Advances in thermochemical conversion of woody biomass to energy, fuels and chemicals

Accepted Manuscript Advances in thermochemical conversion of woody biomass to energy, fuels and chemicals

Shusheng Pang PII: DOI: Reference:

S0734-9750(18)30180-0 https://doi.org/10.1016/j.biotechadv.2018.11.004 JBA 7315

To appear in:

Biotechnology Advances

Received date: Revised date: Accepted date:

30 November 2017 10 November 2018 13 November 2018

Please cite this article as: Shusheng Pang , Advances in thermochemical conversion of woody biomass to energy, fuels and chemicals. Jba (2018), https://doi.org/10.1016/ j.biotechadv.2018.11.004

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ACCEPTED MANUSCRIPT Advances in Thermochemical Conversion of Woody Biomass to Energy, Fuels and Chemicals Shusheng Pang Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand, email: [email protected]

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University, Jiaozuo, China;

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Adjunct Professor, School of Mechanical and Power Engineering, Henan Polytechnic

Adjunct Professor, Henan Centre for Outstanding Overseas Scientists, School of Chemical

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Engineering and Energy, Zhengzhou University, Zhengzhou, China

Abstract

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Biomass has been recognised as a promising resource for future energy and fuels. The biomass, originated from plants, is renewable and application of its derived energy and fuels is close to carbon-neutral by considering that the growing plants absorb CO2 for

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photosynthesis. However, the complex physical structure and chemical composition of the

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biomass significantly hinder its conversion to gaseous and liquid fuels. This paper reviews recent advances in biomass thermochemical conversion technologies for

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energy, liquid fuels and chemicals. Combustion process produces heat or heat and power from the biomass through oxidation reactions; however, this is a mature technology and has been successfully applied in industry. Therefore, this review will focus on the remaining

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three thermochemical processes, namely biomass pyrolysis, biomass thermal liquefaction and biomass gasification. For biomass pyrolysis, biomass pretreatment and application of catalysts can simplify the bio-oil composition and retain high yield. In biomass liquefaction, application of appropriate solvents and catalysts improves the liquid product quality and yield. Gaseous product from biomass gasification is relatively simple and can be further processed for useful products. Dual fluidised bed (DFB) gasification technology using steam as gasification agent provides an opportunity for achieving high hydrogen content and CO2 capture with application of appropriate catalytic bed materials. In addition, multi-staged

ACCEPTED MANUSCRIPT gasification technology, and integrated biomass pyrolysis and gasification as well as gasification for poly-generation have attracted increasing attention. Keywords: Woody biomass, pyrolysis, liquefaction, gasification, bio-oil, liquid fuel, biochemicals

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1.0 Introduction Global energy and chemicals presently consumed are largely derived from fossil fuels, which

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induces two major concerns including uncertainty for sustained supply of future energy and

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greenhouse gas (GHG) emissions. Alternative resources to the fossil fuels have been sought to mitigate these issues, and various renewable resources have been or are being explored which include solar, wind, geothermal and biomass resources. Among these renewables,

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biomass is the abundant and potential candidate which can be processed for energy, fuels and chemicals. Biomass is a term covering organic materials originated from vegetation plants

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and organic wastes. The biomass, while growing, absorbs CO2 through photosynthesis and thus can be regarded as a carrier of energy by the chemical bonds among carbon, hydrogen

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and oxygen molecules.

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The biomass resource is estimated to be 146 billion tonnes per annum. If 10% of the biomass (considering residues from forest harvesting, wood processing and agriculture sector) is used for energy at a conversion efficiency of 50%, then it will generate 3.1 trillion tonnes of oil

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equivalent energy which would be over 200 times the world energy consumption in 2015 (EIA 2017). Alternatively, if 10% biomass is used for production of organic chemicals at a

produced.

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conversion rate of 10%, then 1.6 billion tonnes of such chemical feedstocks could be

However, processing of biomass at full commercial scale are, at present, largely for heat or heat and power generation although some pilot scale and commercial scale plants have been reported in last decade for production of bio-oil. This is largely due to low conversion efficiency, high costs and complexity in processing which are, in turn, attributed to complex physical structure and chemical compositions of the biomass. Extensive research and development have been undertaken over the world in past decades, however, there are still challenges to overcome the above mentioned obstacles (Klass 1998, Bridgwater 2003, Demirbaş 2001, Corma et al. 2007, Ennaet et al. 2016, Sansaniwal et al. 2017).

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There are many routes for conversion of woody biomass to energy, fuels and chemicals, and these have been classified into two major categories: thermochemical processes and biochemical processes. The thermochemical processes include combustion, pyrolysis, gasification and thermal liquefaction while the biochemical processes include fermentation and digestion. This review will present recent advances in research and development on thermochemical conversion technologies of woody biomass for production of energy, fuels

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and chemicals. The combustion process generates heat from the biomass through oxidation combustion reactions. As this is a mature technology and has been in operation at commercial

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scale, this review will focus on the remaining three processes, namely pyrolysis, gasification

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and thermal liquefaction. In order to better understand the difficulties and challenges involved in the biomass conversion, physical structure and chemical composition of the

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biomass are described first.

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2.0 Characteristics of Woody Biomass

Plant stems are the major and most important part of biomass which have relatively

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predictable structures and compositions. The stems in plants serve two functions including (1) physically supporting the plants so that the plant leaves receive needed sunlight for

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photosynthesis, and (2) providing channels for transportation of water and minerals from ground to leaves, and to transport nutrients (sugars and derivatives) for stem growing. In order to serve these two functions, the plant stems have optimised their physical structures

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and chemical compositions through millions of years of evolution. Harrington (2002) has prepared a comprehensive drawing on the macro and microstructure of a tree stem as shown

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in Figure 1.

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Figure 1. Macro and microstructures of the stem of a softwood tree (Harrington 2002).

For biomass conversion, the most influencing characteristics are the microstructure and the

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chemical composition. At micro level, the wood is composed of fibrous elements termed as tracheids which are hollow fibrous cells interconnected to each other. For softwood, these cells have dimensions of 2-4 mm in length, 15-60 m in diameter and cell wall thickness of

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1-6 m (Walker 2006, Kininmonth and Whitehouse 1991). On the cell walls, there are a

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number of sub-layers comprising numerous microfibrils. Hardwood also has cells but the hardwood is characterised by having irregularly large tracheids among the regular cells.

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These fibrous cells provide mechanical strength and stiffness for the plant stem to support the

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crown. Simultaneously these hollow cells allow water to flow upwards from roots to leaves.

For the aspect of chemistry, the wood is composed of cellulose, hemicelluloses and lignin. These chemicals are generated from polymerisation of monosaccharides (glucose, fructose

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and galactose) which are resulted from photosynthesis on leaves (Walker 2006). Cellulose

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has a relatively simple chemical structure consisting of at least ten thousands of -D-glucose units through glucosidic linkages. In this way, the cellulose chemical formula can be

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represented by (C12H20O10)n where n is the number of -D-glucose units.

Hemicellulose is a collective term as it includes a number of different molecules, therefore, in

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many references hemicelluloses are used. The hemicelluloses are mainly branched hydrocarbons with major components being the pentose sugars, the hexoses sugars, Larabinose and D-xylose, D-glucose, D-mannose and D-galactose.

Lignin is the most complied component in wood consisting of aromatic molecules which are insoluble in most solvents. Lignin is the most stable component in wood and is difficult to break down into monomeric units. Softwood lignin is largely composed of cross-linked guaiacylpropane units while hardwood also contains syringylpropane (25-50% mass) as well as a small fraction of dryoxyphenylopropane (Walker 2006).

ACCEPTED MANUSCRIPT In addition to the above three major components, wood also contains extractives such as resin and mineral elements. Extractives of wood are important for wood utilisation which major components include fatty acid, fatty acid esters, resin acids, phenols and unsaponifiables. The fraction of extractives varies from 1% to 20% of wood mass, depending on wood species, position within a tree and location of the tree growing (Uprichard and Lloyd 1980). The mineral elements in wood will result in ash after combustion or gasification. These elements

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contribute to 0.1-0.7% of wood mass and include silica (Si), calcium (Ca), potassium (K), magnesium (Mg) and phosphorus (P) (Misra et al. 1993, Penninckx et al. 2001, Wigley 2016). There are some exceptions for the ash content which needs careful consideration in the

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thermochemical conversion process.

Typically, softwoods have 42±2% cellulose, 27±2% hemicelluloses, 28±3% lignin and 3±2%

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extractives while the corresponding chemical components for hardwoods are 44±2%, 28±5%, 24±4% and 4±3%, respectively (Walker 2006). Each component of the wood affects the thermochemical conversion of biomass in a different way and this will be discussed in the

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following sections.

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For conversion of biomass to energy, fuels and chemicals, approximate and ultimate analyses of the feedstock are also important and a comprehensive review on this topic can be found in

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Tanger et al. (2013). For woody biomass of three species (pine, holm-oak and eucalyptus),

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the analysis results are given by Franco et al. (2003) as listed in Table 1.

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Table 1. Results of proximate and ultimate analyse of three species of woody biomass (Franco et al. 2003) Pine Holm-oak Proximate analysis, wt% 71.5 70.2 16.0 17.8 0.5 2.4 12.0 9.5 Ultimate analysis, wt% ash-free 51.6 51.1 4.9 5.3 0.9 0.9 42.6 42.7 20.2 19.4

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Volatiles Fixed carbon Ash Moisture

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C H N O HHV*, MJ/kg

74.8 13.9 0.7 10.6 52.8 6.4 0.4 40.4 21.3

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Note: * high heating values (HHV) of fuels.

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3.0 Biomass Fast Pyrolysis 3.1

Eucalyptus

Factors affecting bio-oil composition

Biomass pyrolysis is a thermochemical decomposition process in absence of oxygen with

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possible products of liquid (bio-oi), solid (char) and gas. The fractions of these products will

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depend on biomass type, operation temperature, heating rate and residence time. For target product of liquid, fast heating rate, rapid quenching and appropriate operation temperatures of 400 to 650C are required. Extensive reviews have been published on biomass pyrolysis and

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this review will focus on target product of bio-oil and recent advances for improving the quality of the bio-oil (Zhang et al. 2007, Wang et al. 2017, Bridgwater 2012).

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Different component of biomass decomposes at different temperatures and forms different products, but the bio-oil chemical composition is also dependent on heating rate and residence time (Shen and Gu 2009, Wigley 2016). Cellulose decomposes at temperatures between 250 and 350C to form levoglucosan and other anhydrocelluloses. Lignin has the most complicated structure and is most stable among the three major components which decomposes at temperatures from 280 to 500C to form oligomers and monomers of polysubstituted phenols (Mohan et al. 2006, Pandy and Kim 2011). Hemicellulose has highly oxygenated side branches which are relativity easy to break down when being heated (Wigley 2016, Mohan et al. 2006), therefore, hemicellulose is the first component in biomass to decompose at temperatures between 180-320 °C (Wang et al. 2009a, Xu 2002, Aho et al.

ACCEPTED MANUSCRIPT 2007, Lv et al. 2010). The decomposition of hemicelluloses produces acetic acid and other organic acids, sugars, and furans (Murwanashyaka et al. 2002, Nowakowski 2007). Recently, Li et al. (2017) conducted a study to establish correlations between bio-oil compound distribution from fast pyrolysis and biomass feedstock species. They tested 20 types of biomass feedstocks using Pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS) with a final pyrolysis temperature of 550C and heating rate of up to 10000C/s, and found

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that phenolic compounds increased with lignin content, ketones increased with cellulose content and furans increased with hemicelluloses. However, the short chain acids decreased

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with ash content and hydrocarbons decreased with cellulose content in the biomass. In results, bio-oil from biomass pyrolysis has a very complex chemical composition, making it

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problematic at its original state for applications in engine or as chemical feedstock (Mohan et

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al. 2006, Czernik and Bridgwater 2004, Lliopouloua et al. 2012).

Recently, a number of pilot-scale and commercial-scale pyrolysis plants have been constructed

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(Pyrowiki 2018, Cai and Liu 2016). Cai and Liu (2016) reported the performance of a pyrolysis system with biomass (rice husk) feeding of 1000 to 3000 kg/h. In this system, the biooil yield of 48.1% was achieved at pyrolysis temperature of 550 C. BTG Biomass Technology

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Group BV (2018) has constructed a full commercial scale plant (Empyro project) in Hengelo, The

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Netherlands, which produces 20 million liters per year bio-oil from woody biomass. Perkins

(2018) rerviewed latest advances in commercailicsation of biomass pyrolysis technologies at various scales with biomass feeding rates from 1 to 274 tonnes per day (tpd). In all of the

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reported plants, bio-oil is produced for combustion in boilers for generation of process steam or heat and power. In order for the bio-oil to be used as transport liquid fuel or chemicals,

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separation and upgrading are needed.

Bio-oil upgrading

Extensive studies on bio-oil upgrading have been reported in last two decades and comprehensive reviews were recently published by Xiu and Shahnazi (2012), Bridgwater (2012), Zhang et al. (2013) and Wang et al (2013a). The bio-oil from biomass pyrolysis can be upgraded through hydrotreating, hydro-cracking, catalytic cracking, solvent addition and emulsion while the choice is dependent on the target applications of the upgraded bio-oil. For example, if the upgraded bio-oil is to be used as transport fuel, hydrotreating is most common. Hydrotreating can be performed at temperatures of 200 to 400C and pressures of 2-5 MPa

ACCEPTED MANUSCRIPT which process involves oxygen removal through hydro-deoxygenation (HDO), converting the complicated bio-oil to hydrocarbons and water as products. The composition of hydrocarbon product is affected by temperature, pressure and catalysts applied. The upgraded bio-oil has much better properties but it still needs further refining for application as transport liquid fuel. Therefore, the complete process of upgrading of bio-oil and refining is complicated and expensive if the target liquid is for transport fuel (Bridgwater 2012, Wang et al. 2013a).

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However, if the liquid is used as combustion fuel or as heavy engine fuel with better stability and high calorific value as compared with the original bio-oil, one-step supercritical alcohol upgrading is a promising process (Li et al. 2011, Peng et al. 2008, Prajitno et al. 2016, Jo et al.

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2017, Zeb et al. 2017). The supercritical ethanol upgrading can increase the HHV of bio-oil

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from approximately 20 MJ/kg to 36 MJ/kg (Zeb et al. 2017). A separate study by Prajitno et al. (2016) shows that the HHV of upgraded bio-oil was 34.1 MJ/kg with water content of 1.6

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wt.%. These values are similar to the hydrotreating upgraded boil-oil (40 MJ/kg) as reported by Wildschut et al. (2009).

Pretreatment of biomass for pyrolysis

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Recently further research has been undertaken aiming to improve the bio-oil quality and thus to simplify the upgrading process. This includes biomass pretreatment, catalytic pyrolysis and

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combination of both as reported by Wigley et al. (2015, 2016, 2017) who pretreated radiata pine

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wood particles using dilute acid leaching followed by torrefaction. The acid leaching was to remove undesirable inorganic elements which otherwise would promote secondary reactions for pyrolysis of vapours. The torrefaction was to remove water and reduce oxygen content in

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the biomass. It was found that with 1% acetic acid leaching for 4 hours, the inorganic content of the biomass was reduced from 0.41 wt% to 0.16 wt%. With torrefaction of biomass at 270

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C for 20 min following the acid leaching, the bio-oil from pyrolysis of the pretreated biomass had much reduced organic acids and pyrolytic lignin, but was rich in levoglucosan and aromatics as shown in Figure 2 (Wigley et al. 2016, 2017). Most importantly, water content in the bio-oil was reduced by 24 wt% for the untreated wood and by 4.3 wt% for the pretreated wood. Correspondingly, molecular weight of bio-oil was reduced from 327 (g/mol) for untreated wood to 273 (g/mol) for the pretreated wood. Furthermore, the bio-oil yield was increased to 57.8% for the pretreated wood in comparison with 55.3% for pyrolysis of the untreated wood under the same pyrolysis conditions.

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pretreated wood (Preteated bio-oil) (Wigley et al. 2017).

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Figure 2. 1H NMR peak areas for bio-oils from pyrolysis of untreated wood (Raw bio-oil) and

Catalytic pyrolysis

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Catalytic pyrolysis has also attracted great interests for improving the bio-oil quality (Aho et al. 2010, Lu et al. 2010, Nguyen et al. 2013, Zhang et al. 2014, Heracleous et al. 2017). The

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catalytic pyrolysis has advantages that it does not require hydrogen, operates at atmospheric pressure and can upgrade pyrolysis vapours either in-situ or ex-situ (Aho et al. 2010, Heracleous et al. 2017). The key part of this approach is the selection of suitable catalysts and

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optimum operation conditions. The following table (Table 2) lists reported catalysts,

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operation conditions and resultant improvements of bio-oils (Aho et al. 2010, Lu et al. 2010, Nguyen et al. 2013, Zhang et al. 2014). From these studies, the yield and composition of the bio-oil from catalytic pyrolysis of woody biomass are affected by the catalysts applied and

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the operation conditions used. In general, the bio-oil consists of more light hydrocarbons and phenols but less oxygen. However, the results on bio-oil yield are inconsistent, some reported

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that the yield was reduced (Hernando et al. 2017) whie others showed that the bio-oil yield was similar to that of non-catalytic pyrolysis (Aho et al. 2010, Nguyen et al. 2013).

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Challenges and potential opportunities on biomass pyrolysis

Biomass pyrolysis is a simple process to convert biomass to bio-oil but the bio-oil has complicated compositions and it can only be used for direct combustion without further separation and upgrading. If the target application of bio-oil is for transport liquid fuel or engine fuel, upgrading is necessary which is complicated and, consequently, expensive. Pretreatment of biomass and catalytic pyrolysis can improve the bio-oil quality and the corresponding upgrading process could be simplified, however, no report has been found so

ACCEPTED MANUSCRIPT far in literature on upgrading of the bio-oil from catalytic pyrolysis. In addition, separation of the bio-oil may also provide an opportunity to simplify the upgrading process as some compounds separated may be directly used as chemical feedstock. If the target utilization of the liquid fuel is for combustion and heavy combustion engine, one-step upgrading of bio-oil

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using supercritical alcohol may be a promising process.

ACCEPTED MANUSCRIPT Table 2. Effect of catalysts applied in pyrolysis of woody biomass. Biomass

Catalysts

Operation conditions

Results

Aho et al. (2010)

Pine wood (Pinus sylvestris)

Acidic zeolite (Proton form)

Bench fluidised bed reactor; 400C pyrolysis 450C vapour catalytic upgrading

Nguyen et al. (2013)

Canadian white pine (Pinus strobus)

Bench fixed bed reactor, 500C

Lu et al. (2010)

Poplar wood

Zhang et al. (2014)

Sawdust

 Na-faujasite (Na-FAU),  Na0.2H0.8faujasite (Na0.2H0.8-FAU)  H-faujasite (HFAU)  TiO2 (Rutile) (T1),  TiO2 (T2)  ZrO2&TiO2 (T3) Fe(III)/CaO

 Bio-oil yield: 43.5-52.7%  Levoglucosan decreased  Methyl substituted phenols increased  Methoxy substituted phenols decreased  Bio-oil yields: 46.3-56%  Phones and hydrocarbons increased  Carboxylic acids decreased  T1 increased phenols  T3 increased hydrocarbons, light linear ketones and cyclopentanones  Heavy phenols transformed to light phenols  Furans, light and aromatic hydrocarbons increased  Acids, aldehydes and ketones decreased

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Bench fixed bed reactor, 500-600C

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Bench fixed bed reactor, 700C

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References

4.0 Biomass Thermal Liquefaction Biomass liquefaction is similar to biomass fast pyrolysis in the way that both processes aim

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to produce liquid as the target product. However, biomass liquefaction reactions occur in a liquid medium and, in most cases, under pressures. In this way, liquefaction process can

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handle biomass with high moisture content while pyrolysis needs biomass to have a moisture content of below 10% in order to reduce water content in the bio-oil. The biomass liquefaction technologies can be divided into three groups: 1). Hydrothermal liquefaction; 2). Liquefaction with solvents, and 3). Liquefaction with solvent and catalysts. The liquid product from liquefaction has lower oxygen and water contents, and is less complicated. However, the liquefaction process is more complicated as it operates at high pressures and/or uses solvents and catalysts.

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Hydrothermal liquefaction

Hydrothermal liquefaction (HTL) process operates at temperatures between 200 and 400C and pressure of 5-25 MPa (Dimitriadis and Bezergianni 2017; Kumar et al., 2017; Gollakota et al. 2017). In the HTL process, water is normally used as working medium for enhanced heat transfer and biomass decomposition. It is known that water has a critical temperature of 373C and critical pressure of 22.1 MPa, therefore, the HTL process can be operated either at

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subcritical condition (e.g., 250C and 5 MPa) or supercritical condition (e.g., 400C and 25 MPa).

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In past decade, extensive studies have been conducted to understand the mechanisms of the

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HTL process and the effect of operation conditions (temperature, pressure and residence time) on the product yield and composition (Gollakota et al. 2017). The detailed mechanisms of

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biomass liquefaction are still being explored and there is a general agreement that in HTL, the biomass undergoes depolymerisation, decomposition, and recombination and polymerisation (Gollakota et al. 2017, Demirbas 2000). In the initial stage of liquefaction, the

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macromolecules of cellulose, hemicelluloses and lignin are broken down to micellar-like fragments. These fragments are then decomposed to smaller compounds through a series of

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reactions such as dehydration (removal of H2O), dehydrogenation (removal of H2), deoxygenation (removal of O2) and decarboxylation (removal of CO2) as well as deamination

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(removal of amino acids). These compounds, once produced, may rearrange to form new compounds through condensation, cyclization and polymerization. Changing operation conditions and addition of catalysts and solvents may change the above reactions thus

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changing the composition of resultant liquid products.

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Temperature is the most important operation parameter in biomass liquefaction (Dimitriadis and Bezergianni 2017, Cheng et al. 2010, Cheng et al. 2017, Akhtar and Amin 2011) as all of the reactions mentioned above are affected by temperature. In the temperature range of 200 400C, gas yield increases and solid residues decreases with increasing temperature. However, there exits an optimum temperature with maximum liquid yield which varies with biomass species and operation pressure or application of solvent and catalysts (Dimitriadis and Bezergianni 2017). For example, Cheng et al. (2010) found that the liquid yield was 66% for while pine sawdust at operation temperature of 300C without catalyst. Brand et al. (2013) reported the maximum liquid yield of 60% at 400C for red pine sawdust with ethanol as solvent. Pressure is also an important operation parameter which maintains the liquefaction

ACCEPTED MANUSCRIPT medium in the reactor as liquid phase. However, above a certain value, the pressure has insignificant effect on the liquid yield and composition (Akhtar and Amin 2011, Brand et al. 2013). Residence time is another important parameter in biomass liquefaction on liquid yield, and the bio-oil yield increases with reaction time in most cases. Brand et al. (2013) found that with an increase in reaction time from 20 to 60 min, the bio-oil yield increased from 37.1 wt%

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to 52.0 wt%. Further increase in reaction time, up to 240 min, was found to enhance the biooil yield, to some extent, to 59.2 wt%. However, some studies show that the reaction time

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enhanced the bio-oil yield to a certain threshold beyond which the bio-oil yield decreased (Xu and Lancaster 2008, Cheng et al. 2010, Yip et al. 2009, Liu and Zhang 2008). This optimum

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reaction time varied from 20 min to 120 min depending on the biomass species, temperature

4.2

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and application of solvent and catalyst (Dimitriadis and Bezergianni 2017). Application of solvent for biomass liquefaction

Application of solvents in biomass liquefaction have been found to enhance heat transfer and

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biomass decomposition as well as to act as reactant in most cases. Selection of suitable solvents for the biomass liquefaction is important which not only affects the bio-oil yield but also the chemical composition (Liu and Zhang 2008, Biller et al. 2016, Durak and Aysu 2014,

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Cheng et al. 2017). The most common solvent used is water due to its low cost, readily

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availability of property data and solubility of many components of bio-oil at high temperature and high pressure conditions around the critical point (373.95C and 22.06 MPa). In recent years, various other solvents have been tested which include ethanol (Cheng et al. 2010, Xu

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and Etcheverry 2008, Brand et al. 2013, 2014), methanol (Yang et al. 2009, Liu and Zhang 2008, Cheng et al. 2010), acetone (Liu and Zhang 2008, Wang et al. 2013b), glycerol

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(Demirbas 2008, Jin et al. 2011), phenols (Maldas and Shiraishi 1997, Wang et al. 2009b) and ethylene glycol (Yip et al. 2009).

In the biomass liquefaction with solvents, the solvents play two roles: 1). Enhancing heat and mass transfer between the solvents and the biomass particles; 2). Reactants with fragments from biomass decomposition. Zou et al. (2009) conducted studies on liquefaction of poplar wood using three alcoholic solvents including monohydric n-octanol, dihydric ethylene glycol, and trihydric glycerol. Based on thermogravimetry (TG) analysis, they proposed that with presence of solvents, the molecules of solvent were combined with C-OH or L-OH bonds of fragments from biomass decomposition in the first stages of liquefaction. In this

ACCEPTED MANUSCRIPT way, the molecular weight of products was increased and heavy oil was promoted with polyhydric alcohols. These results were supported by TG, DTG and DSC curves and measured heavy bio-oil yields. With applications of monohydric n-octanol, dihydric ethylene glycol, and trihydric glycerol, the heavy oil yields were 11 wt.%, 41 wt.% and 55 wt.%, respectively. Brand and Kim (2015), based on their analysis, proposed that the solvent added retarded the char/tar formation reactions (re-polymerisation) thus increasing the bio-oil yield.

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Some solvents can act as hydrogen donor which would also upgrade the bio-oil by reducing oxygen content (Isa et al. 2017). Such hydrogen donor solvents include alcohol, decalin, glycerol and tetralin, and these solvents are reported to be more effective than injection of

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hydrogen gas as applied for upgrading of biomass pyrolysis bio-oil. Some of these hydrogen

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donor solvents have been used in coal liquefaction (Cronauer et al. 1979, Kouzu et al. 2000)

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which concept has recently been applied to biomass liquefaction (Deng et al. 2015).

Application of catalyst for biomass liquefaction

In biomass liquefaction, catalysts were also applied to further increase the bio-oil yield and to

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improve the bio-oil quality. In reported studies, both homogeneous and heterogeneous catalysts have been investigated (Kumar et al. 2017). The homogeneous catalysts are mainly

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alkali salts such as Na2CO3, K2CO3, KHCO3, NaOH and KOH. These catalysts reduce the tar/char formation and promote water-gas shift reaction thus enriching hydrogen in the

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medium which improves bio-oil quality. In addition, the catalysts provide an opportunity for decarboxylation reaction between biomass hydroxyl groups and formate ions in alkali carbonates to form esters. These esters then promote dehydration, deoxygenation, and

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decarboxylation of fragments from biomass decomposition. A comprehensive study was conducted by Jindal and Jha (2016) who investigate the effect of alkalis (NaOH, Na2CO3,

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KOH and K2CO3) on hydrothermal liquefaction of waste furniture sawdust in a batch reactor operating at 280°C for 15 min, and found that the highest bio-oil yield (34.9 wt%) with low oxygen content was achieved in the presence of 1.0 M K2CO3 catalyst. Similar studies were performed by Nazari et al. (2015) on Hydrothermal liquefaction of birchwood sawdust with a number of alkali salts as catalysts. They found that the best performance catalyst was KOH which increased the bio-oil yield to 40 wt%, more than double the yield of the uncatalyzed liquefaction. Heterogeneous catalysts have also been used in biomass liquefaction which include zeolite based catalysts Zn/HZSM-5 (Cheng et al. 2017a) and Ni/HZSM-5 (Cheng 2017b), which

ACCEPTED MANUSCRIPT were applied in liquefaction of pine sawdust with ethanol as solvent. It is reported that the catalysts applied reduced the content of the undesirable oxygenated compounds (acids, ketones) but increased the desirable hydrocarbon content. Cheng et al. (2017a) also compared effects of different catalysts on the bio-oil yields and compositions as given in Table 3. Table 3. Chemical compositions of bio-crudes before evaporation from different treatments (Cheng et al. 2017a). No catalyst

HZSM-5

5%Zn/ HZSM-5

10%Zn/ HZSM-5

15%Zn/ HZSM-5

20%Zn/ HZSM-5

Phenols Furans Ethers Aldehydes Ketones Esters Alcohols Acids Hydrocarbons Others Bio-oil yield (wt.%) HHV (MJ/kg)

17.8 0.7 2.2 4.5 19.3 16.8 14.5 15.9 4.2 4.0 43.0

9.1 1.4 0.8 3.1 17.2 19.4 8.4 7.3 6.5 26.7 47.0

8.0 0.1 2.1 0.2 17.2 20.3 7.1 5.7 8.3 30.8 51.0

4.2 0.3 1.3 1.6 13.2 33.8 5.9 6.2 15.0 18.4 53.0

6.5 0.1 1.0 1.0 14.1 34.4 7.1 6.3 12.1 17.4 59.0

4.0 0.0 1.9 0.3 17.3 37.5 7.8 5.1 10.2 15.9 54.0

29.0

31.9

32.9

32.9

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33.2

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33.1

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Content (%)

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From the above discussion, the high heating values (HHV) of bio-oils from catalytic liquefaction of biomass are much higher and their chemical composition is much simpler than those of bio-oils of biomass pyrolysis. However, the HHV is still lower and the chemical

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composition are more complex in comparison with those of liquid fuels derived from crude

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oil, therefore, further separation and upgrading are still necessary.

In last decade, some pilot scale and commercial scale hydrothermal liquefaction plants have been constructed. For example, LicellaTM (2018) has constructed staged plants located in New South Wales, Australia, with capacities from 100 to 125000 tonnes slurry per annum. Recently, Steeper Energy (2018) has announced that it would be in partnership with Silva Green Fuel, a Norwegian-Swedish joint venture, to construct a demonstration thermal liquefaction plant with a capacity of about 4000 liters liquid product per day. In the Steeper Energy plant, the biomass material will consist of residual products from the forest industry.

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5.0 Biomass Gasification Biomass gasification is another thermochemical process in which the biomass is converted to a gaseous product, termed as producer gas or product gas, which consists mainly of CO, H2, CO2 and CH4 as well as other hydrocarbon species. The gasification process operates at temperatures from 700 to 1200C. Gasification agents such as O2, air, steam and CO2 or their

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mixtures are used in the biomass gasification process. When O2 or air is used as the gasification agent, partial combustion reactions occur which provide heat needed for other

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endothermic reactions. On the other hand, when steam is used as gasification agent, the

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overall gasification reactions are endothermic thus external heat supply is needed to the gasification reactor.

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Different gasification technologies are available which include fixed bed gasifiers (updraft and down draft), fluidised bed gasifiers (bubbling and circulating) and entrained flow

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gasifiers. Recently new gasification technologies have been reported aiming to reduce tar content, increase hydrogen content in the producer gas and increase energy efficiency of the biomass gasification (Sikarwar et al. 2016, Sansaniwal et al. 2017, Watson et al. 2018). More

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details on description of these gasifiers can be found in Pang (2016) and Knoef (2005), and

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this review only focuses on recently developed new technologies. The objective of new and improved biomass gasification technology is for the producer gas to have desirable composition for further application such as heat and power generation, chemicals, pure H 2,

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synthesis of natural gas or synthesis of liquid fuel. Different applications may require different gas composition, however, high content of H2 and high heating value are desirable

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for most applications. In biomass gasification with air as gasification agent, the producer gas is diluted with N2 which is carried in by the air but is not involved in favourable reactions. Therefore, the heating value of the producer gas is low (4-7 MJ/Nm3) and the energy efficiency is low, too, due to the heat consumed for N2 heating up. When oxygen is used as the gasification agent, the issues with N2 dilution are resolved but oxygen cost is high, compromising the economic benefits.

5.1 Biomass steam gasification in a dual fluidised bed system Steam has been recognised as the ideal gasification agent as it enhances a number of favourable gasification reactions as follows:

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Steam char reaction: C + H2O  H2 + CO

(1)

Water-gas shift reaction: CO + H2O  H2 + CO2

(2)

Steam methane reforming reaction: CH4 + H2O  3H2 + CO

(3)

However, the steam biomass gasification reactions, in overall, are endothermic and thus external heat is needed to maintain the gasification operation at set temperatures.

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Conventional heat transfer techniques using heat exchangers are not suitable as the heat transfer coefficient outside the heat exchanger surface is low. In addition, the flow of biomass

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and gases outside the heat exchanger generates severe wear on the heat exchanger surface.

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In order to solve the above engineering problems, a new gasification technology, dual fluidised bed (DFB) steam gasification, has been developed as illustrated in Figure 3 (Saw

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and Pang 2012) and reported by Pfeifer et al. (2004), Holfbauer and Knoef (2005), Rauch et al. (2013), and Saw et al. (2012). The DFB gasifier comprises two columns, one bubbling

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fluidised bed (BFB) column (BFB gasifier) and one circulating fluidised bed (CFB) column (CFB combustor). It uses steam as gasification agent and supplies needed heat to the endothermic gasification process by internally circulating bed material. In the DFB

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gasification system, solid fuel is fed to the bed of the BFB gasifier and steam is injected from

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the gasifier bottom. The char generated from the steam gasification together with the bed materials flows to the CFB column in which air is introduced and the char is combusted for heating up the bed materials. Then the heated bed materials and the fluegas in the CFB

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column flow up and out of the column to a cyclone which separates the bed materials and the flue gas. The flue gas exits the cyclone from the top for heat recovery and the hot bed material flows out from the cyclone bottom and then into a siphon. The siphon acts as a seal

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to prevent gas cross flow between the two columns. Finally the hot bed material flows back to the BFB gasifier to provide needed heat for the steam gasification. The producer gas from the BFB gasifier flows out of the reactor from the top and then into another cyclone for separation of ash and entrained particles.

The DFB steam gasification system can produce hydrogen-rich producer gas and achieve high overall energy efficiency. However, in the DFB gasification system, bed material circulates through two columns, and flows in siphon and chute, which needs careful operation to maintain steady conditions.

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The DFB gasification system also provides an opportunity to apply various catalytic bed materials top achieve target gas composition. An example is to use calcite as the bed material for increasing H2 content in the producer gas, and for CO2 capture through calcination and carbonation cycles. The fundamental concept is illustrated in Figure 4 in which the calcite bed material acts in two roles, one being the heat carrier as normal bed material and the other

FFB cyclone

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Sampling port Flue gas

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being a CO2 shift agent from the BFB gasifier to the CFB combustor.

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Producer gas Flue gas Flue gas + Bed material Bed material Siphon

Secondary air

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Chute

Bubbling Fluidised Bed (BFB)

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Fast Fluidised Bed (FFB)

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Bed materia charger

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Steam

Sampling port Producer gas BFB cyclone Trap collector

Fuel hopper

Screwfeeder

LPG

Steam Primary air

Figure 3. Sketch of a dual fluidised bed gasifier (Saw and Pang 2012).

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Figure 4. Concept of carbonation and calcination by applying calcite as bed material in DFB

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gasifier (Pfeifer et al. 2009).

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In the BFB gasifier, CaO absorbs CO2 to form calcium carbonate (CaCO3): H298 = - 178.8 kJ/mol

(4)

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Carbonation reaction: CaO + CO2  CaCO3

The carbonation reaction is favoured at temperatures of 700-750C (Lee et al. 2004). When

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the calcium carbonate flows to the CFB combustor with char, the char combustion heats up the calcium carbonate to above 800 - 850C which promotes the calcium carbonate thermal

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decomposition reaction to form CaO and release CO2.

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Calcination reaction: CaCO3  CaO + CO2

H298 = 178.8 kJ/mol

(5)

This thermal decomposition is also called calcination or the reverse carbonation reaction. In

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effect, this carbonation and calcination cycle shifts CO2 from the gasification reactor to the combustion reactor, therefore, the CO2 concentration in the producer gas is decreased and

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that in the flue gas is increased. In addition, due to the CO2 concentration reduction in the gasification reactor, the water-gas shift reaction (Eq.2) is favoured and H2 concentration is further increased in the producer gas. Our research team has conducted experiments at a 100 kW DFB gasifier with different catalytic bed materials and gasification temperatures using radiata pine wood pellets. From these experiments, selected results on producer gas compositions are given in Table 4.

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Table 4. Selected results from experiments on effect of bed materials and operation temperature on producer gas composition. CO

CH4

CO2

H2/CO

LHV, MJ/Nm3

767

29%

32%

12%

23%

0.9

13.3

700

35%

29%

12%

19%

1.2

12.9

694

40%

20%

12%

23%

2.0

12.8

699

40%

23%

12%

20%

713

50%

18%

11%

716

62%

15%

11%

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H2

1.7

13.8

18%

2.8

12.3

11%

4.1

12.5

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100% Silica sand (control) 100% Olivine 50% Olivine 50% Calcite 50% Silica sand 50% Dolomite 50% Silica sand 50% Calcite 100% Calcite

BFB Temp, °C

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Bed Material

From the above results, if the producer gas is to be used for power generation, silica sand can

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be used at high temperature which lower heating value is reasonably high. If the gas is used for pure hydrogen, the calcite bed material can be used with hydrogen content of 62%. When

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the gas is for liquid fuel production using Fischer-Tropsch process, then mixture of 50% olivine sand and 50% calcite is preferred as the ratio of hydrogen to CO is 2 which is deal for

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5.2 Multistage gasification

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the F-T reactions.

Tar contamination in the producer gas has been a technical challenge for commercialisation of the biomass gasification technologies. Tar is a collective term for organics (largely

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aromatics) with molecular weights higher than benzene, produced under gasification of organic material. Tar compounds exist in vapours at gasification temperatures, but condense

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when the temperature is reduced to or below their due points, which would induce problems in down-stream applications of the producer gas. Post-gasification gas cleaning can be achieved using technologies of thermal cracking, catalytic reforming and solvent scrubbing. However, all of these technologies consume energy and increase the process costs. Based on fundamental understanding of the gasification process, a multi-stage gasification technology has been developed (Henriksen et al. 2006, Gómez-Barea et al. 2013, Leijenhorst et al. 2015, Heidenreich and Foscolo 2015). In the two-stage gasification process (Henriksen et al. 2006, Leijenhorst et al. 2015), the first stage is for devolatilization of biomass at lower temperatures, which generates a gas mixture including tar compound vapours and solid char. The gas

ACCEPTED MANUSCRIPT mixture and char undergo a high temperature process in the second stage gasification, which cracks or reforms the tar compounds into simple gas species.

Recently, a three-staged gasification process was reported for further improving the gas quality (Gómez-Barea et al. 2013, Heidenreich and Foscolo 2015). In this system, the first stage gasification is similar to the two-staged gasification process, but the gas mixture is

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separated from the sold char and then undergoes a high temperature reforming reactor as the second stage reactor. The solid char is finally mixed with the cleaner gas from the second stage reactor and is further gasified with steam. Consequently, the producer gas is cleaner and

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its quality is improved. With two-staged biomass gasification, the cold gas efficiency was

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reported to be 93%, the tar content in the producer gas was 15 mg/nm3 and the hydrogen concentration was 32% with HHV of 6.6 MJ/nm3 (Heidenreich and Foscolo 2015). The three-

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staged gasification further reduced the tar content to 10 mg/nm3 but other gas quality paramneters were not as good as those achieved from the two-staged gasification process.

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Further studies are need to improve and optimise the multi-staged gasification process.

5.3 Poly-generation

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In order to reduce production costs for biomass gasification, the concept of poly-generation

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has been proposed (Gassner and Maréchal 2012, Bai et al. 2015, Heidenreich and Foscolo 2015, Parraga et al. 2018). The basic consideration for the poly-generation is that after the target product is produced, the remaining gas can be used for generation of heat and or power

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thus there is no need to separate the remaining gas species. The poly-products can be: (1). synthetic natural gas (SNG) with heat or SNG with heat and power, (2). biofuel with heat or

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biofuel with heat and power or, (3). hydrogen with heat (Gassner and Maréchal 2012, Heidenreich and Foscolo 2015). For achieving maximum energy and conversion efficiencies, biomass-integrated gasification combined cycle process is proposed (Parraga et al. 2018). Furthermore, Bai et al. (2015) proposed a poly-generation system for integration of biomass gasification with concentrated solar energy for co-production of methanol and power. Although these systems show promising potential, further studies are needed to validate these concepts.

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6.0 Conclusion and Future Perspectives Heavily use of fossil fuels causes serious concerns on resource depletion and GHG emissions. Biomass is the most promising renewable resource for future liquid fuel and chemicals to substitute those which are presently derived from fossil fuels. However, due to the nature of biomass physical structure and chemical composition, there are some challenges in commercialisation of biomass conversion technologies to energy, liquid biofuel and

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chemicals. The key challenges include low economic returns, low conversion efficiency, and uncertainty on environmental impacts. Extensive research and development have been

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performed recently on thermochemical conversion technologies with targets to improve the

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product quality and yields.

In the future, it is expected that the conversion technologies will be tailored to fit in the

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available resources and the target product applications. For biomass pyrolysis and liquefaction, liquid products can be simplified through biomass pretreatment and application

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of catalyst and solvents. Separation of the liquid product before upgrading is an important area for further development. It is also expected that the production costs can be reduced and process efficiency improved by considering poly-generation, process integration and

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optimisation.

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ACCEPTED MANUSCRIPT Advances in Thermochemical Conversion of Woody Biomass to Energy, Fuels and Chemicals Shusheng Pang Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand, email: [email protected]

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University, Jiaozuo, China;

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Adjunct Professor, School of Mechanical and Power Engineering, Henan Polytechnic

Adjunct Professor, Henan Centre for Outstanding Overseas Scientists, School of Chemical

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Engineering and Energy, Zhengzhou University, Zhengzhou, China

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There are three major challenges in biomass to energy, fuels and chemicals The challenges are related to biomass physical and chemical properties Pretreatment and catalytic pyrolysis can improve bio-oil quality and yield Supercritical one-step upgrading of bio-oil has potential for heavy engine fuel Solvents and catalyst in biomass liquefaction can simplify liquid composition New gasification technologies provide a promising opportunity

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