Biochar gasification: An experimental study on Colombian

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Biochar gasification: An experimental study with Colombian agroindustrial biomass residues in a fluidized bed Gloria Marrugo, Carlos F. Valdés, and Farid Chejne Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00665 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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Biochar gasification: An experimental study on Colombian agroindustrial

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biomass residues in a fluidized bed

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Gloria Marrugo, Carlos F. Valdés, and Farid Chejne1

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Universidad Nacional de Colombia, Facultad de Minas, Escuela de Procesos y Energía,

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TAYEA Group, Carrera 80 No. 65-223, Medellín (Colombia).

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Abstract

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The effect of lignocellulosic biomass composition on gasification of biochar in a scale pilot

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fluidized bed reactor was studied. First, three biomass samples from agro-industrial sectors

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of Colombia were pyrolyzed. These biochars were then subjected to gasification. The

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biomasses, and biochars, and gasification residues were subjected to various

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physicochemical and morphological analyses to study the evolution of the carbonaceous

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structure and syngas composition. In the case of sugarcane bagasse (SCB) and rice husk

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(RH), the high contents of cellulose and hemicellulose promoted the thermochemical

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conversion of the biochar into syngas; however, in the Palm Kernel Shell (PKS) its high

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lignin content made thermochemical pyrolysis and gasification difficult due to the

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recalcitrance of the lignitic structure, which forms more organized carbonaceous structure.

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Corresponding author: Universidad Nacional de Colombia, Facultad de Minas, Escuela de Procesos y Energía, Medellín, Colombia. Email address: [email protected] 1 ACS Paragon Plus Environment

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Keywords: Biomass; Biochar structure; Lignocellulosic composition; pyrolysis;

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Gasification, Fluidized bed reactor.

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1. Introduction

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In Colombia, around 72 million ton/year of agricultural residual biomass are thought to

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exist, indicating an energy potential of around 332,000 TJ/year1. This high agro-industrial

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activity has created a large supply of available biomass that can satisfy the demand for

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energy if forestry and agricultural residues are managed well. A recent study estimates the

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production of sugarcane bagasse at 7 million tons/year, rice husks at 453,000 ton/year, and

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palm oil residues at 1.6 million ton/year in the country1.

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Biomass such as wood, grass, and agricultural residue consists mainly (85 - 90 wt. %) of

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three lignocellulosic components: hemicellulose, cellulose, and lignin2. The

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thermochemical decompositions of these polymers3 occur between 250 °C - 300 °C, 300

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°C - 350 °C and 300 °C -500 °C, respectively. Lignin has the broadest range because it is a

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highly complex and recalcitrant component4,5. The proportions of these and other

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components vary between plant species and affect thermochemical conversion

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characteristics.

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Of the various thermochemical processes, pyrolysis and gasification have been widely

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studied in the past 30 years since they can produce a large spectrum of products6. In

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pyrolysis, the proportions of the products depend on operating conditions (heating rate,

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particle size, residence time, etc.) and lignocellulosic biomass composition7. These 2 ACS Paragon Plus Environment

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properties produce differences in structure that ultimately affect biomass reactivity during

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its use8; however, very little literature exists regarding the effect of biomass composition on

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the char formed from pyrolysis as intermediate process for biochar gasification.

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Biomass gasification is a complex process consisting mainly of two steps that occur in

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series and simultaneously, depending on operating conditions: (1) pyrolysis to release

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moisture and volatiles and (2) conversion of biochar via gasifying agents and volatiles

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released. The first stage also called devolatilization is characterized by volatile material

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being released and formation of a solid carbonaceous residue (biochar and soot); the

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parametric study of this process have been of great research interest5,7–9. Generally, biochar

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has a morphological structure similar to the starting lignocellulosic material that changes

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with process conditions. In the technical and scientific literature there are many studies on

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the performance of various types of biomass under gasification including rice husks 10–14,

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woody biomass 17–22 and other biomass. Results have demonstrated that the process

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performance also depends on the composition raw material31–34. Additionally it is well-

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established that the conversion of the biochar is the slowest stage and controls the biomass

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gasification process21,35. Because of this, interest toward the gasification of biochars has

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grown considerably 36. However, the effect on the thermochemical transformation of the

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lignocellulosic structure of the biomass and its interaction with the other components is not

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clearly understood 37.

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Sattar et al.36 carried out a parametric study on steam gasification of four biochars in a

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quartz tubular reactor. They found that increases in both steam flow and temperature

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significantly increase the dry gas yield and carbon conversion, but the hydrogen volume 3 ACS Paragon Plus Environment

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fraction decreases at higher temperatures. Additionally, they found that each biochar has a

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unique surface which could affect its behavior during gasification. It is important to note

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that the biochars used in this study had a high content of volatile material; therefore during

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the process there was likely a strong interaction between devolatilization gases, char, and

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the rate of conversion of biochar via gasification.

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On the other hand, Lv et al.38 studied the effect of cellulose, lignin and alkali and alkaline

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earth metals on the characteristics of pyrolysis and gasification. They evaluated six types of

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biomass with compositions of varied lignocellulosic material, as well as cellulose and pure

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lignin. Gasification was evaluated using the biochar obtained from pyrolysis. The results

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show that for biochar obtained from biomass with a high content of cellulose, the

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gasification process is more prolonged in time; this is probably due to differences in

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microstructure of the biochar obtained according to the relationship between the contents of

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cellulose and lignin. For some relationships, the structure is more porous, making the

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gasifying agent more easily diffusible within the particle during gasification; this behavior

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varies from one biochar to another.

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Yip et al.39, investigated the wood biochar structure and its reactivity. They found that

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thermal annealing leads to a significant change in the reactivity of biochar. In the absence

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of catalytic species, the reduction of biochar reactivity is due to the ordering of the carbon

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structure induced by thermal annealing. The presence of alkali and alkaline earth metals

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(AAEM) changes biochar reactivity and the biochar structure. As expected, there is

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retention of AAEM species in the biochars from raw wood after thermal annealing at 750

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and 900 °C. 4 ACS Paragon Plus Environment

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The effect of AAEMs on thermochemical conversion of biomass has been extensively

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investigated 40–43. In general, AAEMs are associated with changes in the structure of

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biochar after pyrolysis and its reactivity44. Some mineral species existing in biomass (Ca,

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K, Mg, and others) have catalytic effects during biomass transformation. It is currently

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agreed that reactivity is not only related to the amount of ash, but also specific metals in

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that ash45. Therefore, the abundance of a particular component may or may not contribute

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to the catalytic effect. In general the trends show more noticeable catalytic effects on the

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biochar gasification step, when performed with steam and/or CO2. The K, Na, and Ca are

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have shown strong catalytic properties46. Formation of H2 can be enhanced by the presence

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of K and Ca oxides21,47, while Si and Al do not contribute significantly to catalysis45.

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Despite information on gasification and pyrolysis of biomass, there practically are no pilot-

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scale studies leading to an understanding of the interaction between the actual

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lignocellulosic structure of the biomass, the environment, the operating conditions, and the

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effects of the same structure on the performance of processes. This challenge is important

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today because there is lack of knowledge regarding the influence of lignocellulosic

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structure on products and yields from gasification. This limitation in phenomenological

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understanding has delayed the use of many biomasses with high energy potential.

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Therefore, it is necessary to further study fundamental changes in the biomass structure

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during pyrolysis and effects thereof on the gasification.

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These studies were carried out in reactors as close to those used at the larger scale,

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particularly fluidized bed reactors that have characteristically high mass and heat transfer.

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This work utilizes a variety of characterization techniques to elucidate the affect that the 5 ACS Paragon Plus Environment

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original lignocellulosic structure of biomass can have on the distribution of gasification

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products. Three biomasses with different physicochemical characteristics were pyrolyzed in

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a fluidized bed, and then the biochar was gasified to minimize the volatile-biochar. The

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effect of AAEMs was also evaluated by loading of metal oxides into the feedstocks.

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Gasification products were characterized and new essential knowledge was discovered

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based on the results.

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2. Experimental methods

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The gasification process was analyzed in two steps, first, biomass samples were pyrolyzed

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with the aim of reducing the interaction between the released volatiles and the gasifying

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agent and then the biochar produced was taken out of the reactor. Second, biochar samples

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obtained from biomass pyrolysis were gasified with steam. The biomass, biochar,

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permanent gases and gasification residue were characterized, in order to establish a

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relationship between biomass and syngas, where biochar is a bridge between them.

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2.1 Biomass characterization

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Three biomasses were selected; samples of Sugarcane Bagasse (SCB), Rice Husk (RH) and

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Palm Kernel Shell (PKS) were obtained from Colombian regions of Risaralda, Tolima, and

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Norte de Santander, respectively. The biomasses were milled and sieved, the particle size

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average/diameter (Dp, mm), used to pyrolysis tests was 0.93 mm.

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Due to the inherent compositional and structural complexity of the biomass, a complete

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characterization of the biomass through multiple analyses is fundamental. The

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physicochemical characterization of these biomasses are presented in Table 1, the

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characterization methods were specified in Supporting Information and more

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characterization details can be found in an earlier work by the authors48.

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Table 1. Physicochemical characterization of biomasses Parameters SCB RH PKS Ultimate analysis (wt.%) (Norm ASTM D5373-14) C 41.39 33.87 46.05 H 4.99 4.57 5.14 N 0.32 0.84 0.62 O 40.86 41.28 45.40 0.11 0.12 0.14 S (ASTM D4239-14) Proximate analysis (wt.%) 7.04 6.92 7.52 Moisture (ASTM D3173-11) 12.34 19.33 2.67 Ash (ASTM D3174-12) 70.47 55.85 69.35 Volatile material (ASTM D3175–11) 10.15 17.90 20.46 Fixed carbon (ASTM D3172) 15114 14089 18962 HHV (kJ/kg) (ASTM D5865-13) Lignocellulosic content (wt.% daf) (NREL/TP-510-42618) Hemicellulose 14.63 24.50 27.06 Cellulose 53.18 39.65 14.64 Lignin 32.19 35.84 58.30 Area (m2/g) BET surface area ( N2) 3.93±1.25 2.40±1.56 4.59±1.10 Area CO2 adsorption 56.17±3.81 48.05±1.93 47.04±3.58 Ash chemical characterization (wt.%) SiO2 58.32 83.76 36.09 K 2O 5.15 6.01 16.70 P2O5 2.14 4.40 4.86 CaO 9.29 2.52 10.93 Fe2O3 13.45 1.11 13.05 MgO 1.64 0.81 4.88 SO3 1.80 9.11 MnO 0.33 Al2O3 7.88 1.53 Cr2O3 1.73 NiO 1.12 Others 1.40 -

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Some results are presented in dry ash-free basis (acronym “daf”).


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The means contents of carbon and oxygen were 42 wt. % dry ash-free basis (acronym

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“daf”) and 44 wt. % daf, respectively. These values are characteristic of lignocellulosic

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biomasses due to the presence of highly oxygenated groups inside their structures.

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The SCB had the highest content of volatile material, followed by the PKS. This is

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important because is the fraction of biomass that is volatilized during the pyrolysis and

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while more material is released; the carbonaceous matrix is more exposed, which can

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facilitate the gasification process. The fixed carbon content is of special interest since this

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indicates the potential for char generation once the biomass has been devolatilized.

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Therefore, it is expected that the PKS and the RH will have a bigger char yield (see Table

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1). However, the RH has a high amount of ash (19.33 wt. %), so that the yields should lower

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than PKS.

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Lignocellulosic composition gives an estimate of the degradation of biomass. In this case, it

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is expected that SCB and RH are the easiest to degrade at low temperatures compared to

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the PKS because they are mainly cellulose and hemicellulose. PKS has high lignin content,

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and thus it requires a more severe degradation temperature as lignin can form recalcitrant

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structures when heated 4,5; RH and SCB can be considered soft by comparison, but

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although they (RH and SCB) have similar distributions of lignocellulosic materials

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(cellulose > lignin > hemicellulose), their physical structures are quite different.

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The SCB has the largest cellulose content and the lowest lignin content (see Table 1),

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whereas the PKS has the opposite behavior with the lowest cellulose content and highest

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content of lignin. The large amount of lignin present in the PKS gives it its characteristic

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hardness and a greater capacity for char generation once it has been devolatilized. From the 8 ACS Paragon Plus Environment

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surface area analysis, it was concluded that the biomasses studied are essentially nonporous

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materials.

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As for the catalytic species content of the ash (K2O, MgO and CaO), SCB, RH, and PKS

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have totals of 16.08 wt. %, 9.34 wt. % and 32.51 wt. %, respectively. These species will

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affect thermochemical processes. However, around 65 wt. % of the ash is comprised of

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non-catalytic species such SiO2 and Al2O3. All these results are in agreement with those

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reported in scientific literature for similar biomasses48.

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2.2 Experimental equipment description

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The biochar preparation by means of biomass pyrolysis and biochar gasification were

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performed in a pilot scale fluidized bed reactor operating at atmospheric pressure, see

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Figure 1. The stainless steel reactor has an internal diameter and height of 10 cm and 50

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cm, respectively, at the reaction zone (bed) and 14 cm in diameter and a height of 100 cm

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in the free zone (freeboard). The unit is isolated by a ceramic material. Power is supplied to

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the reactor through a set of three electric heaters (1500W each), located along the reactor.

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Temperature control was adjusted using a PID temperature controller and was monitored

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using three K-type thermocouples.

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The feeding system is composed for a hopper of approximately 6 kg of capacity, with a

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screw feeder that can be regulated with a variable frequency driver, discharging the solids

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at the upper of the reactor. Prior to the tests, a calibration of the screw feeder for each

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material was done, in order to achieve a desired solids feeding rate. The liquid produced in 9 ACS Paragon Plus Environment

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pyrolysis test was collected in a condenser cooled to water at room temperature. To the gas

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cleaning a system with solvent and silica gel was used. The feeding and condenser systems

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were used only to biomass pyrolysis and the water supply was used only to biochar

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gasification.

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Figure 1. Scheme of experimental equipment 2.3 Biochar preparation

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Biomass was feeding continuously to reactor; the average particle size was 0.93 mm

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utilized for. Silica sand was the fluidizing medium with average particle size of 0.6 mm and

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was charged to inside the reactor before it got heating. The tests were developed at

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temperatures between 750 °C - 800 °C. The carrier gas for pyrolysis was N2 and was

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continuously preheated to 600 °C, the flow rates assured in each case good fluidization

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conditions (1.25 times the minimum fluidization velocity). Once the reactor reached the

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desired experimental temperature, the biomass was feed at a rate fixed for each biomass.

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The pyrolysis provided enough biochar to perform biochar gasification in the second phase.


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n each experimental trial, the global mass balance was determined in order to find the

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product yields based on biomass “daf” feed quantity. The bio-oil was collected in the

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condenser at the end of each experimental test was weighed. Likewise, the biochar was

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recovered in the cyclone and inside the reactor. The gas yield was determined by difference

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in the mass balance based on the known yields of biochar and oil. The humidity of the gas

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was determined by psychometrics measuring the dry bulb and wet bulb temperature.

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The influence of biomass minerals on the gas composition was evaluated. For this the mass

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quantity of catalytic species (Ca, K, Mg in oxide form) / mass quantity of biomass “daf” fed

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was related with the mass quantity of species (H2 and CO) in dry gas /mass quantity of

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biomass “daf” fed. Also, the ratio between the biomass with the highest amount of H2 and

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CO produced and its quantity of catalytic species was established to validate if there is a

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proportional variation in the yields of these gases for the other biomasses and their

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respective contents of catalytic species.

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2.4 Biochar gasification in fluidized bed

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Biochar gasification tests were performed at 850 °C. The tests, of two-hour process were

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performed in batches. The bed consisted of mixtures of biochar and silica sand (average

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particle size 0.6 mm). The material proportion in bed was determined by a previous fluid

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dynamic study, and the material proportion of 50 wt. % - 50 wt. % was the highest ratio

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accepted for maintain a good fluidization. For the mixtures, the minimum fluidization

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velocity was determined to ensure that process gas flow is beyond the minimum

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fluidization velocity without entraining particles outside the reactor. 11 ACS Paragon Plus Environment

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The mixture of biochar and silica sand was changed inside the reactor before the heating.

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The reactor heating started under N2. Once the reactor reached the desired experimental

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temperature, the steam was turned on. Water was vaporized in a preheater to 600 °C and

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entrained with the flow of N2 to the reaction zone; once the process time is over, the water

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flow was closed and the N2 flow was maintained until cooling to room temperature. The

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steam/biochar ratio (SB, kg/kg) was established based on the initial carbon content of each

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biochar for total consumption in two-hour of processing. The SB was fixed for the three

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biochars in order to make comparisons of the performance between them. The principal

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tests were developed with SB=1 and later was increased (SB=2) to evaluate the syngas

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quality.

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The tests were conducted for triplicate for each SB ratio; the conditions are presented in

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Table 2. The reproducibility of results was validated from acceptance of satisfactory tests

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in which the differences between measurements (temperatures, pressure, flows, and global

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mass balance) showed an error less than 5%. The error bars correspond to the mean

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standard deviation of realized tests.

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Table 2. Summary of gasification process Parameters Number of test Average temperature ,°C Feed loaded, kg SB initial ratio Carrier gas (N2 + Steam), kg/h

Biochar BRH BSCB 6 6 850 ± 2 852 ± 1 0.25 0.15 1.0 and 2.0 10.06 6.73 6.39

BPKS 6 852 ± 2 0.38

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Gasification tests results and products characterization gave the global mass balances, as

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well as the products yields (based on the biochar “daf” fed quantity). The solid residues

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were recovered in the cyclone and inside the reactor and were weighed at end test. The 12 ACS Paragon Plus Environment

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global yield and moisture in syngas was determined as described previously for pyrolysis

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tests, without the t bio-oil part. The cold syngas efficiency (Ecg) and carbon conversion

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(CC) were calculated. Ecg is defined as the relation between the energy exit in the syngas

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and the energy of biochar fed and the CC was defined as the carbon conversion from

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biochar to dry syngas, using Eq. 2 and Eq. 3, respectively: =

, ∙ ∙ 100 , ∙

Eq. 2

,

∙ 100 ,

Eq. 3

= 261

where FS, dry and CFS, dry represent the volumetric flow (Nm3/h) and carbon mass flow

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(kg/h) of dry syngas. FB, daf and FCB, daf, are the mass and carbon mass flow (kg/h) of

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biochar “daf”. LHVs and LHV B represent the lower heating value of the syngas (MJ/Nm3)

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and biochar (MJ/kg), respectively.

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Similar to pyrolysis, the influence of the minerals on syngas was investigated. It was

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assumed that biochar retained most of the mineral species present in the biomass after

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pyrolysis. It was found that the relative amounts of alkalis released strongly depended on

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the nature of biomass 49,50. Added to that, a study by Okuno et al.51 found that pyrolysis of

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SCB in a wire-mesh reactor the retention of K decreased only by 10% and those of Ca and

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Mg remained unchanged the retentions. Davidsson et al.49 found a significant negative

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influence of increasing particle size on the release of minerals such as K. It is believed that

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the release of K is facilitated by the presence of Cl while suppressed by that of Si, leading

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to formation of K-silicates that are much less volatile than KOH and KCl as well 51. In this

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study, biomass with a particle size of 1 mm was used and the characterization showed that


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these biomasses contain high contents of Si but do not have Cl. For all of the above, the

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volatilization of mineral species was considered insignificant during pyrolysis.

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2.5 Product analysis

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After each test, the pyrolysis biochar and gasification solid residue were separated from the

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silica sand. The separation of the material was not so easy and to guarantee that there is no

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silica in the sample is difficult, however, 200 g of the mixture were cold fluidized in an

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acrylic fluidizer for 20 minutes until separation of the materials was observed. Only the top

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of the bed was taken. The process was repeated until separated all the biochar required for

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gasification and characterization and only characterization in the case of gasification solid

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residue. In the case of SCB and RH it was easy to observe the separation by the shape of

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the biochar. Subsequently they were subjected to electrostatic separation. A part of these

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samples were milled and sieved and subjected to analysis, the rest of pyrolysis biochar was

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used to the gasification tests.

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Proximate (ASTM D3172) and ultimate (ASTM D5373-14 and ASTM D4239-14) analysis

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were done. For the high heating value (HHV), the procedures described in ASTM D5865-

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13 guidelines were used. The study of morphological was done through a Scanning

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Electron Microscopy SEM (JEOL JSM 5910 LV with an SEI detector). A characterization

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of the physical structure of the samples was carried using a Nikon Eclipse LV100 optical

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microscope with both white reflected light (WL), transmitted blue fluorescence light (FL).

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Additionally the presence of minerals was corroborated through polarized light (PL) in the

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optical microscope. The determination of the surface area was done by absorption of N2 14 ACS Paragon Plus Environment

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and CO2 adsorption in a Micrometrics TriStar II Plus, the test was done by duplicate. The

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protocols are similar to used for biomass characterization, described in Supporting

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Information.

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The pyrolysis gas and syngas were characterized during each test with a MicroGC (Agilent,

304

Model 3000) with two TCD detectors and an injector backflush type. Two columns were

305

used, a 5A 10 m x 0.32 mm molecular sieve column using Ar as carrier gas and a 8 m x

306

0.32 mm U Plot Column using He as a carrier gas. The columns temperatures and pressure

307

were 80°C and 30 psi. An analysis method was set upon the system, based on calibration

308

curves with standards gas mixtures, to quantify concentrations v/v of H2, CO, CO2, CH4

309

and CnHm (C2H4, C2H6 and C3H8). The mean composition of gas species was determined

310

from the gas analysis during the two hours of process. The error bars in the results

311

correspond to the mean standard deviations between the realized tests and following the

312

rules of uncertainty propagation. The HHV of pyrolysis gas and syngas were determinates

313

as the sum of the calorific contribution of each combustible species in the N2-free gas.

314 315

Additionally, in order to determine the transformation of biomass through gasification and

316

pyrolysis processes, changes in the functional groups of the chemical structure of biomass,

317

biochar, and residue of gasification were analyzed. Identification of the main functional

318

groups present in the structure was carried-out using an Attenuated Total Reflection Fourier

319

Transformed Infrared spectrometry (FTIR-ATR) in a Perkin Elmer Spectrum Two device.

320

Each sample was scanned from 4000 cm-1 to 600 cm-1 and 128 spectra were recorded at a

321

resolution of 2 cm-1 and processed using the Spectrum10TM Spectroscopy software.

322


Energy & Fuels

323

3. Results and discussion

324 325

3.1 Biochar preparation: Biomass pyrolysis

326 327

Mean global composition of species in pyrolysis gas is shown in Figure 2. CO was the

328

main product of pyrolysis of all biomasses considered in this investigation; this implies that

329

the CO forming reactions government at these temperatures. There was also notable

330

presence of H2, CO2, and CH4 for SCB and RH. Mean composition, Vol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 41

60% 50% 40% 30% 20% 10% 0%

331 Hemicellulose Cellulose Lignin

332 333 334 335

SCB

RH

14.63 wt.% 53.18 wt.% 32.19 wt.%

25.48 wt.% 39.65 wt.% 31.84 wt.%

PKS 27.06 wt.% 14.64 wt.% 58.30 wt.%

Figure 2. Mean composition of pyrolysis gas ■H2; ■CH4; ■CO; ■CO2; ■CnHm

336

It is considered cellulose and hemicellulose release CO during primary pyrolysis and at

337

high temperature secondary reactions 52,53. Tar can be decomposed into H2, CO, and CH4

338

before leaving the reactor 54. In all cases, the CO is high because the pyrolysis temperatures

339

promoted secondary reactions in the homogeneous phase (volatile cracking and gasification

340

reactions) and in the heterogeneous phase (gasification of char with steam or carbon

341

dioxide)7. Furthermore, alkyl-ether groups from lignin structure may be detached and

342

decompose to carbon monoxide in the homogeneous phase44. CO and CO2 primarily come 16 ACS Paragon Plus Environment

Page 17 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

343

from the bond breaking of C-O-C, C=O, CHO, and COOH, while the CH4 and H2 are

344

mainly products of breaking of O-CH3 and dehydrogenation of aliphatic hydrocarbons;

345

respectively, all from the outer functional groups of cellulose, hemicellulose, and lignin.

346

However the CO y CO2 can also be generated due to oxidation reactions56 that they also

347

take place during the process.

348

The low value of CO2 in gas composition could be due to char gasification reactions with

349

CO2, that they favor the formation of CO at these temperatures; as well it also due to the

350

Water-Gas Shift (WGS) reactions; which generates more CO and additional H2.

351

Additionally, in a fluidized bed reactor decomposition of tars is common in the freeboard

352

and even within the structure of the particle, given its large size. Additionally, there may be

353

reactions of volatiles with char promoting dehydration, decarboxylation, depolymerization,

354

and other reactions.

355 356

The PKS did not behave as expected. The composition of permanent gases from PKS

357

pyrolysis may imply that lignin pyrolysis is not effective due to its inherent recalcitrant

358

behavior4,5,57. Although methane production is high, high amounts of H2 are not present. It

359

appears that the methoxy groups react, but not the aromatic backbone of lignin, being likely

360

that the CH4 comes from weak bonds of the methoxy group (-OCH3), such as was reported

361

by Brebu and Vasile58. Even though SCB and PKS have a similar content of volatile

362

material, it can be assumed that most of these in PKS are derived from lignin59.

363

Additionally, it is possible that volatilized lignin material is condensed in pores and favors

364

the generation of CO2 under gasification conditions54.

365


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

366

Regarding the energy content in the pyrolysis gas, a higher HHV (N2-free gas) was

367

obtained for SCB pyrolysis gas with an average value of 18.02 MJ/Nm3; this is 6% higher

368

than RH (16.86 MJ/Nm3) and PKS (16.83 MJ/Nm3). This difference is caused by the high

369

content of CO and H2 from degradation of cellulose (main component) and hemicellulose-

370

lignin interactions which are evenly distributed in SCB.

371 372

The average yields of products from biomass pyrolysis are presented in Table SI2 in

373

Supporting Information. It is noted that SCB generated the most gas yield, because this

374

biomass has the higher volatile matter content due to reaction severity; those volatiles can

375

undergo cracking reactions at these high temperatures which are between traditional

376

definitions of pyrolysis and gasification. Additionally, its lignocellulosic composition is

377

rich in cellulose and hemicellulose which promoted the generated more gas during

378

thermochemical conversion than the other biomasses54. The high heating rate in the

379

fluidized bed reduces devolatilization time and exhibits greater devolatilization60,61, where

380

fundamentally components rich in H and O are released leaving a carbonaceous structure

381

coming from lignin, principally. This explains the lower yield of permanents gas and bigger

382

biochar yield in PKS pyrolysis because it is a lignin-rich biomass and has the highest fixed

383

carbon (see Section 2.1). The RH has similar amounts of cellulose and hemicellulose to

384

SCB, but has less volatile material so a smaller quantity of gas and bio-oil was produced.

385

During PKS processing the highest water quantity was generated such moisture in gas

386

(22.36 wt.%) and bio-oil (10.51 wt.%), may be due to cracking of hydroxyl-aliphatic

387

groups from side chains lignin55.

388


Page 18 of 41

Page 19 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

389

The effect of the catalytic species in the biomass on the pyrolysis process is shown in

390

Supporting Information (Figure SI3). It is known that AAEMs enhance production of H2

391

CO2 and inhibit the production of CO, with a negligible effect over CH4

392

observed in this research, due to the all the gas species presented a similar behavior for the

393

biomass (see SI3 a). In the case of the PKS, which has less amount of catalytic species, a

394

lower yield of species in gases was found, but this biomass also has a the lignitic structure

395

that hinders its degradation. Although RH is the biomass with the highest amount of

396

catalytic species due to its high ash content, however the H2 and CO2 yields do not have the

397

major values; the catalytic effect could be limited due to the high presence of non-catalytic

398

species such as SiO2 and Al2O347. For SCB with an intermediate amount of catalytic

399

species, higher gas yields were found. For this biomass there are competitive influences:

400

the lignocellulosic structure facilitates thermochemical degradation and catalytic AAEMs

401

promote the formation of H2 and CO2. There was no linearity in the effects of these metals

402

based on loading (see Figure SI3 b, c, d and e in Supporting Information).

62

, but this not was

403 404

3.1.1

Biochar characterization

405 406

The proximate and ultimate characterizations of biochar were performed, as shown in

407

Figure 3 for the biomasses and their respective biochars. Based on the volatile matter

408

contents, it is clear that complete conversion was not achieved. There was residual volatile

409

material in the SCB biochar (BSCB), RH biochar (BRH) and biochar PKS (BPKS);

410

however, over than 90 wt. % of volatile material was released, except for the case of PKS.

411

BPKS has a content of residual volatile matter close to 20 wt. % of the total content of the

412

original raw material. This indicates that this material could need a higher pyrolysis time or 19 ACS Paragon Plus Environment

Energy & Fuels

413

that volatile matter was encapsulated within the carbonaceous structure during

414

thermoplastic transformations due to its high lignin content. Evidence of this behavior has

415

been presented by Dufour et al4 and Montoya et al.44 who found that the lignin has a

416

pseudoplastic behavior at high temperatures which generates melted structures57. The high

417

temperatures induce fusion and coalescence of the structure which could encapsulate

418

materials and prevent them from being transported to the surface during pyrolysis. C (df)

O (df)

80% 60%

2.0

40%

Atomic rerlation, H:C

Composiction, wt %

100%

20% 0% SCB BSCB

Fixed carbon (df)

RH

BRH

PKS BPKS

Volatile material (df)

Ash (df)

100% Composition, wt %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

SCB RH PKS

1.5

BSCB BRH BPKS

1.0 0.5 0.0 0.0

80% 60%

0.2

0.4 0.6 0.8 Atomic rerlation, O:C

1.0

Van Krevelen diagram

40% 20% 0% SCB BSCB

419 420

RH

BRH

PKS BPKS

Figure 3. Evolution of chemical composition of biomass pyrolysis *df: dry free basis

421 422

The carbon and fixed carbon contents increased with thermochemical transformation,

423

which leads to an increase in the energy content that is evident in the Van Krevelen

424

diagram. Here we can see decreasing O/C and H/C ratios, leading them to values similar to

425

coal. This is because the release of volatile material reduces the oxygen content. The ash

426

content increases due to the concentration of this material. For the RH, the ash content


Page 21 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

427

increased to 57.66 wt. % on a dry free basis (df); with this ash content, the BRH have a

428

fixed carbon content similar to BPKS on a dry ash free basis (daf).

429 430

As part of biochar characterization, optical and morphological analyses were performed.

431

With SEM microscopy, it was possible to verify the degree of deterioration of the material

432

with formation of hollow, melted structures (see Figure 4). In Figure 4a images of

433

morphological exploration of BSCB show destruction of the superficial structure, compared

434

to the raw biomass (see Figure S1), from the severity of thermochemical transformation.

435

Shaped structures with characteristic SCB channels are exposed, which mostly have smooth

436

surfaces and show the presence of small hollow areas.

437 438

Figure 4.SEM images with three magnifications of (a) BSCB, (b) BRH and (c) BPKS

439

Morphological exploration of BRH was also made, as seen in Figure 5b. This shows the

440

laminar structure with many exfoliations in the surface layers due to the severity of the

441

thermal process. In general it is possible to say that the morphological structure of RH is


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

442

preserved (see raw biomass in Figure SI1), although it seems to have been softened without

443

a predominance of porous forms.

444 445

BPKS, shown in Figure 5c, has a compact and smooth structure with few holes and

446

negligible porosity; the structure of the original raw material is preserved (see raw biomass

447

in Figure SI). Interestingly, the exposure of mineral material on the surface of the biochar

448

can be seen. Presumably, these are clusters of silica oxides that are the main components in

449

the ash of raw material.

450 451

Similarly, optical microscopy with transmitted and reflected light reveals the presence of

452

uncarbonized substances, minerals, and condensed liquids without maturation (tars, oils).

453

These condensates appear dark gray color under WL, greenish yellow under FL, and dark

454

in PL. The minerals look like bright colors under polarized light (see Figure 5). In the last

455

case, according to the recent findings by Montoya et al. 20178, the presence of mineral

456

matter is critical for lignocellulosic materials to retain their original morphology. We

457

repeated the studies on raw bagasse and acid washed bagasse. The raw bagasse retains its

458

morphology; the acid washed bagasse shrank dramatically, but was not completely

459

converted into a liquid. Our results suggest that in addition to hemicellulose with the

460

presence of ash, other biomass fractions or the different temperatures at which the different

461

fractions melt and solidify could also be contributing to maintain the structure of the

462

lignocellulosic material during pyrolysis. Therefore, the retention of them during the

463

devolatilization process, despite the severity of the process, contributes to the preservation

464

of the original structure of the biomass.

465 22 ACS Paragon Plus Environment

Page 22 of 41

Page 23 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

466

In Figure 5a, the BSCB microphotograph shows no condensed oil or raw material

467

remaining in biochar. This confirms a good devolatilization in the SCB, for which it was

468

possible eliminate more than 90% of the original volatile material. This indicates that for

469

subsequent gasification process of BSCB, the volatile-biochar and volatile-steam

470

interaction is non-interfering. Observations in optical microscopy with both transmitted and

471

reflected light for BRH (see Figure 5b) and BSCB support the conclusion that there is no

472

existence of unprocessed material, the colors to raw biomass under FL were yellow - green

473

(see Figure S1) and now in biochars is gray color, and the presence of condensed oil is

474

insignificant.

475 476

The compact and smooth structure of BPKS, however, seems to be from pyrolysis at high

477

temperature. The lignin is melted and exhibits thermoplastic properties to coalesce on the

478

structure; this gives the biochar its smooth surface5 which does not allow for release of all

479

volatile material in the PKS. The FL microphotograph (Figure 5c) shows that the hollows

480

filled with greenish yellow material that is reactive to FL, characteristic of hydrocarbons

481

generated for heating of volatile matter this color is similar to showed in the raw biomass

482

(see Figure S1). When viewed under white and PL, they have a dark gray color.

483


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

484 485 486

Figure 5. Microphotograph of (a) BSCB; (b) BRH; (c) BPKS. Left to right: White light (WL), fluorescent light (FL), and polarized light (PL).

487

Complementary to the morphological examination, the BET surface area and CO2 analysis

488

for biochar samples was performed (see Table 3). The greatest surface area was seen for

489

BSCB. Comparing the results with those obtained from raw biomass (See Table 1), it is

490

clear that the SCB and RH develop surface area as volatiles are released and the surface is

491

oxidized. However, in PKS the surface area of biochar is only slightly greater than the

492

surface area of raw biomass (4.58 m2/g); this confirms that there has not been complete

493

release of volatile material and shows the effect of formation of liquid, which forms

494

secondary char, with less porosity.

495

Table 3. BET surface and CO2 area of biochar Area, m2/g Sample BET CO2 BSCB 124.60 ± 2.53 423.91± 2.20 BRH 28.75 ± 3.06 229.63 ± 4.42 BPKS 5.78 ± 1.71 375.25 ± 3.80

496


Page 24 of 41

Page 25 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

497

The CO2 surface area analysis (see Table 3) provided information about the narrowest

498

micropores corresponding to those pores below 0.7 nm63. In the BPKS case, the surface

499

area is mainly from narrow micropores. The BSCB and BRH have narrow microporosity,

500

but unlike the BPKS these biochar have meso and macropores as shown by BET area.

501

Therefore, these biomasses can have an easier steam access during gasification.

502 503

3.2 Biochar gasification in fluidized bed

504 505

Biochar were subjected to steam gasification, the products characterization and yields are

506

presented.

507 508

3.2.1

Syngas composition

509 510

In Figure 6 the thermal profile of the process and typical composition of syngas (N2-free

511

gas) during the development of tests with SB=1 is presented. It is evident that the process

512

temperature for each of the gasification tests showed adequate stability maintained on

513

average at 850 °C throughout the test. It is important to note that the thermal steady-state

514

during gasification is not only correlated to the equipment’s thermal profile; it is also

515

dependent on parallel reactions of several processes. In this particular case, we have

516

pyrolysis of residual volatile material, gasification, cracking, and likely others. These

517

reactions determine the product distribution and go before catalytic reactions and

518

interactions between char and ash.


Energy & Fuels

900

40%

500

30% 20%

300

10%

Composition, Vol

700

Temperature, °C

60% 50%

b)

70% 60%

700

50% 40%

500

30% 20%

300

10%

0%

100 20

40 60 80 Time, min

100

100 0

20

c)

70% Composition, Vol

0%

120

40 60 80 Time, min

100 120

900

60% 700

50% 40%

500

30% 20%

300

Temperature, °C

0

10% 0%

100 0

519 520

900 Temperature, °C

a)

70% Composition, Vol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

20

40 60 80 Time, min

100 120

Figure 6. Syngas composition, SB=1. (a) BSCB; (b) BRH; (c) BPKS ♦ H2; ■ CH4; ▲ CO; ●CO2; -T

521 522

Another important aspect to note in Figure 6 is evidence of low material consumption

523

during the process progression, since the species compositions do not decrease significantly

524

over time. Given that gasification is performed only with steam, there is a predominance of

525

H2, CO, and CO2 as result of interactions between gasification reactions 1, 2, 3 and to a

526

lesser extent the reaction 4 (shown below).

527

C + H O ↔ CO + H C0 + H O ↔ CO + H C + CO ↔ 2CO C + 2H ↔ CH

Primary gasification Water - gas shift CO2 gasification (Boudouard) H2 gasification

528


(Reaction 1) (Reaction 2) (Reaction 3) (Reaction 4)

Page 27 of 41

529

For the particular case of BPKS gasification, the meaningful yield of CO2 is due to

530

interactions between the high content of residual volatile material and the carbonaceous

531

biochar structure, along with the simultaneous occurrence of gasification reactions of

532

biochar rich in lignin54.

533 534

In Figure 7 the mean composition of species in the syngas is presented. It was determined

535

that BSCB generated a syngas with better features by having increased production of H2

536

and CO, whose participation are 50 vol. %, and 30 vol. %, respectively. BHR gasification

537

has a similar trend to BSCB; however, the gas production is lower due to the lower

538

availability of carbonaceous material to react. In contrast, the BPKS gasification generates

539

a syngas rich in H2 and CO2 (42 vol. % and 47 vol. %, respectively), which as mentioned

540

above is generated from the lignitic material structure. 60% Mean composition, Vol %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

50% 40% 30% 20% 10% 0% BSCB

BRH

BPKS

541 542 543 544

Gasified sample ■ H2; ■ CH4; ■ CO; ■CO2 Figure 7. Mean composition of species in syngas

545

The HHV of the syngas (N2-free gas) obtained reflects the composition of syngas, which is

546

why the higher HHV is obtained from BSCB gasification with 10.80 MJ/Nm3, followed to

547

BRH of 10.26 MJ/Nm3 and BPKS of 7.27 MJ/Nm3. The latter, have a high concentration


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

548

of CO2 that goes against the energetic quality of product gas, although this compound

549

mainly of H2.

550 551

With the increment in SB ratio is expected there is better gasification due to more

552

consumption of biochar. In the case of BSCB (see supporting information S3) with

553

increasing SB ratio the H2 and CH4 participation rise. It is evident that both CO and CO2

554

tend to decrease. This behavior can be explained by the development of simultaneous and

555

competing reactions. The development of primary gasification and CO reforming reactions

556

explain the H2 production. These reactions also reduce CO but favor the CO2 and H2

557

formation. The increase in CH4 content can be due to methanation reactions that are not

558

very significant as there H2 consumption is negligible. During gasification of BHR, the

559

production of H2 and CO was favored by increasing the SB ratio, with a remarkable effect

560

on consumption of CO2 and some CH4. In this case, primary gasification reactions favor

561

CO and H2 production; the CO2 reaction with carbon and CH4 with steam favor CO and H2

562

production.

563 564

The increased steam ratio did not change the behavior of BPKS gasification CO2 and H2

565

remain the main products. Although, no significant differences were observed according

566

statistically point of view, it was perceived a trend towards an increase in the average

567

production of CO2, as SB ratio is increased. This may be because of vapor reactions that

568

allow the tars and volatile material trapped in biochar structure to get out more easily and

569

be converted to CO254. Also, increasing the SB ratio slightly favors the formation of CH4,

570

which may be due to the increased methanation reactions that consume H2.


Page 28 of 41

Page 29 of 41

572

It was found that the SB ratio had little effect on HHV syngas. In the BSCB case, the HHV

573

increased only a 5% and for BRH the increase was 15%, due to the H2 concentration was

574

increased; while for BPKS, the HHV decreased by 4% because the CO2 is high for SB=2.

575

This is likely due to the gas dilution and heat-absorbing properties of excess water.

576

578

production of a gaseous species 34,47,64,65. However, in our biochar a behavior similar to the

579

pyrolysis process was found, in terms of the effect of the minerals on syngas (see Figure8).

580

BSCB gasification exhibited minimal effect on the yield of H2 and CO gases from minerals,

581

whereas for RH the effect of non-catalytic species is dominated by its high SiO2 content.

582

For BPKS, the reactivity under gasification is low due to low content catalytic species and a

583

structure that limits the mass and heat transfer.

400

a)

350 300

H2

250 200 150 100

CO

50 0

0

50 100 150 200 g Catalytic species / kg biochar daf

b)

50 40 30 20 10 0 0

H2 of B SCB / H2 of X

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

CO of SCB / CO of X

It was previously mentioned that minerals naturally present in biochar can increase the

g of CO / kg de biomasa daf

577

g H2 in syngas / kg biochar daf

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10 20 30 40 50 Catalytic species in SCB / Catalytic species en X

c)

14 12 10 8 6 4 2 0 0

2 4 6 8 10 12 14 Catalytic species in SCB / Catalytic species in X

Figure 8. Relation between catalytic species in biochar and syngas composition X is PKS or RH ♦PKS; ♦ SCB; ♦RH


Energy & Fuels

584

Similarly, it was found that there was no linearity in the behavior between the biochars with

585

catalytic species content. In fact, it was determined that the lignocellulosic structure had a

586

more significant effect on the processes than the minerals present.

587 588

3.2.2

Gasification residue characterization

589 590

Conclusions made from the gas analysis were supported by elemental and proximate

591

analysis of the solid residue of biochar gasification after two hours process. In Table 4, the

592

compositions of solid residues obtained from BSCB gasification (RSCB), BRH gasification

593

(RRH), and BPKS gasification (RPKS) are presented. As expected, there were decreases in

594

carbon, fixed carbon, and volatile matter contents from the gasification reactions. Similarly

595

we see an increase in oxygen and ash content from the release of the other constituents.

596

Table 4. Ultimate and proximate analysis of gasification solid residue Sample Parameters RSCB RRH RPKS 95.24 94.82 92.80 C 0.50 0.41 2.08 H 0.11 0.00 0.54 N 3.95 4.51 4.47 O 0.20 0.25 0.12 S Ultimate analysis (wt.% daf)

597

Proximate analysis (wt.% df)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

Ash

22.15

62.45

9.08

Volatile matter

6.03

4.41

14.76

Fixed carbon

71.82

33.14

76.16

598 599

In a fluidized bed, the particles are continuously subjected to heating and annealing

600

processes can coincide with gasification reactions in the char39. This is consistent with other


Page 31 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

601

research that has demonstrated that high temperatures induce fusion and coalescence of the

602

original material, which appears as a single particle with bright, compact, and smooth

603

surfaces5. These carbon transformations impact reactivity. Even with all difficulties of

604

biochar annealing, gasification is governed by the biochar structure itself and behaviors of

605

certain minerals that may or may not act as catalysts (inhibitors or accelerators) and

606

generate active sites for development of surface reactions21,40. A morphological examination

607

of the solid residue product was performed in order to verify modifications on structure due

608

to these properties. Morphologic observations of RSCB are shown in Figure 9a. Here, we

609

can see that the carbonaceous residue exhibits a severe attack on the surface, as well as

610

destruction of characteristic canals of SCB due to the thermochemical process. Hollows are

611

well-defined, smooth and exfoliated surfaces throughout the particle.

612

613 614 615

Figure 9. SEM images of gasification residue (a) RSCB; (b) RRH; (c) RPKS


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

616

Similar observations can be seen for RRH show a severe attack of surface, where one can

617

see a predominance of fragmented structures, holes and great exfoliation of surface layers

618

(see Figure 9b), which give an idea of the degree of process progress. This corroborates the

619

results of the chemical analysis of gases and residues. However, contrary to behavior of

620

RSCB and RRH, in RPKS we can see compaction and smoothing of the structure with

621

large holes and lack of porosity (see Figure 9c). This shows the difficulty for BPKS

622

gasification to be effective, because gasification of solids require solid-oxidant interaction.

623 624

To further understand these residues, optical microscope observations were performed

625

using WL, FL, and PL. Photomicrographs of RSCB (Figure 10a) validate the absence of

626

oils and partially transformed material, which verifies the efficacy of the gasification

627

process. Absence of oily material on RRH surface is validated by photomicrographs taken

628

under FL (see Figure 10b). The high mineral content exposed by the process is readily

629

apparent under PL; as in the other case, there is no evidence of oils condensed on the

630

surface from residual volatile matter.

631 632

Similarly, in the case of RPKS (see Figure 10c), the absence of condensed oil on the

633

surface or unprocessed material was confirmed , but it was obvious an exposure of mineral

634

material that looks brilliant in PL, yellowish green in FL, and different shades of gray in

635

WL.

636 637 638 639


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

640 641 642 643 644

Energy & Fuels

Figure 10. Microphotography of gasification residue (a) RSCB; (b) RRH; (c) RPKS. Left to right: White light (WL), fluorescent light (FL), and polarized light (PL).

Table 5. BET and CO2 surface area of gasification residue Area, m2/g Sample BET CO2 RSCB 244.53 ± 2.09 469.29 ± 2.18 RRH 182.11 ± 2.90 259.60 ± 4.59 RPKS 7.02 ±1.26 400.60 ± 2.14

645 646

To study porosity, BET surface area and microporosity by CO2 adsorption analyses were

647

performed (see Table 5). It was established that there is highest BET surface area in the

648

RSCB. The RPKS has the lowest surface area, confirming the difficulty of PKS to be

649

processed by gasification due to its lignitic origin. With respect to the narrow

650

microporosity, it was determined that the RSCB has the most area, followed by RPKS

651

residue; however the RRH developed more microporosity during gasification (13%),

652

compared with RSCB and RPKS (11% and 7%, respectively). This same behavior occurred

653

for the development of BET area, which can be explained by the destruction of soft 33 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

654

structures of cellulosic and hemicellulosic origin, which are majority in RH and SCB. Table

655

5 also shows that BSCB and BRH have high areas with N2 and CO2, indicating structures

656

with micro, meso, and macro pores which facilitate its degradation on gasification.

657 658

The microporosity developed by RPKS is greater than raw material (see Table 1), but lower

659

than the residues. In the absence of significant development of surface area in RPKS but

660

high CO2 adsorption, one can see a lot of microporosity but low accessibility. This explains

661

why the steam had little penetration during gasification. The great recalcitrant ability

662

inherent to BPKS due to high lignin also makes degradation difficult53. The behavior of

663

RPKS may make it useful for CO2 capture and storage.

664 665

3.2.3

Chemical functional group changes in biomass, biochar, and gasification residue

666

Another tool used to study the transformation of biomass through pyrolysis and gasification

667

was FTIR, which allows for analysis of the functional groups in the carbonaceous structure.

668

In Figure 11, the FTIR spectra of biomass, biochar, and residue are presented. In the

669

biomass, the main characteristics of the spectra are attributed to the presence of the lignin,

670

hemicellulose, and cellulose through the bond stretching of functional groups such as C-

671

phenolic, C=C, O-CH3, C=O, and others52,67,68.

672 673

There is an initial wide band at around 3365 cm-1 corresponding to O-H stretching that is

674

characteristic of lignin and carbohydrate phenolic (Ph), alcoholic groups, carboxylic

675

functional groups, and hydrogen bonds. The peak at 2942 cm-1 is associated with vibration

676

of C-H functional groups and elongations of -CH2 and -CH3.


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Transmittance, %

a)

3600

3100

2600

2100

1600

1100

600

Trasnmittance, %

b)

3600

3100

2600

2100

1600

1100

600

c) Transmittance , %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3600

3100

2600

2100

1600

wavenumbers (cm-1)

1100

600

Figure 11. FTIR spectra of (a) SCB; (b) RH; (c) PKS. − Biomass; − Pyrolized; − Gasification solid residue

677 678 679 680 681

The band at 2870 cm-1 is attributed to the vibration of O-CH3, which is commonly present

682

in lignin; it is weak for SCB and stronger for the PKS coinciding with the high content of

683

lignin in the latter. The peak around 1730 cm-1 corresponds to C=O in conjugate or non-

684

conjugate systems (carbonyl/carboxyl) and acetyl groups present in the hemicellulose. The

685

bands at 1604 and 1509 cm-1 are attributed to C-Ph and C=C, respectively, and are usually

686

found in the lignin aromatic structure; these peaks are stronger in the PKS than for the other

687

biomass. The deformations of aliphatic C-H corresponding to the polysaccharides are 35 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

688

present in all the spectrums (1456 cm-1 and 1426 cm-1). There is a common lignin band at

689

1375 cm-1 and another signal at 1230 cm-1 from C-H flexing69.

690 691

The peak at 1170 cm-1 is from C-O-C present in the lignin and the polysaccharides of

692

cellulose and hemicellulose. There are bands of C-O, C=C, and C-C-O stretching for

693

cellulose, hemicellulose, and lignin at 1051 cm-1; these bands overlaps with the signal

694

inorganics for this reason it signals is wider for RH, given that at these wavelengths there is

695

a strong adsorption of the C-Si type bonds70, characteristic of this biomass. Finally, the

696

signal in 897 cm-1 is attributed to the β-glycosidic links between monosaccharide units in

697

biomass71–74 of hemicellulose and cellulose. The signals around 850-870 cm-1 are attributed

698

to C-H bending of aromatic compounds.

699 700

As can be seen, many signals decrease in intensity (becoming almost unnoticeable) after

701

process of pyrolysis and gasification. This is due to the loss of functional groups during

702

devolatilization, steam attack on carbonaceous matrix, the thermal annealing and plastic

703

transformation. This leads to structures more ordered, which are composed primarily of

704

aromatic ring compounds with the simultaneous release of gases with functional groups

705

rich in O75. These rigid structures emit low vibrations.

706 707

3.2.4

Products yields for biochar gasification

708 709

Mass balances allowed for determine of product yields for each gasification biochar

710

process, the mean results for tests with SB = 1 are presented in Table 6.


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Energy & Fuels

712

Gasified sample BSCB BRH BPKS 713

Table 6. Yields of biochar gasification, SB=1 Products yield, kg biochar daf/ kg product CC (%) Solid residue, Syngas, “dry” “daf” 8.76 ± 1.83 15.21 ± 2.30 84.39 ± 0.52 18.98 ± 1.02 14.54 ± 1.13 85.26 ± 1.41 0.73 ± 0.98 2.40 ± 1.02 97.15 ± 0.84

Ecg (%) 7.55 ± 1.84 7.13 ± 1.34 0.45± 0.62

*dry free basis (“dry”)

714

The highest gas yield was for gasification of BSCB, confirming the process efficacy in

715

compared with other biochar treated. This is consistent with SCB structure (raw material),

716

which is composed basically of cellulose and hemicellulose. Its transformation requires less

717

thermochemical severity to make gas. The BRH with its high ash content has the lowest

718

syngas yield, as its available carbon content is lower. This restricts its use for gasification.

719

The BPKS exhibited the worst syngas yield, for reasons demonstrated by microscopic

720

analysis: the biochar structure fused and coalesced leading to a high yield of solid residue.

721

BPKS comes from a raw material with low ash content. Due to, the BPKS requires

722

increased process temperature and use of other oxidizing agents such as O2, CO2, or air for

723

a higher degradation of the lignitic structure. On the other hand, the low cold gas

724

efficiencies found for all biochars studied are evidence of poor gasification represented by

725

low syngas flow and low energy quality.

726 727

4. Conclusions

728 729

PKS has a high lignin content that conferred recalcitrant properties that hindered

730

transformation during pyrolysis and subsequent steam gasification of biochar. Therefore,

731

mixtures of oxygen with the water vapor may enhance gasification yields. In the case of


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

732

biomass rich in cellulose, such as SCB, the porous structures developed during pyrolysis

733

combined with a possible catalytic effect of mineral species facilitated the effect of the

734

gasifying agent on the carbonaceous matrix. Similar performance was observed for RH,

735

whose structure is rich in cellulose and hemicellulose, process yields can be affected

736

negative by the high ash content.

737 738

For the three biomass analyzed, it was observed that the process of biochar gasification is

739

made difficult by several factors:

740 741

•

Encapsulation of volatile material during the metaplastic phase.

742

•

Melted structures that clog pores and decrease the effect of process gas.

743

•

Large particle size that promotes the development of intraparticle secondary

744

reactions with which the release of volatiles is reduced favoring the formation of

745

biochar of organized structure of low reactivity.

746 747

The softening and melting of the lignocellulosic biomass components during pyrolysis

748

depends on the interaction between them, the heating rate, and other factors which induce

749

thermoplasticity. Thermoplasticity impacts further thermochemical processing, as seen with

750

PKS.

751 752

The authors consider that it is important to carry out further studies to understand the effect

753

of minerals on gaseous products through the implementation of techniques that allow the

754

removal of mineral species but preserving intact the lignocellulosic structure of biomass or


Page 38 of 41

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Energy & Fuels

755

biochar. Further, the properties of RPKS char residues may make it possible to use for CO2

756

capture and storage.

757 758

Acknowledgements

759

Acknowledgments to the Vice – Dean of Investigation and Extension of the Facultad de

760

Minas of Universidad Nacional de Colombia – Medellín Branch, for funding the project

761

code 24674, through the national program of support to the post – graduate students for the

762

strengthening of the investigation, creation and innovation, 2013 – 2014. To Brennan

763

Pecha of Washington State University, Pullman for his valuable contributions to this

764

research.

765 766

Supporting Information

767

Biomass characterization techniques and results, detailed biomass pyrolysis results and

768

product characterization, and SB ratio effect on syngas composition.

769 770

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Biochar gasification: An experimental study on Colombian

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