Conversion of Biomass to Hydrocarbon-rich Bio-oil via Microwave-assisted Catalytic Pyrolysis: A Review

Wang Yunpu; Zhang Shumei; Yu Zhenting; Jiang Lin; Liu Yuhuan;Ruan Roger,4; Fu Guiming

(1. Nanchang University, State Key Laboratory of Food Science and Technology, Nanchang 330047; 2. Nanchang University, Engineering Research Center for Biomass Conversion, Ministry of Education, Nanchang 330047;3. Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640; 4. Center for Biorefining and Department of Bioproducts and Biosystems Engineering, University of Minnesota, USA)

Abstract: The method for pyrolysis of biomass to manufacture hydrocarbon-rich fuel remains challenging in terms of conversion of multifunctional biomass with high oxygen content and low thermal stability into a high-quality compound,featuring high content of hydrocarbons, low oxygen content, few functional groups, and high thermal stability. This study offers a promising prospect to derive hydrocarbon-rich oil through microwave-assisted fast catalytic pyrolysis by improving the effective hydrogen to carbon ratio (H/Ceff) of the raw materials. The proposed technique can promote the production of high-quality bio-oil through the molecular sieve catalyzed reduction of oxygenated compounds and mutagenic polyaromatic hydrocarbons. This work aims to review and summarize the research progress on microwave copyrolysis and microwave catalytic copyrolysis to demonstrate their benefits on enhancement of bio-oils derived from the biomass. This review focuses on the potential of optimizing the H/Ceff ratio, the microwave absorbent, and the HZSM-5 catalyst during the microwave copyrolysis to produce the valuable liquid fuel. This paper also proposes future directions for the use of this technique to obtain high yields of bio-oils.

Key words: biomass, microwave pyrolysis, H/Ceff, microwave absorbent, HZSM-5 catalyst, hydrocarbon-rich bio-oil

1 Introduction

With the rapid consumption of fossil fuel and the deterioration of the environment, the fast pyrolysis of biomass has attracted an increasing attention from researchers. However, the oil obtained by using this technology possesses high oxygen content, low heat value, poor thermal stability, high viscosity, high acidity, and strong corrosiveness, which restrict their further application[1-4]. The microwave-assisted fast catalytic pyrolysis is used to improve the quality of bio-oil. Prior to bio-oil condensation, the oil produced through the combination of fast pyrolysis and catalytic reforming exhibits improved quality and properties,such as low oxygen content and high hydrocarbon content. The catalysts used are mostly microporous,mesoporous, and macroporous ones (HZSM-5,MCM-41, LOSA-1, SBA-15, CNT, and modified derivatives)[5-9]. This technology can convert the biomass with low energy density to the hydrocarbonrich oil with high energy density through an inexpensive continuous process. The reduction in the volume of biomass can facilitate the storage and transportation, which are two of the most economic and industrial prospects of technology.

Microwave is a new heating method used in the fast catalytic pyrolysis of biomass. As a limited band in the electromagnetic spectrum, microwave is located between the infrared and the uhf radio waves and has a frequency range of 0.3―30 GHz and a wavelength range of 1 mm―1 m. Microwave is highly penetrable,and microwave irradiation can convert electromagnetic energy into thermal energy via dipole polarization dielectric response in the material and in and out of materials (microwave dielectric heating). In particular,the polar molecules or polar parts in the material are polarized into dipole under the action of microwave.The dipole is oscillating with the microwave alternating electric field and rubbing against each other to produce heat, causing rapid simultaneous warming in the inside and outside of the material. The essence of microwave heating is the energy dissipation of microwave in the material. In comparison with the traditional heating method, microwave heating exhibits the following advantages: (1) even heating, (2) energy saving, (3) lack of hysteresis effect, (4) easy to operate, and (5) safe and pollution free. Microwave heating technology has developed rapidly. Ruan successfully applied microwave heating technology to biomass pyrolysis oil and developed the microwave-assisted catalytic fast pyrolysis (MACFP)[10-12]. In the international field of biomass MACFP oil, various research work was conducted in the Washington State University of America[13-15], the University of Florence in Italy[16-20],the University Malaysia Sarawak in Malaysia[21-22], the University of Leeds in Britain[23], and the University of British Columbia in Canada[24].

The main challenge of microwave-assisted catalytic fast pyrolysis of biomass for hydrocarbon-rich fuel method is the conversion of biomass with high oxygen content and low thermal stability into a high-quality compound with high content of hydrocarbons, low oxygen content,and high thermal stability (Figure 1). The research and application of the microwave-assisted catalytic pyrolysis for conversion of hydrocarbon-rich bio-oil have become increasingly relevant.

Figure 1 The main challenge for biomass MACFP technique

2 H/Ceff Ratio for Biomass Pyrolysis

2.1 Introduction of H/Ceff ratio

Selecting an appropriate thermal cracking raw material model is important for obtaining the high-quality hydrocarbon-rich bio-oil. Pyrolysis of biomass produces renewable fuel with high oxygen content and low calorific value because of the lower hydrogen to carbon effective ratio (H/Ceff) of the raw material. Chen proposed the following formula for determining the hydrogen to carbon effective ratio of raw materials[25]:

where C, H, O, N, and S are the mole percentage of the corresponding element in the feedstock. Lignocellulosic biomass has a low H/Ceffratio (0―0.3) and is considered to be a hydrogen-deficient matrix; by contrast, waste oil and alcohol have high H/Ceffratio (greater than 1.5). A hydrogen-rich matrix is generally added to the biomass prior to its catalytic pyrolysis into the hydrocarbon-rich bio-oil.

2.2 Effect of H/Ceff ratio on biomass pyrolysis for hydrocarbon-rich bio-oil

When used as raw material, the lignocellulosic biomass with a H/Ceffratio of less than 1.0 is difficult to pyrolyze into hydrocarbons in the presence of the HZSM-5 catalyst[18]. In this regard, researchers attempted to add the hydrogen-rich matrix to the biomass pyrolysis system to obtain high-quality fuel. Nonedible oils (waste oil,vegetable fats, and soapstock)[26-27], waste plastics (PE,PP, PS, and PVC)[28-30], and waste tires[31-32], which have high H/Ceffratio, have been used for catalytic pyrolysis of biomass. The results showed that the copyrolysis reaction of the hydrogen-rich matrix is beneficial to the increase of hydrocarbon content in the pyrolysis fuel, reduction of the coke yield, and extension of the service life of the catalyst.Researchers conducted thermogravimetric and microreaction experiments to systematically investigate the catalytic conversion properties of more than 10 biomass derivatives with different H/Ceffvalues[33-34]. The H/Ceffratio and olefin content showed a strong correlation with the productive rate of aromatics and the deactivation rate of the catalyst; in this regard, the researchers proposed increasing the H/Ceffratio of the raw material to boost the yield of the target product. The addition of a material with high H/Ceffratio and the biomass cocatalyzed pyrolysis are effective methods used to improve the H/Ceffratio of raw materials. A previous study investigated the use of saturated monohydric alcohols (with an H/Ceffratio of 2) and fats (with an H/Ceffratio of 1.5). Huber, et al.used isotope tracer to study the cocatalytic pyrolysis of13C methanol and12C biomass and demonstrated the presence of hydrocarbon sharing during transformation[35].However, the prices of alcohols and fats are high; thus,scholars aim to search for other inexpensive biomass with high H/Ceffratio for cocatalyzed pyrolysis.

The H/Ceffratio of a material is related to the yield of the target product (hydrocarbons, including aromatic hydrocarbons and olefins) obtained via the catalytic pyrolysis. The H/Ceffratio reflects the composition of macro-elements but not the structural characteristics of raw materials. Saturated monohydric alcohols possess an H/Ceffratio of 2 but behave differently during the catalytic pyrolysis over the HZSM-5 zeolite. Du, et al. studied the relationship between the H/Ceffratio and the target product yield and reported that the yield of hydrocarbons derived from some raw materials through the catalytic pyrolysis over HZSM-5 molecular sieve did not increase with an increasing H/Ceffratio[4]. A simple linear relationship was found between the carbon yield of the total chemical products (aromatic hydrocarbons + olefins) and the H/Ceffratio of the raw material, but the fitting effect is not ideal[36].Therefore, the definition of the H/Ceffratio of a raw material must be modified to reflect the composition of macroelements and the structural characteristics of the material.

3 Microwave Absorbents for Biomass Pyrolysis

3.1 Microwave absorbing properties of materials

Simple microwave heating of biomass is not advisable because of its poor microwave absorption. Therefore,adding materials with good absorption properties can help achieve the fast pyrolysis of biomass. Carbon dioxide (activated carbon, silicon carbide, and graphite),which is often used as a microwave absorber for biomass microwave pyrolysis, can reduce the resistance to electromagnetic waves and can be heated by using microwave. The traditional biomass microwave pyrolysis system increases the temperature and causes negative effects, such as “hot spot,” which leads to secondary pyrolysis of volatiles; such effects can be effectively reduced using microwave absorbers[37-39].

The addition of microwave absorbing material can change the warming behavior of biomass and reduce the negative effect such as hot spot, which is caused by the rapid warming of traditional microwave pyrolysis[40]. The microwave absorber-assisted microwave-heating catalyst technology can achieve the dual effects of bio-absorbing heat and wave-absorbing wave-assisted heating, enhance the efficient transmission and absorption of microwave energy, solve the problem of low thermal conductivity during heating, and reduce the loss of energy. The ability of the material to absorb microwave can be characterized by the tangent of the loss angle δ (tanδ). The greater the tanδ value is, the stronger the absorption capacity of the material to microwave will be. If tanδ＞0.5, it would indicate that the absorption capacity of microwave is very strong; if 0.5＞tanδ＞0.1, it would indicate that the absorption capacity of microwave is moderate;and if tanδ＜0.1, it would indicate that the microwave absorption capacity of the material is very low. The tanδ values for common carbon-based materials are shown in Table 1. The so-called “out of control effect”will be generated in microwave heating applications.Microwave will increase the material temperature and then loss control, resulting in a vicious cycle and damage to the material; this phenomenon can be prevented by cutting off the supply of microwave energy or removing the material from the heating zone. Thus far, no report is available about the presence of thermal runaway during the microwave pyrolysis of biomass. Although most researchers believe that the addition of absorbents during microwave pyrolysis is necessary, they have experimentally demonstrated that the process can be achieved by relying solely on moisture in the material without adding a microwave absorbent; however,sufficient microwave power must be supplied[41-43].

Table 1 The tanδ values for common carbon-based materials [44]

3.2 Effect of microwave absorbents on biomass pyrolysis for hydrocarbon-rich bio-oil

Scholars conducted an in-depth study on the use of microwave absorbents in the fast pyrolysis of biomass.Microwave absorbing materials usually include activated carbon, silicon carbide, and graphite. Russell,et al. used activated carbon beds to pyrolyze the highdensity polyethylene under microwave conditions and demonstrated that the activated carbon beds could produce more hydrocarbon products than conventional coke beds. In this experiment, activated carbon not only acted as a microwave absorber for absorbing heat, but also played a catalytic role[45]. To compensate for the limitations of a carbonaceous material, Li, et al. used the porous activated carbon ball (PACB) as a carrier and Fe3+ions and Co2+ions to modify the activated carbon.The results indicated that the CoFe2O4-Co3Fe7-PACB composites are lightweight and efficient microwave absorber with high specific surface area and excellent microwave absorbing properties, which thereby can be used on a large scale[46]. Ruan confirmed the effectiveness of microwave, microwave absorber (SiC), and catalyst(HZSM-5) on the fast pyrolysis of biomass and presented its broad application prospects; the developed pulse width modulation device could effectively solve the microwave absorber’s auxiliary microwave suction heating out of the control phenomenon and could be used to automatically control the start and shut-down of the microwave reactor.Some other researchers also studied the microwaveassisted catalytic fast pyrolysis of biomass, with the results summarized in Table 2.

Table 2 Catalysts and products referred to in biomass microwave-assisted catalytic fast pyrolysis process

4 HZSM-5 Catalyst for Biomass Pyrolysis

4.1 Effect of HZSM-5 catalyst on biomass microwave pyrolysis for production of hydrocarbon-rich bio-oil

A series of reactions may occur during the preparation of hydrocarbon-rich bio-oil via the microwave-assisted fast catalytic pyrolysis of biomass; these reactions include dehydration, depolymerization, isomerization,aromatization, and decarboxylation. The occurrence of these reactions significantly affects the yield of final product. Thus, researchers have used various methods for improving the experimental parameters and conditions to obtain bio-oil with a high quality.

Catalysts play an important role in the microwaveassisted reactions. The HZSM-5 zeolite is the most effective catalyst for deoxidation; this material also catalyzes the pyrolysis of oxygenaceous species in the original pyrolysis gas to form hydrocarbons and to remove oxygen atoms in the form of H2O, CO,and CO2

[3,55-56]. The oxygen compounds of the original pyrolysis gas undergoing dehydration, decarbonylation,and decarboxylation reactions over the HZSM-5 zeolite with straight channels, chord-type channels (in nanometer scale) and two-way cross-micropores can be completely subject to catalytic deoxidation in these channels. Thing,et al. found that the HZSM-5 promoted the deoxygenation process during the pyrolysis of lignin[57]. Mullen, et al.also studied the catalytic pyrolysis of four lignin biomass samples and concluded that the addition of the HZSM-5 catalyst improved the yield of aromatic hydrocarbons[58].Zhang Bo, et al. conducted the microwave-assisted fast catalytic pyrolysis of biomass for producing the hydrocarbon rich bio-oil and confirmed the effectiveness of microwave, the microwave absorber (SiC), and the catalyst (HZSM-5)[59-60].

The HZSM-5 zeolites are widely used in copyrolysis of biomass and hydrogen-rich substrates. Dorado, et al.studied the copyrolysis of biomass and waste plastics catalyzed by HZSM-5 under the action of helium through Py-GC/MS and focused their investigation on six aromatic hydrocarbons (toluene, ethylbenzene, o-xylene,p-xylene, naphthalene, and dimethylnaphthalene). A synergistic effect was observed between the biomass and plastic catalyzed by HZSM-5, and the underlying mechanism was found to be the Diels-Alder reaction[61].Zhang Huiyan, et al. used a fluidized bed to catalyze the copyrolysis of pine wood with polypropylene,polyethylene, and polystyrene. The catalytic performance of LOSA-1 (with its main component consisting of ZSM-5) is better than that of Al2O3. When the mass ratio of polyethylene to pine wood was set at 4:1, 71%of liquid fuel (composed of 36% of aromatics and 35%of olefins) was obtained at 600 °C[62]; moreover, the biomass and plastic play a synergistic role in copyrolysis.Zhang Bo, et al. by using Py-GC/MS techniques also studied the copyrolysis characteristics of corn stalk and kitchen waste to obtain high yield of the target product,namely: aromatic hydrocarbons. A synergistic effect was found in the promotion of pyrolysis between corn straw and kitchen waste. Upon considering its high hydrogen content, the kitchen waste can provide sufficient hydrogen to corn straw during the co-pyrolysis process and promote the conversion of corn stalks into hydrocarbons.Moreover, oxygenaceous substances in the pyrolysis products of corn stalks can promote the breakage of long-chain substances in kitchen waste and promote its pyrolysis[63].

4.2 Coking on HZSM-5 catalyst

Coking on the HZSM-5 catalyst is a problem experienced in the biomass-based MACFP oil making technology.Coke is mainly formed by polymerization of the pyrolysis products on the surface of the molecular sieve catalyst.The formation of the pyrolysis products is a series of deep dehydrogenation of the relevant reaction products.The coke formed thereby is mainly composed of macromolecular-fused aromatic hydrocarbons that contain more carbon and less hydrogen. The coking pathway is shown in Figure 2. The two channels of the HZSM-5 catalyst are very small; thus, the inner surface does not have coke growth extension space, and coke with a large molecular structure can only be formed on the outer surface of the catalyst. The formation of the coke leads to clogging of the catalyst pores, limiting the diffusion of the reactants into the catalyst pores and destroying the catalytic activity of the catalyst.

The yield of coke is related to the supply of hydrogen and the acidity of the HZSM-5 catalyst used in the process.

Figure 2 Reaction mechanism for biomass MACFP process over HZSM-5 zeolite[64]

Adequate hydrogen supply can essentially alleviate the coking of acidic catalysts, but applying additional sources of hydrogen (such as hydrogen and alcohols)is expensive; therefore, adding high-quality hydrogen sources to the biomass catalytic pyrolysis processes is not ideal. Moreover, the effect of catalysts with overacidity to coke formation is significant; hence, the acidity of the catalyst must be decreased to slow down the coking process. In a previous study, the adoption of a proper hydrothermal treatment can optimize the acidity and activity of the HZSM-5 catalyst by dealumination to reduce the coke yield and the oxygen content in the pyrolysis vapor in order to increase the relative content of hydrocarbons[65]. The acid sites on the outer surface of the HZSM-5 catalyst are mainly coking-prone sites, and the acid sites on the inner surface are mainly catalytic sites that have no effect on coking. If the acidity of the outer surface is weakened and the acidity of the inner surface is retained, then the yield of the coke reduces, the catalyst life is prolonged, and the catalytic performance of the HZSM-5 catalyst in terms of the target product yield can be ensured[66-68].

5 Future Directions

Although the microwave-assisted catalytic fast pyrolysis technology can be used for uniform heating of a material and promoting hydrocarbon generation, the technology has several limitations in terms of feedstock, heat transfer effect, and catalyst. (1) As the raw material, biomass lacks sufficient hydrogen and oxygen elements, and then other hydrogen sources must be added to improve the quality of bio-oil. (2) During the microwave absorption heating, the biomass exhibits disadvantages, such as low microwave absorption factor, low dielectric constant,and slow heating rate. The improvement of hydrocarbon quality and yield during pyrolysis is hindered by the slow heating rate, which leads to the low temperature state of the reactants for a long time. Therefore, a microwave absorber must be added to enhance the heating effect.(3) The HZSM-5 catalyst can be easily subject to coke buildup, loses activity during in-situ catalysis, and exhibits reduced service life. By controlling an effective hydrogen-carbon ratio of the microwave absorber, the HZSM-5 catalyst can increase the catalytic coupling efficiency to improve the fuel quality and reduce the coke yield in the future; in this regard, the synergistic mechanism of biomass and hydrogen-rich matrix in the microwave catalytic copyrolysis must be determined;furthermore, modification of the definition of H/Ceff ratio must be investigated to regulate pyrolysis fuel components.

6 Conclusions

This review focuses on microwave copyrolysis and microwave catalytic copyrolysis by presenting their benefits on the enhancement of the quality of bio-oil derived from biomass. This paper also discusses the potential of optimizing the H/Ceffratio, the microwave absorbent, and the HZSM-5 catalyst in microwave copyrolysis to produce valuable liquid fuel. The emerging microwave catalytic copyrolysis technique can promote the clean production of bio-oil through the reduction of undesired oxygenated compounds and mutagenic polyaromatic hydrocarbons. The HZSM-5 catalyst can enhance the yield and selectivity of desirable hydrocarbons in commercial production. The microwave catalytic copyrolysis has opened a new era for future explorations because of its superior performance in upgrading the bio-oil.

Acknowledgements:We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21766019, 21466022), the Key Research and Development Program of Jiangxi Province (20171BBF60023),the International Science & Technology Cooperation Project of China (2015DFA60170-4), the Science and Technology Research Project of Jiangxi Province Education Department (No.GJJ150213), and the Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development Program (No. Y707sb1001)”.