Autothermal CaO Looping Biomass Gasification for

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Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Autothermal CaO Looping Biomass Gasification for Renewable Syngas Production Hongman Sun†,‡ and Chunfei Wu*,†,‡ †

School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT7 1NN, U.K. School of Engineering and Computer Science, University of Hull, Hull, HU6 7RX, U.K.

‡

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S Supporting Information *

ABSTRACT: Biomass gasification is regarded as a promising alternative to fossil fuels for producing sustainable and clean value-added products. However, the challenges including low energy efficiency, CO 2 emission, and ash agglomeration significantly delay the deployment of the technology. Herein, we first proposed a novel autothermal CaO looping biomass gasification (Auto-CaL-Gas) technology, in which CaO-based materials react with flue gas with a high concentration of CO2 (>30 vol %) to produce heat inside the gasifier, simultaneously providing energy for low-temperature biomass gasification using CO2 as a gasification agent. Upon use of this concept the syngas production exhibited a significant increase from 0.21 kg/h to 0.90 kg/h in the Aspen simulation results and more than 3-fold improvement in the experimental results.

1. INTRODUCTION The utilization of fossil fuels for energy demand has significant challenges due to the future shortage of these fuels and environmental concerns including greenhouse gas emissions.1−4 Compared to nonrenewable fossil fuels, biomass is regarded as a sustainable alternative feedstock in the sector of heat and power.5−8 Gasification is a key technology to convert biomass wastes into syngas with the advantage that the produced gas is easy to handle and store and has the potential to be converted into transport fuels.9−11 However, biomass gasification has several challenges as follows: 1. Energy efficiency: The overall energy efficiency increase during biomass gasification is the key to the success of this technology.12 Thus, many efforts including developing low-cost feedstock, improving processing efficiency, and reducing installation and operation costs as well as socio-environmental impacts are devoted to biomass gasification.13,14 Compared to other existing technologies, such as landfill and incineration, biomass gasification has the advantages that multiple useful products (e.g., fuels) other than heat and power can be produced.13 2. CO2 emission: Although the net carbon emission of biomass gasification is claimed as zero due to the new growth of biomass sequestering carbon, the climate impact of CO2 from temporary changes cannot be ignored. Thus, further removing CO2 from biomass gasification would be necessary to mitigate the impact on climate change.13,14 Recently, Fermoso et al. reported © XXXX American Chemical Society

an integrated CO2 capture and biomass gasification process at atmospheric pressure and intermediated temperature around 650 °C. The yield of high purity hydrogen will be largely enhanced to 80−93%.14 3. Ash agglomeration: the formed alkali silicates and sulfates deposited on gasifier walls result in bed sintering and even shutdown of the gasifier.15,16 Ma et al. studied the effects of different gas compositions including air and H2 on the agglomeration and defluidization phenomena, which provides a novel understanding of the agglomeration mechanisms during the biomass gasification process.17 However, agglomeration remains one of the major challenges for the biomass combustion and gasification. Coupling CO2 capture with the gasification process for the enhancement of energy efficiency and the reduction of CO2 emission, as well as the ash agglomeration has been largely investigated. The carbon gasification promoted by integrating CO2 capture was proposed as early as 1880. The removal of CO2 can shift the equilibrium toward the positive side, sequentially enhancing the conversion of CO.18 In addition, biomass gasification is an endothermic reaction, and the required heat is 206 kJ/mol. Therefore, the exothermic carbonation reaction with the released heat of 177.8 kJ/mol can provide heat for biomass gasification inside the gasifer. Received: Revised: Accepted: Published: A

March 12, 2019 June 14, 2019 June 26, 2019 June 26, 2019 DOI: 10.1021/acs.est.9b01527 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 1. Schematic diagram of the traditional biomass gasification (a) and Auto-CaL-Gas technology (b).

Figure 2. Schematic diagram of the novel Auto-CaL-Gas technology reactor system.

30 vol % CO2 at a temperature around 650 °C contains the energy of around 96 kJ/mol (obtained from Aspen Hysys calculation). The activation energy required for biomass CO2 gasification is around 200 kJ/mol,25 which is much lower than the energy input (∼273.8 kJ/mol) from the reaction of CaO and CO2 and the internal energy of flue gas. In addition, there is around 14 vol % O2 inside the flue gas. The oxidation of biomass components or biomass decomposition compounds could further release energy for the endothermic biomass gasification with CO2. Therefore, in principle, the concept of using the reaction between CO2 and CaO for autothermal biomass gasification is feasible. From the early work of Kyaw, the reaction system can increase reactor temperature from 500 to 1000 °C.26 The detailed schematic diagram of the traditional biomass gasification and the novel Auto-CaL-Gas technology is displayed in Figure 1. New CaO-based materials will be developed and injected into a gasifier. The material streams (CaO and CO2) react to produce heat inside the gasifier providing energy for biomass gasification with a high concentration of CO2 (>30 vol %) as the gasification agent. When the reaction between CaO-based materials and CO2 is finished or the released energy cannot sustain the biomass gasification, the used CaO-based materials will be transferred to a separate reactor for regeneration. The proposed technology will provide solutions to the above challenges of biomass gasification. (1) High calorific value syngas will be produced, and direct injection of air to gasification is avoided to enhance energy efficiency. (2) As biomass combustion is

Although proposals for biomass gasification combined with in situ CO2 capture have received intensive attention, the concept of using the heat generated from CaO carbonization for biomass gasification has not been reported to the best of our knowledge.19 Acharya et al. investigated the biomass steam gasification for hydrogen production with the addition of CaO adsorbents. Compared to the biomass gasification in the absence of CaO adsorbents, H2 concentration increased to 54.43%, meanwhile CO2 concentration decreased to 93.33% when the ratio of CaO to biomass was 2.20 A further study of the integrated biomass gasification with calcium looping process was proposed for the H2-enriched syngas production. The results displayed that H2 concentration was increased to 81% at 575 °C, while CO2 concentration was dramatically decreased to 5%. The energy efficiency of this proposed integrated process is predicted to be around 78.77%.21 In addition, this sorption enhanced process has also been widely used in the chemical looping steam reforming of ethanol.22,23 For the reported CaO-sorption-enhanced biomass gasification, it is focused on the use of CaO for the in situ CO2 capture, thus enhancing the concentration of syngas and promoting the production of hydrogen. These works normally ignore the heat released from the reaction between CaO and CO2. Herein, we first propose an autothermal CaO looping biomass gasification (Auto-CaL-Gas) technology to promote the development of biomass gasification with a novel internal heat generation for the biomass decomposition. In principle, the reaction between CaO and CO2 can release 177.8 kJ per mole of CaO.24 The injected flue gas with a concentration of B

DOI: 10.1021/acs.est.9b01527 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article


Figure 3. Comparison between the traditional biomass gasification (no CaO), the novel Auto-CaL-Gas technology (CaO:Bio), and the introduction of oxygen to Auto-CaL-Gas using Aspen simulation (a); influence of CaO/biomass ratio (b). Other experimental conditions: 1 kg/h CaO; 1 kg/h CO2; 0.1−0.5 kg/h biomass (guaiacol); 0.1 kg/h O2; inlet temperature of 650 °C for CaO, CO2, and O2 and 100 °C for biomass.

Table 1. Comparison between Traditional Biomass Gasification (no CaO), Novel Auto-CaL-Gas Technology (CaO:Bio) and the Introduction of Oxygen to Auto-CaL-Gas Using Aspen Simulation feed

total outlet mass flows (kg/h)

CaO (kg/h)

CO2 (kg/h)

guaiacol (kg/h)

CaCO3 (kg/h)

H2 (kg/h)

CH4 (kg/h)

CO (kg/h)

H2O (kg/h)

oxygen (kg/h)

no CaOa CaO/Biob = 2 oxgenc no CaO CaO/Bio = 5 oxgen no CaO CaO/Bio = 10 oxgen

1.5 2.5 2.6 1.2 2.2 2.3 1.1 2.1 2.2

0 0.40 0.42 0 0.37 0.83 0 0.58 1

0.88 0 0.08 0.84 0.13 0.66 0.89 0.43 0.98

0.39 0 0 0.05 0 0 0 0 0

0 1.06 1.04 0 1.12 0.30 0 0.75 0

0 0 0.02 0 0.01 0 0 0 0

0.03 0.13 0.06 0.04 0 0 0.03 0 0

0.21 0.90 0.98 0.28 0.55 0.45 0.18 0.31 0.17

0 0 0 0 0.01 0.05 0 0.03 0.05

0 0 0 0 0 0 0 0 0

cold gas T (°C) efficiencyd (%) 94.7 504.5 687.8 110.0 733.9 803.4 297.8 804.1 962.9

17.0 74.4 71.2 56.7 83.8 64.4 74.4 85.5 44.4

No CaO means the traditional biomass gasification process. The process flow diagram is displayed in Figure S1. bCaO/Bio = 2, 5, and 10 indicate introducing CaO into the biomass gasification process and the ratios of CaO to biomass are 2, 5, and 10, respectively. The process flow diagram is displayed in Figure S2. cOxygen means the novel Auto-CaL-GaS technology with oxygen introduction. The process flow diagram is displayed in Figure S3. dCalculated as a

cold gas efficiency% =

∑ gas × mass flow × 100 Guaiacol × inlet mass flow

2.2. Experimental Work of CaO−CO 2 Reaction Supported Biomass Gasification. Lignin and calcium oxide were purchased from Sigma-Aldrich without any further purification. The performance of this novel Auto-CaL-Gas process was carried out in a fixed-bed reactor under atmospheric pressure. An online gas analyzer was utilized to analyze the product distributions as shown in Figure 2. The fixed bed reactor which consists of a stainless steel tube with a length of 50 cm and inner diameter of 0.635 cm was heated by an electrical, tubular, horizontal furnace purchased from Elite Thermal Systems Limited. A K-type thermocouple was utilized to control the temperature in the fixed bed reactor. The flow rates of gases including CO2, N2, and O2 were controlled by mass flow controllers provided by Omega. In a typical experiment, the physical mixture of 0.5 g of powdered CaO and 0.1 g of lignin was loaded into the stainless-steel tube fixed by the quartz wool. The temperature of the fixed bed reactor was increased to 650 °C with a heating rate of 10 °C/min and held 30 min in different atmospheres with 100 mL/min total flow rate to observe the product compositions.

largely avoided, less CO2 will be produced. In addition, using flue gas as a biomass gasification agent will convert CO2 partially into CO contributing to syngas production. Thus, the extra benefit of carbon emission can be obtained. (3) A novel temperature control mechanism will be applied in the proposed Auto-CaL-Gas technology. The low-temperature flue gas (∼650 °C) and the controlled release of heat from the CaO−CO2 reaction will largely mitigate ash agglomeration because temperature is a key factor for ash agglomeration.27,28 In this work, the ratios of CaO to biomass, biomass feed temperature, CO2 content, and oxygen content were studied to investigate the feasibility of the proposed Auto-CaL-Gas technology to enhance syngas production using the Aspen Plus simulation and a fixed-bed gasifier.

2. MATERIALS AND METHODS 2.1. Simulation Using Aspen Plus. Simulation of the novel Auto-CaL-Gas technology (guaiacol used as a model compound) was carried out using Aspen Plus, and the process flow diagram is displayed in Figures S1−S3 of the Supporting Information. Typically, a Gibbs reactor is included with a feed containing 0.1−0.5 kg/h biomass (guaiacol), 0.2−2 kg/h CO2, 1 kg/h CaO, and/or 0.025−0.1 kg/h O2.

3. RESULTS AND DISCUSSION 3.1. Simulation of Biomass Gasification. The comparison between traditional biomass gasification (no CaO), novel C


Article


Figure 4. Influence of CO2 content (a) and oxygen content (b) on the key product flow rates.

Table 2. Influence of CO2 Content on Product Flow Ratesa CO2 content (kg/h)


CaO (kg/h)

CO2 (kg/h)

guaiacol (kg/h)

CaCO3 (kg/h)

H2 (kg/h)

CH4 (kg/h)

CO (kg/h)

H2O (kg/h)

oxygen (kg/h)

T (°C)

cold gas efficiency (%)

0.2 0.5 1 2

1.425 1.725 2.225 3.225

0.93 0.68 0.47 0.33

0 0.03 0.25 1.04

0 0 0 0

0.13 0.56 0.95 1.19

0 0 0.01 0

0.05 0.03 0 0

0.32 0.41 0.53 0.59

0 0 0.02 0.06

0 0 0 0

484.5 684.8 761.0 811.6

69.1 63.8 79.3 80.4

a Other experimental conditions: 1 kg/h CaO; 0.2 kg/h biomass (guaiacol); 0.025 kg/h O2; inlet temperature of 650 °C for CaO, CO2 and O2 and 100 °C for biomass.

However, it should be noticed that the CO outlet mass flow is increased per unit of biomass (Figure 3b insert picture) because the high ratio of CaO to biomass can provide adequate energy to promote the biomass gasification, which is in agreement with the increase of cold gas efficiency from 74.4% (CaO/Bio = 2) to 85.5% (CaO/Bio = 10). After the introduction of oxygen when the ratio of CaO to biomass is 5 or 10, the mass flow of CO is decreased from 0.55 kg/h and 0.31 kg/h to 0.45 kg/h and 0.17 kg/h, respectively. This occurs beause less CO was generated at a high ratio of CaO to biomass with the same amount of oxygen (0.1 kg/h). Thus, CO and H2 can be oxidized as seen in eq 2 and eq 3, respectively, which is in agreement with the increased outlet mass flows of CO2 and H2O.31

Auto-CaL-Gas technology (CaO:Bio), and the introduction of oxygen to Auto-CaL-Gas is shown in Figure 3a. With the addition of CaO, the outlet mass flow of CO is dramatically increased by more than 3 times from 0.21 kg/h to 0.90 kg/h (Table 1), while the CO2 outlet mass flow decreases from 0.88 kg/h to 0 kg/h when the ratio of CaO to biomass is 2. In addition, the cold gas efficiency which is defined as the ratio between the flow of energy in the syngas and the energy contained within the fuel is dramatically increased from 17.0% to 74.4%. A similar trend is obtained when the ratio of CaO to biomass is increased to 5 and 10. It is suggested that the introduced CaO reacts with CO2 to produce heat inside the gasifier and then provides energy for biomass gasification. This phenomenon can also be confirmed by the increasing temperature of the reactor, which is increases from 94.7, 110.0, and 297.8 °C to 504.5, 733.9, and 804.1 °C, respectively, after the introduction of different amounts of CaO. It is known that at a higher gasification temperature, the conversion of biomass feedstock will be increased. In addition, CO2 in the flue gas is partially converted into CO sequentially contributing syngas production.29 In comparison to the Auto-CaL-Gas without oxygen, the introduction of oxygen slightly increases the outlet mass flows of CO and H2 at a low ratio of CaO to biomass as shown in Figure 3a, which is in agreement with the increased temperature from 504.5 to 687.8 °C. Both the mass flow of CO2 and H2O are increased due to the reaction between CH4 and O2 (eq 1), corresponding to the decreased mass flow rate of CH4 from 0.13 kg/h to 0.06 kg/h. In addition, other volatiles derived from biomass decomposition can also be oxidized with the introduction of oxygen.30 However, the cold gas efficiency exhibited a slight decrease from 74.4% to 71.2% when the ratio of CaO to biomass was 2. Increasing the ratio of CaO to biomass from 2 to 10 results in a decreased mass flow of CO, as shown in Figure 3b.

CH4(g) + 2O2 (g) → CO2 (g) + 2H 2O(g), ΔHr,298K = −802.3kJ/mol

(1)

2CO(g) + O2 (g) → 2CO2 (g), ΔHr,298K = −565.6 kJ/mol

(2)

2H 2(g) + O2 (g) → 2H 2O(g), ΔHr,298K = −483.6 kJ/mol

(3)

The influence of biomass feed temperature is investigated when the feeding rate and inlet temperature of CaO are 1 kg/h and 650 °C, respectively. The flow rate of the biomass inlet is fixed at 0.2 kg/h. The performance of Auto-CaL-Gas process is displayed in Figure S4 and Table S1. Almost no difference can be observed when the feed temperature of biomass feedstock varies from 50 to 200 °C. This is expected as the sensible energy is only changed slightly with the increase of biomass feeding temperature. In addition, the influence of CO2 content and oxygen content is shown in Figure 4. The syngas gas D


Article

Environmental Science & Technology Table 3. Influence of the Oxygen Content on Product Flow Ratesa oxygen (kg/h)


CaO (kg/h)

CO2 (kg/h)

guaiacol (kg/h)

CaCO3 (kg/h)

H2 (kg/h)

CH4 (kg/h)

CO (kg/h)

H2O (kg/h)

oxygen (kg/h)

T (°C)

cold gas efficiency (%)

0.025 0.05 0.1

2.225 2.25 2.3

0.47 0.58 0.83

0.25 0.38 0.66

0 0 0

0.95 0.74 0.30

0.01 0.01 0

0 0 0

0.53 0.51 0.45

0.02 0.03 0.05

0 0 0

761.0 779.7 803.4

79.3 74.4 64.4

Other experimental conditions: 1 kg/h CaO; 0.2 kg/h biomass (guaiacol); 1 kg/h CO2; inlet temperature: 650 °C for CaO, CO2 and O2, and 100 °C for biomass.

a

Figure 5. Product distributions for (a) traditional biomass gasification (0.1 g lignin) and (b) novel Auto-CaL-Gas technology (0.1 g of lignin and 0.5 g of CaO) in 46% CO2 balanced with N2(GA1).

Figure 6. Product distributions for (a) traditional biomass gasification (0.1 g lignin) and (b) novel Auto-CaL-Gas technology (0.1 g of lignin and 0.5 g of CaO) in 15% CO2 balanced with N2 (GA2).

production is enhanced with the increase of CO2 content from 0.2 kg/h to 2 kg/h, and especially, the CO production increases from 0.32 kg/h to 0.59 kg/h as shown in Figure 4a. This occurs because the reaction between CaO sorbent and highly concentrated CO2 releases more heat, which is further proven by the temperature increase from 484.5 to 811.6 °C (Table 2). In the presence of CO2, with the increase of oxygen content, the mass flows of CH4, H2, and CO are decreased, while the outlet mass flow of CO2 is increased due to the oxidation property of oxygen (Figure 4b and Table 3). The temperatures are increased with the increase of oxygen content due to the exothermic combustion reactions.32 However, the cold gas efficiency exhibits a decrease from 79.3% to 64.4%

when the oxygen content increases from 0.025 kg/h to 0.1 kg/ h. 3.2. Lab-Scale Biomass Gasification Assisted by CaO− CO2 Reaction. Experimental studies have been carried out on the effect of introducing CaO into the traditional biomass gasification process in a fixed bed reactor. Four types of gasification agents/atmosphere were used including (1) 46% CO2 balanced with N2, (2) 15% CO2 balanced with N2, (3) pure N2, and (4) 50% O2 balanced with N2. These four types of atmosphere are assigned as GA1, GA2, GA3, and GA4, respectively. Compared to the traditional biomass gasification process without the introduction of CaO as shown in Figure 5a, the CO production exhibits a 3-fold increase (from 3.6 vol E


Article


Figure 7. Product distributions for (a) traditional biomass gasification (0.1 g lignin) and (b) novel Auto-CaL-Gas technology (0.1 g lignin and 0.5 g CaO) in pure N2 (GA3).

Figure 8. Product distributions for (a) traditional biomass gasification (0.1 g lignin) and (b) novel Auto-CaL-Gas technology (0.1 g lignin and 0.5 g CaO) in 50% air balanced with N2 (GA4).

CaO during the biomass gasification process using air as the gasification agent (GA4) shows negative impacts on syngas production as shown in Figure 8 due to the absence of CO2. The CO2 concentration is increased using GA4 in Figure 8a attributing to the enhanced combustion reactions. The concentration of CO2 exhibits a dramatic decrease in Figure 8b because the introduction of CaO can eliminate the CO2 emission using GA4. Shen et al. also demonstrated that the higher system efficiency can be obtained in the in situ coal gasification chemical looping combustion power plant when using CO2 instead of steam as a gasification agent. This is attributed to the energy consumption for steam generation can be eliminated in this proposed new process.36 Therefore, we first propose a novel Auto-CaL-Gas concept for renewable syngas production, in which the material streams react to produce heat inside the gasifier providing energy for biomass gasification with high concentration CO2 as the gasification agent. Compared to the traditional biomass gasification process, the proposed novel Auto-CaL-Gas technology significantly increases the CO production from 0.21 kg/h to 0.90 kg/h in the Aspen simulation results and more than three times in experimental results. The proposed innovative idea has the following advantages: (1) Air is not directly required for biomass gasification. Thus, the produced syngas has a much higher energy density which is important to produce liquid fuels from syngas. (2) Flue gas from different

% without CaO to 12.2 vol % with CaO) by the interaction between CaO and GA1 using the novel Auto-CaL-Gas concept (Figure 5b), which is in agreement with the simulation results. However, the positive influence on syngas production is decreased after switching to a lower CO2 concentration in the proposed Auto-CaL-Gas technology. For example, the syngas production is only increased by 2 times after the introduction of CaO in GA2 as shown in Figure 6. The decrease of CO2 concentration shown in Figure 5b and Figure 6b is attributed to the reaction between CaO sorbent and the CO2 gasification agent, which is the source of energy inside the gasifier.33 Dou et al. also illustrated that the biomass gasification integrated with continuous sorption-enhanced steam reforming performed in a fluidized bed reactor can not only reduce the CO2 emission but also enhance the hydrogen production.34 Using the novel Auto-CaL-Gas technology, the concentration of outlet CO balanced with N2 is decreased from 12.2 vol % in GA1 (Figure 5b) and 4.8 vol % in GA2 (Figure 6b) to 3.3 vol % in GA3 (Figure 7b). It is clearly indicated the importance of CO2 concentration on biomass gasification when CaO is introduced into the gasifier. A comparative analysis of technology and economy in biomass gasification with and without carbon capture was proposed by Zang et al. It was found that the cost of power plant can be reduced by the introduction of carbon capture technology when the CO2 credit is as high as 90 $/ton.35 In addition, the introduction of F


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combustion processes can be used to provide CO2 for reacting with CaO and as a gasification agent, thus a lower greenhouse gas emission is anticipated. (3) A key energy input obtained from the reaction between CaO-based adsorbents and CO2 gasification agent promotes the biomass gasification at a low gas feeding temperature (∼650 °C) and sequentially reduces the ash agglomeration, which needs to be further studied. However, there are some assumptions and challenges for the development of the proposed Auto-CaL-Gas technology: (1) Novel cost-effective CaO-based materials need to be developed with high capture capacity, fast reaction kinetics, and cyclic stability for reacting with CO2. (2) Detailed understanding of the interactions between biomass, CaO, and flue gas inside the gasifier is required, because CaO particles will gain weight from the reaction with CO2, and biomass particles will lose mass and shrink from decomposition and reforming reactions inside the gasifier. (3) The energy provided by the reaction between the CaO sorbent and the CO2 gasification agent depends on the reactivity and the amount of CaO materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b01527. Process flow diagram of traditional biomass gasification and novel Auto-CaL-Gas technology, as well as the influence of the biomass feed temperature (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongman Sun: 0000-0001-9225-0362 Chunfei Wu: 0000-0001-7961-1186 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the China Scholarship Council (CSC, no. 201606450016) for financial support. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 823745.

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REFERENCES

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