SolarâWindâBio Ecosystem for Biomass Cascade Utilization with

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A solar-wind-bio ecosystem for biomass cascade utilization with multi-generation of formic acid, hydrogen, and graphene Zhao Sun, Liang Zeng, Christopher Russell, Shiyi Chen, Lunbo Duan, Wenguo Xiang, and Jinlong Gong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05546 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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ACS Sustainable Chemistry & Engineering

A solar-wind-bio ecosystem for biomass cascade utilization with multi-generation of formic acid, hydrogen, and graphene

Zhao Sun †, ‡,1, Liang Zeng †,1, Christopher K. Russell §, Suttichai Assabumrungrat ⁄⁄, Shiyi Chen ‡, Lunbo Duan ‡, Wenguo Xiang *, ‡, Jinlong Gong *, †

†

Key Laboratory for Green Chemical Technology of Ministry of Education, School of

Chemical Engineering & Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, No. 92 Weijin Road, Tianjin 300072, China ‡

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School

of Energy and Environment, Southeast University, No. 2 Sipailou, Nanjing 210096, China §

Department of Civil and Environmental Engineering, Stanford University, 450 Serra Mall,

Stanford, CA 94305, USA ⁄⁄

Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand 1

These authors contributed equally to this work.

*Corresponding author: [email protected]; [email protected]

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ABSTRACT This paper describes the development of a 20MWth solar-wind-bio distributed energy system and its viability of achieving biomass cascade utilization, water resource conservation, waste heat recovery, and CO2 mitigation while coproducing hydrogen, formic acid, and graphene. The proposed ubiquitous energy system model is developed by using ASPEN Plus® for the operating parameter optimization and system-wide heat assessment. The system consists of (i) a biomass gasification module, (ii) a biochar utilization module, (iii) a chemical looping hydrogen generation module, (iv) an oxy-syngas combustion module, (v) a CO2 electro-reduction module, and (vi) heat recovery units. Bioenergy is progressively converted through biomass gasification, chemical looping hydrogen generation, syngas combustion, and CO2 electro-reduction. The role of solar energy and wind power is to induce the biomass gasification and CO2 electro-reduction, respectively. The system has been proved to be water saving with a water recycle efficiency of 70.1% from steam condensation and recovery from the outlets of i, iii, and iv modules. The synergistic effect of parameters such as CO2/biomass mass ratio, operation temperatures, and oxygen carrier (OC)/syngas mole ratio is optimized. Furthermore, the carbon migration pathway, water/steam consumption and conservation, energy transformation and heat supplement of the system are investigated, achieving an optimized system energy efficiency of 59.2%.

KEYWORDS: wind-solar-bio energy system; biomass cascade utilization; CO2 reduction; formic acid; chemical looping.


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INTRODUCTION Renewable energy utilization has been determined as an approach to reduce the negative impacts of fossil fuel combustion, most notably carbon dioxide emission, which has been shown to be the primary cause of climate change

1-4.

Additionally, biomass may play a vital role in the

future energy supply framework due to its abundance around the world, and the possibility of near5-6.

zero life cycle carbon emission

Biomass gasification is one of the most attractive

thermochemical processes for biofuel/chemical production

7-8.

However, many gasification

products (i.e. H2, CO, CH4, CO2, C2-C4 and bio-tar) have not been effectively implemented in power generation processes at a large scale

9-10.

The efficient conversion of biomass-derived

syngas into high-quality, clean, energy becomes one of the most crucial issues yet to be solved. Hydrogen has been conceptualized as a clean energy carrier and a promising solution for replacing conventional fuels and reducing carbon emissions 11-12. Currently, the hydrocarbon steam reforming remains the dominant technology for H2 production, where CO and CO2 are inevitably produced 13. Consequently, complicated and costly separation processes are required to eliminate COx from the product stream, typically consisting of desulphurization, hydrocarbon steam reforming, water-gas shift (WGS) reaction, and pressure swing adsorption (PSA), with an estimated cost of 2.27 $/kg and 2.08 $/kg with and without carbon capture and sequestration, respectively, by using methane as the feedstock 14-15. Moreover, the system complexity results in a high capital investment and makes the system sensitive to natural gas qualities. Thus, renewable energy-based hydrogen production is a long-term strategy as the quantity and quality of fossil fuels decline and global climate change attracts more attention 16-18. Chemical looping gasification (CLG) has been investigated as a method for efficient syngas production via biomass conversion

19-23.

In place of molecular O2, an oxygen carrier (OC),


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typically a metal oxide, is used to provide the lattice oxygen for biomass gasification in the fuel reactor (FR) 24. The reduced OC is then fed into the air reactor (AR) and oxidized by air, releasing sufficient heat to maintain the auto-thermal operation of the looping system, or with external heating and steam regeneration, can be used for H2 production (Figure 1a). It has been shown that hydrogen and biomass-derived syngas can be co-generated from the biomass gasification through the metal oxide redox cycle (Figure 1b). The selection of an oxygen carrier capable of being reduced and oxidized over a long period of time against deactivation is a key issue for the CLG process 25. Generally, the oxygen carrier cannot maintain a high activity after multiple redox cycles, primarily due to the sintering, agglomeration, as well as the poisoning of OC by bio-tar and H2S, leading to the OC pore blockage and the redox performance deactivation 26-31. Here, we propose to decouple the chemical looping gasification of biomass into biomass gasification and chemical looping hydrogen generation (CLHG). In such a scheme, harmful products of biomass gasification can be removed before transferred to the fuel reactor, extending the oxygen carrier’s lifetime (Figure 1 c). Moreover, the steam in the outlet gas can be condensed and collected between solar gasifier and chemical looping reactor, which is favorable for the deep reduction of oxygen carriers. Products from biomass gasification, specifically biochar, have also been considered to be a viable precursor for graphene production. Graphene related materials have been widely studied and used due to the remarkable electronic, thermal, and mechanical properties derived from their unique 2D structure 32-35. Primo et al. prepared the large-area and high-quality N-doped graphene from biomass wastes under the condition of 800 °C and argon atmosphere, which proves the feasibility of the graphene preparation from biochar 36. The electrochemical methods to reduce CO2 to valuable chemicals have been paid great attention. The CO2 electroreduction has the advantage of taking place under ambient pressure and



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at room temperature 37. Moreover, hydrogen is not required as the feedstock because water could provide the protons needed for reducing CO2. Under such conditions, a variety of substances such as C2H4, CH4, HCOOH, CO, and alcohols can be produced 38-39. Among the chemicals obtained through CO2 reduction, formic acid is the simplest carboxylic acid with a wide range of market in pharmaceutical synthesis, pesticide industry, and chemical production

40.

Recently, a very high

faradaic efficiency of 95% obtained using a circulation flow cell proves its feasibility to be utilized at an industrial scale 41.

Figure 1. Schematic illustration of a) chemical looping gasification of biomass; b) chemical looping gasification of biomass with syngas and hydrogen generation; c) chemical looping hydrogen generation using biomass-derived syngas as reducing agents.

In this work, we present a renewable energy supply system which achieves the cascade utilization of biomass with hydrogen, formic acid, and graphene poly-generation. A 20MWth scale solar-wind-bio distributed energy system is selected after the comprehensive consideration of the availability of feedstocks, system performance, product yields, and capital/operational cost. The system provides effective approaches for i) biomass cascade utilization with high purity hydrogen and graphene production; ii) CO2 mitigation by solar-thermal-electro reduction of CO2 with CO 6 ACS Paragon Plus Environment

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and formic acid generation; iii) water sources recycling and re-utilization; iv) and the reduction of bio-tar, NH3, as well as H2S from the outlet of solar gasifier (Figure 2). The objectives of this study are to a) investigate the biomass cascade utilization strategy; b) optimize the operating parameters; c) verify the feasibility of the whole system; and d) evaluate the system performance using ASPEN Plus.

Figure 2. Illustration of the integrated operation mode of the proposed ubiquitous energy system.

SYSTEM MODEL System descriptions. The biomass cascade and CO2 solar-thermal-electro reduction system consists of five parts: the solar gasifier (I), solar-thermal reduction furnace (II), three stage fuel reactor (III-I) and steam reactor (III-II), the oxy-syngas combustor (IV), and the electro-reduction setup (V), as shown in Figure 3. Biomass gasification and solar-thermal reduction of CO2 generate syngas and biochar occur in the solar gasifier. The produced biochar is then fed to the solar-thermal reduction furnace and used for the graphene production. Syngas and water are fed from the gasifier to the fuel and steam reactors, respectively, where the oxygen carrier is reduced by the biomassderived syngas and oxidized by steam. The outlet syngas from the fuel reactor is then combusted 7 ACS Paragon Plus Environment


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to produce heat to ensure the auto-thermal operation of the overall system and to provide CO2 for electro reduction and as a gasifying agent in the solar gasifier. Feedstock and oxygen carrier. The biomass feedstock is a beetle eroded pine wood, a solid waste common in Wyoming, USA (Table 1). Calcium ferrite (Ca2Fe2O5) was selected as the oxygen carrier for chemical looping due to the abundances of Ca and Fe, its low cost, superior performance in anti-carbon deposition, tar abatement, and redox durability

29, 42-44.

Many

researchers have reported the faradaic efficiencies more than 80% from CO2 electroreduction to formic acid 45-47. Thus, faradaic efficiencies of 80% and 20% were adopted for simulation of CO2 reduction to formic acid (HCOOH) and carbon monoxide (CO), respectively. Energy input and output. Solar, wind, and biomass-based energy powers the process. Solar energy powers graphene oxide reduction and biomass gasification, which is subsequently cascaded through the chemical looping reactor (syngas partial oxidation), syngas combustor (syngas complete oxidation) and solar gasifier/electroreduction cell (CO2 solar-thermo/electro reduction). Wind energy powers the air separation and CO2 electro-reduction units. No.1 heat recovery unit is located at the outlet of the solar gasifier cyclone which plays important roles in water recovery and bio-tar separation; No.2 heat recovery unit is at the outlet of steam reactor for heat recovery and steam recycling; and No.3 heat exchanger is a kind of coil which wraps around the outside of the system module (solar gasifier, multi-stage fuel reactor, and syngas oxy-combustor) to provide sufficient heat with simultaneously the achievement of steam condensation and water recycling. To evaluate the performances of the solar-wind assisted biomass cascade utilization and CO2 reduction system, ASPEN Plus® is applied for simulation. The reactor models RYield, RGibbs, and Rstoic were used, and the detailed flowsheet of the system is shown in Figure 4. The solar gasifier is comprised of a DECOMP reactor and GASIFIER reactor, which correspond


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to the blocks of RYield and RGibbs, respectively. The generated biochar from biomass gasification was collected through the model CYCLONE 1 (Ssplit). The reactors RGibbs are also adopted for the three-stage fuel reactor (RFUEL1, RFUEL2, and RFUEL3), steam reactor (RSTEAM), and oxy-syngas combustor (RCOMB) to calculate the chemical equilibrium and phase equilibrium, based on the minimization of Gibbs free energy of each reaction mixture. The electro-reduction setup (CELL) was simulated by using Rstoic reactor since the faradaic efficiency is known while the electrochemical reaction kinetics data are not available.

Figure 3. Schematic illustration of biomass cascade utilization and CO2 solar-thermal and electroreduction with solar-wind-bio energy supply system for formic acid, hydrogen, and graphene generation.

Table 1. Proximate and ultimate analysis of pine wood. Proximate analysis (% by weight) Fixed carbon 16.50 Volatile 78.12 9 ACS Paragon Plus Environment


Moisture 5.09 Ash 0.29 Ultimate analysis (% by weight) Carbon 50.36 Hydrogen 6.20 Nitrogen 0.33 Oxygen 43.06 Sulfur 0.05

Figure 4. ASPEN Plus® flowsheet simulation of the biomass cascade utilization and CO2 photothermal-electro reduction system. Part I: biomass gasification with CO2 solar-thermal reduction model; Part III-1: three-stage fuel reactor for oxygen carrier (OC) reduction; Part III-2: the steam reactor for reduced OC oxidation; Part IV: syngas combustor; Part V: CO2 electro-reduction setup for HCOOH production.

Simulation assumptions and illustrations. The following assumptions are considered in the modeling of the system: i) the gaseous products during biomass gasification contain H2, CH4, CO, CO2, H2O, C2H6, C3H8, NH3, and H2S; ii) H2S, NH3, and un-recycled H2O can be removed and separated from the other gases; iii) the chemical intermediates, bio-tar and other heavy hydrocarbons, are completely converted during biomass gasification; iv) biochar is composed of 10 ACS Paragon Plus Environment

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carbon and ash, and ash is considered as an inert which does not participate in the reactions; v) the graphene preparation from biochar is a refined material synthesis process, thus not included in the system simulation; vi) all the reactors involved can be operated in a steady-flow state; vii) the afterheat of the outlet gas from the solar gasifier and steam reactor is used to heat the water to the steam of 300 °C; and viii) the final temperatures of outlet gas in solar gasifier, steam reactor, and syngas oxy-combustor are cooled down to 320 °C for water preheating and then cooled down to 20 °C to facilitate the steam condensation to water. Based on the assumptions above, the reactions considered during the pine wood gasification are summarized as presented from (R1) to (R10). In addition to the above assumptions, the following reaction mechanisms are assumed in the corresponding reactors. Biomass pyrolysis:

CH1.477O0.641  H 2  CH 4  CO  CO2  H 2O  C2 H 6  C3 H 8  C4 H10

(R1)

bio-tar  bio-char  Ash Bio-tar reforming: C x H y  CO2  CO  H 2

(R2)

CO2 biochar gasification:

C  CO2 ƒ 2CO

(R3)

Steam biochar gasification:

C  H 2O  CO  H 2

(R4)

Water-gas shift:

CO  H 2O ƒ CO2  H 2

(R5)

Steam methane reforming:



CH 4  H 2O ƒ CO  3H 2

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(R6)

Dry reforming of methane:

CO2  CH 4 ƒ 2CO  2 H 2

(R7)

Methanation:

C  2 H 2 ƒ CH 4

(R8)

Hydrogen sulfide formation:

S  H2  H2S

(R9)

Ammonia formation:

N 2  3H 2  2 NH 3

(R10)

The main reactions regarding Ca-Fe based chemical looping hydrogen production are displayed from (R11) to (R17) considering that the oxygen carrier is excessive and FeO is produced: Multi-stage fuel reactor:

Ca2 Fe2O5  CO  2 FeO  2CaO  CO2

(R11)

Ca2 Fe2O5  H 2  2CaO  2 FeO  H 2O

(R12)

Ca2 Fe2O5  CH 4  2CaO  2 FeO  CO  2 H 2

(R13)

Steam reactor:

2 FeO  2CaO  H 2O  Ca2 Fe2O5  H 2

(R14)

Oxy-syngas combustor:

2 H 2  O2  2 H 2O

(R15)

2CO  O2  2CO2

(R16)


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CH 4  2O2  CO2  2 H 2O

(R17)

Electro-reduction setup: Cathode : CO2  2 H   2e-  HCOOH

(R18)

Anode : 4OH -  4e-  2 H 2O  O2

(R19)

Cathode : CO2  2 H   2e   CO  H 2O

(R20)

Anode : 4OH   4e   2 H 2O  O2

(R21)

Data analysis. The carbon dioxide to biomass mass ratio, carbon fixation efficiency, CO2 solar-thermal reduction efficiency, system CO2 reduction efficiency, H atom utilization efficiency, water net consumption rate, water recycle efficiency, and system efficiency were calculated according to the following equations. Carbon dioxide to biomass mass ratio:

RCO2 to Biomass 

(E1)

Inlet mass flow of CO2 Inlet mass flow of biomass

Carbon fixation efficiency:

carbon 

(E2)

Moles of carbon in biochar 100% Moles of carbon in biomass

CO2 solar-thermal reduction efficiency:

CO ，solar reduction  2

Inlet CO2 flow rate - outlet CO2 production rate Inlet CO2 flow rate

(E3)

System CO2 reduction efficiency:

CO ，reduction  2

Moles of CO2 ( solar thermal  electro) reduction 100% Total CO2 production form syngas combustor

H atom utilization efficiency:


(E4)


H  2

Moles of H in generted H 2 100% Moles of H in H 2O fed int o the steam reactor

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(E5)

Water net consumption rate: Cnet , water  Csteam  Celectro - Rsolar - R fuel - Rsteam - Rsyngas

(E6)

where Cnet , water is the net water consumption rate of the whole system, C steam is the steam consumption rate in the steam reactor, Celectro represents the water consumption rate in the electroreduction cell, R solar , R fuel , R steam , and R syngas denote the water recycle rate in the solar gasifier, fuel reactor, steam reactor, and oxy-syngas combustor, respectively. Water recycle efficiency of the whole system:

 water 

Water recycle rate of the whole system 100% Water  steam consumption rate of the whole system

(E7)

Single day system efficiency:

 system 

mH 2  LHVH 2  mHCOOH  LHVH 2  mC  LHVC mbiomass  LHVbiomass  Esolar  t  Pwind

100%

(E8)

where mi and LHVi represents the molar flow rate (in moles/day) and lower heating value of species ‘i’, E solar and Pwind are the is the solar and wind power supply, respectively, and t is the radiation time, calculated as 8 hours per day.

RESULTS AND DISCUSSION Solar gasification. The simulation results under the conditions of biomass flow rate at 1kg/s, CO2/biomass mass ratio at 0.5, and temperature at 800 °C indicate that the main components of the flue gases from biomass gasification are H2, CO, CO2, and H2O at production rates of 103.6 kmol/h, 150.3 kmol/h, 12.7 kmol/h, and 8.0 kmol/h, corresponding to the mole fraction of 37.43 14 ACS Paragon Plus Environment

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vol.%, 54.33 vol.%, 4.58 vol.%, and 2.90 vol.%, respectively. Besides, the outlet gases from the solar gasifier include 0.59 vol.% CH4, 0.15 vol.% N2, as well as a trace amount of NH3 and H2S. It is supposed that the produced NH3 and H2S can be removed and cleaned up before sending the gases to the chemical looping reactor. The effect of temperature and CO2/biomass mass ratio on syngas production characteristics for the gasification of pine wood is presented in Figure 5a. It can be noted that the CO2/ flow rate shows a significant effect on the gas production rate and the gas composition. Solar-thermal reduction efficiency was optimized by adjusting the feed mass ratio. Increasing the CO2/biomass ratio unsurprisingly leads to a higher outlet concentration of CO2, but also leads to a significantly increased rate in CO production due to greater biochar oxidation. The CO2 solar-thermal reduction efficiencies of 55.7%, 68.8%, 72.9%, and 66.2% are achieved with CO2/biomass mass ratios of 0.25, 0.50, 0.75, and 1.00, respectively, which indicates that the CO2 solar-thermal reduction efficiency rises first and then decreases when the mass ratio of CO2 to biomass reaches to a certain level. The synergistic effect of CO2 to biomass mass ratio and gasification temperature on the biochar production rate is also investigated as presented in Figure 5b. It is found that high CO2/biomass mass ratio and gasification temperature promote biochar conversion, leading to a relatively low biochar production rate.



Figure 5. Effect of gasification temperature and CO2/biomass mass ratio on a) gas production rate and product distributions; b) biochar production rate.

The operating conditions are further optimized based on the amount of required solar energy to maintain the heat balance of solar gasifier (Q1), relatively low CO2 concentration from the outlet stream of the solar gasifier, and high CO2 solar-thermal reduction efficiency (See Figure S1 to Figure S3). Here, a series of process requirements are imposed to screen suitable range of gasification temperatures and CO2/biomass mass ratios. First, the solar gasifier’s heat requirement, Q1, should be no more than 6.00 Gcal/h, considering that higher solar energy requests more solar concentrator with a higher cost; 2) Secondly, the CO2 outlet concentration from the solar gasifier should be lower than 10 vol.%. A high CO2 concentration would affect the oxygen carrier reduction equilibrium, and thus result in a relatively low hydrogen production rate; 3) CO2 solarthermal reduction efficiency should be higher than 70% to guarantee the efficient CO2 solarthermal reduction. As can be seen from Figure 6a, Figure 6b, and Figure 6c, the dark blue areas reveal the simulation results which fail to meet the requirements. For the qualified area, the heat balance, CO2 concentration, and CO2 solar-thermal reduction efficiency distributions are


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calculated and presented. Figure 6d displayed the recommended area by comprehensively considering the above mentioned three aspects, thereby concluding recommended operating conditions for CO2 biomass gasification. Chemical looping hydrogen generation. Chemical looping-based hydrogen generation is performed with a multi-stage fuel reactor and a steam reactor. Meng, et al. reported that a multistaged reaction can increase the coal’s total residence time, resulting in higher char conversion and releasing more energy from the fuel 48-49. Here, a three-stage fluidized bed reactor (fuel reactor) is adopted for the efficient enhancement of gas and solid conversions under the gas-solid countercurrent mode and sufficient utilization of biomass-derived syngas 50. The effect of different OC reduction temperatures (800 °C, 850 °C, 900 °C, and 950 °C) on the compositions of the oxidized syngas under each stage is displayed in Table 2. Within each OC reduction stage, higher temperature promotes the equilibrium constraint to the oxidation of syngas, thereby generating more CO2 and H2O with Ca2Fe2O5 being the oxygen carrier in CLHG. It is also concluded that the reduction stages would also affect the reaction equilibrium and thus converts more H2, CO, CH4 to CO2 and H2O with the increasing number of reduction stages. Combining the simulation results as well as our previously reported experimental results indicates 900 °C is the optimal condition for balancing thermodynamic equilibrium and kinetic characteristics 42.



Figure 6. Operating condition optimization: a) Required heat from solar; b) CO2 concentration distributions of the produced gases; and c) CO2 solar-thermal reduction efficiency; and d) recommended area.

The effect of the syngas/OC mole ratio is discussed during the OC reduction stage as shown in Figure 7a. It is noted that Fe0 can be generated using low OC: syngas mole ratios (< 0.33). Moreover, the Ca2Fe2O5 oxygen carrier is more likely reduced to be Fe2+ when the Ca2Fe2O5/syngas mole ratio is more than 0.33. The red pentagram simulates the theoretical hydrogen production rate under the circumstance that excessive steam is added for Fe0/Fe2+ oxidation. It is found that the hydrogen production rate is not influenced much by the reduction degree of Fe3+ (Fe0 or Fe2+), instead, a high Ca2Fe2O5/syngas mole ratio results in a high hydrogen


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production rate. The over conversion/oxidation of the biomass-derived syngas can result in a low heating value of the syngas, leading to insufficient heating via oxy-syngas combustion. Thus, the selection of Ca2Fe2O5 flow rate for the CLHG process is a crucial step for the subsequent system operation.

Table 2 Product distributions by using three-stage fuel reactor [feed rate of Ca2Fe2O5: 120.0 kmol/h, reduction temperature: 800-950 °C, syngas flow rate: 276.7 kmol/h]. 　 Fuel reactor 800 °C

Fuel reactor 850 °C



Products　 H2 CH4 CO H2O CO2 H2 CH4 CO H2O CO2 H2 CH4 CO H2O CO2 H2 CH4 CO H2O CO2

RFuel-1 (kmol/h) 86.8 0.37 130.8 19.3 31.9 85.7 0.11 132.9 20.9 30.0 84.4 0.03 134.6 22.4 28.4 83.0 0.01 136.0 23.8 27.0

RFuel-2 (kmol/h) 63.3 0.04 93.1 43.4 69.9 69.7 0.03 109.2 37.1 53.8 67.5 0.01 111.5 39.3 51.6 65.5 0.00 113.5 41.3 49.5

RFuel-3 (kmol/h) 57.7 0.03 84.4 49.1 78.6 53.8 0.01 85.2 53.1 77.8 51.3 0.00 87.7 55.5 75.3 49.1 0.00 89.9 57.7 73.1

Effect of steam to OC mole ratio and oxidation-stage temperature on H2 production rate, H atom utilization efficiency, and the heat balance distribution in the steam reactor is illustrated in Figure 7b, Figure 7c, and Figure 7d, respectively. The OC oxidation process is an exothermic reaction thus a relatively low temperature promotes steam conversion and H2 production. It is also 19 ACS Paragon Plus Environment


observed that lower temperatures are expected to lead to a higher H atom utilization efficiency and release more heat from OC oxidation under the same OC/steam mole ratio. Taking the kinetics of OC oxidation into consideration, 820 °C is chosen as a typical condition for steam reactor operation. It was determined that increasing the steam: OC ratio enhanced the H2 production rate within a certain reasonable range while decreasing H atom utilization efficiency. Thus, a 200.0 kmol/h feed rate with steam/OC mole ratio of 1.67 for the steam reactor performed with high hydrogen yield while ensuring acceptable H atom utilization efficiency. Simulation results of the entire system. Based on the discussion and optimization, the recommended parameters of the entire system are summarized in Table 3. For Part IV, the partially oxidized syngas from fuel reactor with the production rate of H2 51.3 kmol/h, CO 87.7 kmol/h, H2O 55.5 kmol/h, and CO2 75.3 kmol/h were fed into the oxy-syngas combustor (RGibbs) for burning. The oxygen required for oxy-syngas combustion is primarily supplied by the anode of the CO2 electro-reduction cell, with the air separation unit providing the remainder, ensuring the complete conversion of syngas to CO2 and H2O. The oxy-syngas combustor effluent (CO2 and H2O, 950 °C) is then used to heat the system by flowing through the coil. The energized stream was then sent to the heat exchanger units for heat recovery and steam condensation with in-situ high purity CO2 capture. The produced CO2 is divided into two streams, a portion of CO2 is fed into the solar gasifier, accomplishing biomass gasification and CO2 solar-thermal reduction; the other portion of CO2 is used for electrocatalysis conversion of CO2 to HCOOH and CO. Results indicate that the 123.0 kmol/h CO2 fed into the electro-reduction unit produces 98.4 kmol/h HCOOH and 24.6 kmol/h CO from cathode. It is found that the theoretical O2 production rate from the anode of the CO2


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electro-reduction cell is 61.5 kmol/h, which is inadequate to meet the demand of the oxy-syngas combustion, thus the air separation unit is used to supply the additional oxygen.

Table 3. Typical operation conditions and corresponding simulation results. Basic information

Part I Solar gasifier

Part III-1 Fuel reactor

Part III-2 Steam reactor

Part IV oxy-syngas combustor

Part V Electroreduction setup

Type of biomass System pressure (bar) Atmosphere temperature (°C) Biomass feeding rate (kg/s) Lower heating value of biomass (MJ/kg) Gasifier temperature (°C) Solar energy duration (h/day) CO2/biomass mass ratio Syngas production rate (kmol/h) Biochar production rate (kmol/h) Final temperature of the outlet gas (°C) Reactor temperature (°C) Calcium ferrite flow rate (kmol/h) Number of the fuel reactor stage Fe production rate (kmol/h) FeO production rate (kmol/h) Reactor temperature (°C) Steam temperature (°C) Steam/OC mole ratio H2 production rate (kmol/h) H atom utilization efficiency (%) Final temperature of the outlet gas (°C) Combustor temperature (°C) O2 consumption rate (kmol/h) O2 flow rate from air separation (kmol/h) CO2 production rate (kmol/h) Final temperature of the outlet gas (°C) Faradaic efficiency for HCOOH production (%) Faradaic efficiency for CO production (%) Wind power duration (h/day) CO2 flow rate for electro-reduction (kmol/h) CO production rate (kmol/h) HCOOH production rate (kmol/h) O2 production rate (kmol/h) Water consumption rate (kmol/h)


Pine wood 1.013 20 1.0 19.05 800 8 0.50 276.7 19.8 20 900 120.0 3 0.0 240.0 820 300 1.67 96.8 48.4 20 950 69.8 8.3 163.0 20 80 20 16 123.0 24.6 98.4 61.5 98.4


Figure 7. a) Effect of Ca2Fe2O5/syngas mole ratio on Fe0 and Fe2+ production rate of Part III-1; and synergistic effect of oxidation temperature and Steam/OC mole ratio on b) hydrogen production rate; c) H atom utilization efficiency; d) heat balance distribution.

The heat consumption and heat supplement of the system are listed in Table 4. Based on the operating conditions given in Table 3, the heat balance in the solar gasifier, fuel reactor, steam reactor, and oxy-syngas combustor is calculated, which are -5.55 Gcal/h, -4.76 Gcal/h, 2.02 Gcal/h, and 8.48 Gcal/h. The solar thermal furnace and electro-reduction cell are assumed to have no additional heating requirement beyond that provided by solar and wind power, respectively. To ensure stable auto-thermal operation of the system, the required heat from the syngas combustor 22 ACS Paragon Plus Environment

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should be 5.20 Gcal/h supposing 3.00 Gcal/h of solar energy is provided. As mentioned, the OC flow rate plays a significant role in the system heat balance, thus the effect of different OC flow rates is further investigated as presented in Figure 8 and the simulation details are provided in Figure S4. Figure 8a shows the heat balance of the system under the circumstance of no solar energy provided, considering the existence of sunless weather. It is observed that unassisted autothermal operation is viable with a flow rate of 120.0 kmol/h Ca2Fe2O5, while for Ca2Fe2O5 flow rate at 150.0 kmol/h, the system cannot achieve the auto-thermal operation. In order to maintain continuous operation, the oxy-syngas combustor must be sized to meet the power requirement of the system when solar power is unavailable. Thus, the maximum heat that oxy-syngas combustor could supply is calculated and given in Figure 8b.

Figure 8. Effect of Ca2Fe2O5 flow rate on the system heat distributions and the heat requirement from syngas combustion: a) heat balance without solar supply; b) required heat from syngas oxycombustor.

Water consumption is minimized by recycling water in the various reactors. Detailed parameters are displayed in Table 5. The outlet streams from the solar gasifier, steam reactor, and



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oxy-syngas combustor are fed into a heat exchanger for water preheating and steam generation as well as for the water in the outlet stream recovery, theoretically saving 3.5 kmol/h, 101.6 kmol/h, and 104.1 kmol/h of water, respectively. The system achieves a water recycle efficiency of 70.1%, carbon fixation efficiency of 13.1%, and system CO2 reduction of 92.3%, respectively, and an overall energy efficiency of 59.17% under typical conditions as presented in Table 6.

Table 4. Parameters on heat consumption and heat supplement of the system.

a b

Parameters Heat balance (without solar) in solar gasifier Q1 (Gcal/h) Heat balance in solar-thermal furnace Q2 (Gcal/h) Heat balance in fuel reactor Q3 (Gcal/h) Heat balance in steam reactor Q4 (Gcal/h) Maximum heat provided by syngas combustor Q5 (Gcal/h) Heat balance in electro-reduction cell Q6 (Gcal/h) Solar supplied energy Qsolar (Gcal/h)

Results -5.55 0.00 -4.67 2.02 8.48 0.00 3.00

Required heat from syngas combustor Q5-1 (Gcal/h) a

5.20

Required heat from syngas combustor Q5-2 (Gcal/h) b

8.20

Syngas saving amount compared with no solar (kmol/h)

47.74

solar energy (Qsolar = 3.00 Gcal/h) is provided; no solar energy is provided.

Table 5. Parameters on steam/water consumption and water recycle rate of the system. Steam/water consumption (kmol/h) Water recycle (kmol/h) Total consumption (kmol/h)

Steam reactor Electro-reduction setup Solar gasifier Fuel reactor Steam reactor Oxy-syngas combustor Total recycle amount Theoretical consumption Net consumption

Table 6. Performance summary of the ubiquitous energy system. Water recycle efficiency (%) Carbon fixation efficiency (%) System CO2 reduction efficiency (%)

70.1 13.1 92.3 24

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200.0 98.4 3.5 0.0 101.6 104.1 209.2 298.4 89.2

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System energy efficiency (%)

59.2

CONCLUSIONS A solar-wind-biomass powered H2, HCOOH, and graphene cogeneration system has been proposed and simulated using ASPEN Plus®. Simulation results revealed that the system operated auto-thermally via bioenergy cascade utilization, achieving the water recycle efficiency, system CO2 reduction efficiency, and overall system energy efficiency at 70.1%, 92.3%, and 59.2%, respectively with low bio-tar and H2S emission. Operating parameters (i.e. temperature, CO2/biomass mass ratio, OC/syngas mole ratio, and steam/OC mole ratio) were investigated to determine their influence on product generation and heating requirements, and basic guidelines for optimization. The following conclusions can be drawn from the simulation results and discussions: 1) Substituting H2O with CO2 as the gasifying agent for biomass gasification reduced water demand, and a CO2 solar thermal reduction efficiency of 68.3% for the solar gasifier was achieved with a biochar production rate of 19.8 kmol/h. 2) A CO2/biomass mass feed ratio of 0.50 and a temperature range of 750 °C to 800 °C are expected as a feasible regime for the solar gasifier. 3) The synergistic effects of chemical looping temperature, OC/syngas mole ratio, and steam/OC mole ratio are studied, and it was demonstrated that the oxygen carrier flow rate plays a vital role in biomass-derived syngas cascade utilization and system energy transformation. A hydrogen production rate of 96.8 kmol/h and H atom utilization efficiency of 48.4% can be obtained under the following conditions: OC/syngas mole ratio of 0.43, steam/OC ratio of 1.67, and fuel reactor and steam reactor temperatures at 900 °C and 820 °C, respectively.



4) The oxy-syngas combustor burns the value-reduced syngas from the outlet of the fuel reactor, generating sufficient heat (8.48 Gcal/h) for the entire ubiquitous energy system while generating CO2 (163.0 kmol/h) and H2O (106.9 kmol/h). Part of CO2 (123.0 kmol/h) is fed into the electro-reduction cell for production of HCOOH (98.4 kmol/h), CO (24.6 kmol/h), and O2 (61.5 kmol/h).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI XXXX. Distribution of required heat by solar; distribution of CO2 concentration; distribution of CO2 solar-thermal reduction efficiency; and heat balance calculation details.

AUTHOR INFORMATION Corresponding Authors Jinlong Gong: [email protected] Wenguo Xiang: [email protected]

Author Contributions Zhao Sun and Liang Zeng contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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We acknowledge the National Key R&D Program of China (2016YFB0600901), the National Natural Science Foundation of China (21525626, 51761145012, 51576042, U1663224), and the Program of Introducing Talents of Discipline to Universities (B06006) for financial support.

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Synopsis This paper describes a renewable energy supply ecosystem for sustainable biomass conversion to formic acid, hydrogen, and graphene.


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