthermore, an economic assessment needs to be carried out to implement this new process.