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Johnson Matthey Technol. Rev., 2021, 65, (3), 352

doi:10.1595/205651320x15899664199207

Process Intensification: Activated Carbon Production from Biochar Produced by Gasification

Highly porous carbon substances with low production costs

  • Ahmet Erdem
  • Environmental Engineering Department, Gebze Technical University, Gebze, 41400, Turkey
  • Murat Dogru*
  • Environmental Engineering Department, Gebze Technical University, Gebze, 41400, Turkey; Gasification Consultancy Ltd, No. 24, Newcastle upon Tyne, NE5 2RS, UK
  • *Email: m.dogru@gtu.edu.tr
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Article Synopsis

The recent increase in the number of policies to protect the environment has led to a rise in the worldwide demand for activated carbon, which is the most extensively utilised adsorbent in numerous industries and has a high probability to be used in the energy and agriculture sectors as electrodes in supercapacitors and for fertiliser production. This paper is about the production of activated biochar from oak woodchips char generated by an updraft fixed bed gasifier reactor. Following this, using steam as activating agent and thermal energy from produced synthesis gas (syngas), the resulting highly microporous carbonaceous biomaterial was subjected to physical activation at 750ºC. The properties of activated biochar include adsorption or desorption of nitrogen to identify the physical adsorption and surface area measurement, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The biochar surface area, generated as a result of the gasification process, showed substantial improvement after steam activation. Also, significant discrepancies were obtained from the surface volume and areas of biochar byproducts from the gasifier and activated biochar obtained by steam activation after the gasification treatment (total pore volume 0.022 cm3 g−1 and 0.231 cm3 g−1, Brunauer–Emmett–Teller (BET) surface area 21.35 m2 g−1 and 458.28 m2 g−1, respectively). The two samples also yielded noteworthy differences in performance. As a consequence, it may be concluded that the kinetics of steam gasification is quicker and more efficient for the conversion of biochar to activated carbon. The pore sizes of the carbon produced by steam activation were distributed over a wide spectrum of values, and both micro- and mesoporous structures were developed.

1. Introduction

1.1 Activated Carbon

The adsorption of air and water contaminants on an activated carbon surface is frequently used in air and water treatment systems. Conventional biological treatment processes are efficient in removing biodegradable organics. These processes, however, have limited ability to remove lignins, humic substances, pesticides and residual colour and odour-producing organic compounds. The performance of activated carbon in removing such organic compounds has been proven. Activated carbon filters used for water treatment in homes are usually made of either powdered activated carbon (PAC) or granular activated carbon (GAC). PAC is made of powdered carbon with a particle size of <0.18 mm and diameter in the range of 0.25–1.15 mm. It is generally used in batch type reactors and is subsequently filtered off. It is used in the treatment of liquids and cleaning of flue gases. GAC is produced from 0.2–5 mm particle size irregular carbon. For liquid and gas phase substances, it is applied on adsorption columns (13).

1.2 Biochar

The increase in production and accompanying energy demand which has emerged in the last century with the increasing population is becoming important. The environmental impacts caused by the increase in production draw attention and higher consumption creates problems for our limited natural resources. The necessity of treatment for the prevention of soil, air and water pollution is among the topics that are especially emphasised at international conferences. It is necessary for humanity to take joint decisions to increase the sensitivity of countries on environmental issues and to implement the necessary legal measures (4).

Biochar is a byproduct with high carbon content produced through the conversion of biomass into syngas by advanced thermal gasification using partial oxidation. Intensified gasification of biomass through partial oxidation is a recently developed, environmentally clean and renewable energy technology (5). Biochar obtained as a byproduct from gasification of woody biomass mainly contains carbon (85 wt%), nitrogen, hydrogen, phosphorus, calcium, magnesium and iron. Production of activated carbon from biochar is possible. To convert biochar into activated carbon, it is necessary to remove the specific contaminants to complete the activation.

Biomass is a renewable energy source but can cause environmental issues when it is not properly utilised. Therefore, it needs to be dealt using appropriate technology as compared to other renewable energy sources. Since thermal utilisation of biomass is carbon neutral, it does not cause global warming, and due to the quantities available it has high energy potential. Thermochemical methods are generally used to obtain energy from biomass. Known thermochemical methods are combustion, pyrolysis and gasification. The gasification process has several advantages over other thermal processes: the ability to dissolve material at lower reactor volumes, the formation of low amounts of contaminants and more efficient utilisation of the produced syngas. Compared to pyrolysis, it has the advantage of working autothermally without the need for external energy. Compared to other processes, gasification of woody biomass is stated to be one of the most suitable options for optimising the conversion efficiency of the fuel’s chemical energy (67). The stoichiometric reaction of woody biomass with partial oxygen results in flammable product gases and thus gasification. The basic equation of the gasification of biomass is as in Equation (i):

(i)

Various types of reactors are available for gasification. To not digress from the subject, the kinetics of gasification reaction was not elaborated. Fixed-bed biomass gasification reactors are used for syngas generation and biochar production. Fixed bed gasification reactors efficiently convert biomass to syngas and are the type of reactor suitable for the production of biochar as a byproduct with high carbon content (85–90 wt% carbon) (89).

The biomass fed into the gasification system passes through four sections in the reactor and is converted into syngas and biochar as a byproduct. The biomass passes through the drying, pyrolysis, reduction and oxidation zones in the reactor and is converted into biochar at elevated temperatures (800–1100ºC). The amount of biochar produced depends on the biomass type and the operational conditions. 10–20% of the biomass fed to the reactor is produced as biochar (1012).

Recently, researchers have begun to be interested in biochar produced as a byproduct from biomass by thermal conversion pathways (>700ºC), such as pyrolysis, thermal carbonisation, flash carbonisation and gasification (13). Many studies have suggested that biochar can be used in CO2 adsorption, soil remediation and air pollution removal (1416). Additionally, the use of the syngas produced by thermochemical conversion of biomass as a renewable energy source provides an advantage while the efficiency potential of biochar produced as a byproduct will make these systems very attractive (17).

1.3 Steam Activation

Physical activation improves the surface pores of biochar and affects the chemical properties of the surface (such as surface functional groups, hydrophobicity and polarity). The physical activation methods used for biochar are generally steam and gas activation.

The steam activation of the biochar is usually carried out after the thermal carbonisation of the biomass. The surface porosity of biochar increases after pyrolysis. Also, with steam activation, the activated biochar will gain higher porosity. The chemical formulae are shown in Equations (ii) and (iii) (18):

(ii)

 

(iii)

With the H2O (steam) and carbon reaction during the activation process, three outcomes emerge: (a) removal of the volatiles and tar from the surface; (b) formation of new micropores; and (c) expansion of existing pores (1920).

Studies on steam activation have shown that it increases the biochar’s surface area and micropores. For example, the surface area of biochar (136–793 m2 g−1) obtained by rapid pyrolysis (800ºC, 45 min) was increased as a result of steam activation. All these measurements were performed with BET isotherm (21). In another study, biochar (700ºC) from tea residue biomass was activated by steam and its surface area was obtained as 576.1 m2 g−1, pore volume as 0.109 cm3 g−1 and pore diameter as 1.998 nm (before activation, the surface area was 342.2 m2 g−1, pore volume 0.022 cm3 g−1 and pore diameter 1.756 nm) (22). The steam activated biochar formed at 700ºC indicated the highest absorption capacity (37.7 mg g−1) at pH 3, with a 55% growth in absorption ability compared to non-activated biochar produced under the same conditions. Consequently, activation with steam has potential to enhance the adsorption capability of biochar. At 700ºC, produced biochar derived from plant-based biomass detected relatively low surface areas (9.27 m2 g−1). Due to the formation of tar during thermal decomposition, the pores in the biochar are blocked (23). Steam activated biochar derived from bamboo waste had a larger surface area compared to non-activated bamboo biochar. Optimum conditions for activation were 850ºC and 120 min activation time. Under these conditions, the BET surface area of activated biochar was 1210 m2 g−1 and total pore volume was 0.542 cm3 g−1. This study showed that bamboo waste could be used to arrange new micropores in activated biochar through steam activation (24). In further research, activation of rice husk biochar was carried out at 800ºC using steam. Micro- and mesoporous structured biochar were produced and 1365 m2 g−1 surface area was obtained at the end of 15 min (25).

Another paper presents a study into the effect of different activation conditions and adsorption characteristics of biochar evaluated from tyre rubber waste. Steam was used as an oxidising agent and total micropore volumes and BET surface areas increased to 0.554 cm3 g−1 and 1070 m2 g−1, respectively. Consequently, steam was observed to generate a narrower extensive microporosity (26). An experimental test represents the production of activated biochar from barley straw using steam activation. Activation was conducted to maximise the micropore volumes and BET surface area of the biochar. Optimal conditions for steam activation were a hold time of 1 h at 700ºC. The micropore volume and surface area of the activated biochar were 0.2304 cm3 g−1 and 552 m2 g−1, respectively (27). A further study investigated activated biochar produced from date stone wastes by steam activation. The effect of activation hold time on surface textural structure properties of raw date stone and biochar were studied. The results indicated the presence of cellulose and hemicellulose in the raw material, and the predominance of carbon content. The highest microporous volume was 0.716 cm3 g−1 and specific surface area was 635 m2 g−1, obtained through biochar activated under steam at 700°C for 6 h (28). Another study was conducted to investigate the effect of sulfur in activated biochar prepared from apricot stones by steam activation. The activation temperature and time tested were in the ranges of 650–850ºC for 1–4 h. The experimental results revealed that the surface area of the biochar was 1092 m2 g−1 at activation conditions of 800°C for 4 h. The experimental results indicated that commercial production of porous activated biochar from apricot stones is reasonable (29). The significance of the nature and composition of biomass was demonstrated by the steam activation of three different biomass sources including wheat straw, coconut shell and willow (30). For these three biomass substances, activated carbons with specific surface areas of respectively 246 m2 g−1, 626 m2 g−1 and 840 m2 g−1 were produced.

Although physical activation using steam significantly increases surface area and porosity, there is a disadvantage of using steam activation. This is the loss of aromaticity and polarity in steam activated carbons compared to genuine biochar (31). However, using a combination of CO2 and steam activation has been reported to produce activated carbons with better surface area and pore structure compared to using only CO2 or steam for activation. In one study, activated carbon was produced from biochar obtained from olive kernel biomass using CO2, steam and a combination of CO2 and steam. A combined CO2 and steam activation under similar experimental conditions was reported to produce a higher surface area (1187 m2 g−1) compared to the specific surface area obtained by using only CO2 (572 m2 g−1) or steam (1074 m2 g−1) (32). From the aforementioned studies, it can be concluded that the reaction of steam with carbon occurs in a shorter period compared to the reaction of CO2 with carbon. While generally micropore activated carbons are produced by using CO2 in physical activation, meso- and macropores are formed in the structure during activation with steam (33). The reason for the difference in pores is attributed to the conversion of developed micropores into wide meso- or macropores and the faster reaction of the fixed carbon in the biomass structure of the steam. Also, it is possible to produce more pores using steam as it can penetrate the inner surface of the fixed carbon. On the other hand, CO2 stagnates in its reaction with the fixed carbon and therefore more homogeneous micropores are added to the structure due to activation using CO2. Determining the specified exposure time of the biomass at high temperatures with the activation agent is a critical decision to achieve high surface properties. The significance of biochar steam activation time was emphasised in some studies. It has been determined that there is a decrease in the surface pores of biochar exposed to long activation time (34).

Chemical and physical processes are needed to increase the utilisation value of biochar. Several processes are required such as separation of biochar into suitable granule size, obtaining carbon black, activation of pores by chemical processes and the sizing of activated carbon. The failure to use biochar produced as a byproduct of biomass gasification constitutes a disadvantage in all respects considering the spread of such technologies. Biochar produced as a byproduct by gasification of woody biomass is only used as a soil conditioner in small amounts. It is generally burned inefficiently in combustion boilers. To this end, this study investigates the conversion of biochar into activated carbon, which is a widely used valuable product. Several studies have emphasised that surface area and pore volume increase with the conversion of biomass at high temperatures by thermal methods (35). Thus, it is envisaged in the present study that the process of converting biochar obtained by gasification at high temperatures (800–1000ºC) into activated carbon by specific activation methods may have many advantages. The need for extra energy from external sources such as utilisation of fossil fuels to reach these temperatures in conventional carbon conversion systems causes high operational costs. Considering all this, it is expected that high quality activated carbons obtained from biochar will be available in the market at low costs.

2. Experimental

2.1 Materials and Methods

In this experimental study, oak woodchip was gasified in an updraft gasifier. Byproduct biochar was activated in the activation unit integrated into the gasification system.

The proximate and ultimate analyses of the oak woodchips were conducted, and the results are presented in the Results and Discussion section. Proximate and elemental analyses were performed to determine the usability of biochar produced after the gasification of biomass for the production of activated biochar. Proximate analyses of moisture (ASTM D7582-12), ash (ASTM E1755-01(2020)), fixed carbon (ASTM D3172-13) and volatile matter (ASTM D7582-12) were made according to standard methods. The lower heating value was determined using ASTM D5865-13 standard method. The amounts of carbon, hydrogen, nitrogen and sulfur were determined in the element analyser and the amount of oxygen was determined using the standard ASTM D3176-09.

Using a thermal analyser, the thermogravimetric carbonisation analysis of oak woodchips was performed. Approximately 10 mg samples with an average particle size of 0.25 mm were heated at 10ºC min−1 from room temperature to 800ºC under nitrogen flow. Throughout the measurements, the nitrogen flow was kept constant at 10 cm3 min−1.

Activation of the biochar produced in the gasification system in the Gebze Technical University (GTU) Gasification Laboratory was performed. For the preparation of the biochar and activated biochar, oak woodchips were first carbonised by gasification at 800–1000ºC for 1–2 h. Then, the resultant biochar produced as a byproduct from the gasifier was activated by steam at three different temperatures (700ºC, 750ºC and 850ºC) utilising thermal heat generated from a syngas burner at different activation times (30 min, 60 min and 90 min). 5.5 kg byproduct biochar was used in each run. During the final process, the reactor was cooled under inert injection of nitrogen gas and then the activated carbon was removed from the reactor and weighed in order to determine the burn-off undergone in the reaction. For steam activation, a stainless-steel vertical reactor illustrated in Figure 1 was used and integrated to the system to heat each 5.5 kg biochar sample. Throughout the experiment, an exact heating rate and steam flow rate (~20ºC min−1, 1.3 kg min−1, respectively) were applied. Referring to Equation (ii), the optimal stoichiometric steam amount was determined and calculated for the system.

Fig. 1

Schematic diagram of the experimental biochar activation setup with gasifier: A main biomass feeding hopper; B updraft gasifier; C biochar byproduct; D syngas exit pipe; E syngas burner; F air; G thermal heater; H exhaust waste heat; I pressure regulator; J steam generator; K jacket heater and insulation; L biochar activation reactor; M nitrogen inert gas; N stack gas; O gas clean-up; P stack

Schematic diagram of the experimental biochar activation setup with gasifier: A main biomass feeding hopper; B updraft gasifier; C biochar byproduct; D syngas exit pipe; E syngas burner; F air; G thermal heater; H exhaust waste heat; I pressure regulator; J steam generator; K jacket heater and insulation; L biochar activation reactor; M nitrogen inert gas; N stack gas; O gas clean-up; P stack

2.2 Characterisation of Biochar and Activated Biochar

The nitrogen adsorption or desorption isotherms were determined at 77 K by means of an automatic adsorption instrument to identify the textural properties of the produced biochar and activated biochar. Before the gas adsorption measurements, the samples were degassed at 300ºC under vacuum for 5 h. The N2 adsorption isotherm was achieved over a relative pressure, P:P0, ranging from roughly 10−6 to 1. The BET and t-plot methods were employed to determine the surface area, micropore surface area and pore volume of the biochar and activated biochar, respectively. Relative pressures in the 0.01–0.15 range were applied to evaluate the BET surface areas. The total pore volumes (Vt, cm3 g−1) were considered to be the liquid volumes of N2 at high relative pressure near unity (~0.99) (3637).

SEM analysis was performed to investigate the surface, textural, porosity and structural properties of activated biochar produced under different conditions. SEM analyses were taken by enlarging ×500 to observe changes in surface morphological structure before and after activation.

To examine the crystal structure, XRD profiles of each sample were obtained at room temperature, using a copper Kα X-ray source, under 40 kV and 30 mA analysis conditions. Diffraction data were taken on a scale ranging from 2θ = 0–90º.

In order to qualitatively determine surface functional groups, FTIR spectra were obtained at room temperature, with the support of diamond orbital attenuated total reflection (ATR) accessory, by scanning 128 times in the range of 500–4000 cm−1 band at 4 cm−1 separation sensitivity. Samples were placed directly on the diamond crystal and were analysed by applying pressure to allow it to fully interact with the diamond crystal.

3. Results and Discussion

Figure 2 shows the complete activation of biochar produced from the gasification system at GTU Gasification Experimental Rig (Figure 3). The biochar obtained by the gasification of oak woodchips (Figure 4) in the gasification system at GTU was treated with the physical partial activation method integrated to the gasification system. There are different wood-based sources for biochar production. The highest carbon content is provided from oak woodchip feedstock. Therefore, in the present study, biochar produced from oak woodchip feedstock was used for the production of activated carbon materials. Table I summarises the composition of the oak woodchip feedstock. In addition, the proximate and elemental analysis of the oak woodchip is given in Tables II and III respectively, and Figure 5 represents the TGA tracings for oak woodchips.

Fig. 2

(a) Oak woodchips; (b) gasification byproduct biochar; (c) activated biochar with steam activation

(a) Oak woodchips; (b) gasification byproduct biochar; (c) activated biochar with steam activation

Fig. 3

Gasification System in Gebze Technical University, Turkey

Gasification System in Gebze Technical University, Turkey

Fig. 4

Oak woodchips gasification byproducts (biochar)

Oak woodchips gasification byproducts (biochar)

Table I

Analysis Results of Oak Woodchip Feedstock

Analysis Unit Analysis results Method
Original based In air dry based Dry based
Moisture wt% 45.01 2.95 ASTM D7582-12
Ash wt% 0.22 0.4 0.41 ASTM E1755-01
Volatile matter wt% 44.92 79.27 81.68 ASTM D7582-12
Fixed carbon wt% 9.85 17.39 17.91 ASTM D3172-13
Total sulfur wt% 0.06 0.1 0.1 ASTM D4239-13
Sulfur in ash wt% 1.38 ASTM D4239-13
Lower heating value cal g−1 2330 4533 4688 ASTM D5865-13
Higher heating value cal g−1 2749 4852 5000 ASTM D5865-13
Table II

The Results of Elemental Analysis of Oak Woodchip Feedstock

Analysis Unit Oak woodchip Method
C wt% 52.38 ASTM D5373-14
H wt% 6.57 ASTM D5373-14
N wt% 0.27 ASTM D5373-14
S wt% 0.1 ASTM D4239-13
Ash wt% 0.41 ASTM E1755-01(2020)
O wt% 40.27 ASTM D3176-09
Table III

Halogen and Ash Analysis Results of Oak Woodchip Feedstock

Analysis Unit Oak woodchip Method
F % <0.05 Ion chromatography
Cl % <0.02 ISO 587:2020
Bulk density kg m−3 367.8 ISO 787-11:1981
SiO2 % 14.5 Ash analysis ASTM D2795-95
Al2O3 % 4.3
Fe2O3 % 3.3
CaO % 37.3
MgO % 11.6
SO3 % 3.6
Na2O % 3.2
K2O % 19.3
TiO2 % 0.4
P2O5 % 2.5

Fig. 5

TGA diagram for oak woodchips

TGA diagram for oak woodchips

According to the proximate and elemental analysis data given in Tables I and II, it is understood that oak woodchip has good potential and represents an ideal feedstock to produce suitable biochar via gasification due its high carbon content and higher heating values. In addition, it appears to be an excellent source of feedstock for the gasification process due to its low ash content.

According to the halogen and ash analysis given in Table III, the mineral and metal content in the oak woodchip feedstock caused the formation of a cratered shape biochar, which also acted as a natural catalyst during the gasification reactions, thereby providing the desired or better syngas yield.

Figure 5 shows the results from TGA carried out on oak woodchips samples. The thermal degradation of oak woodchips takes place in three stages. The first stage, which occurs at temperatures ranging between 30ºC and 100ºC, involves the loss of moisture content in the wood with approximate weight loss of 10 wt%. The second stage with nearly 20 wt% weight loss occurred while temperature rose from 150ºC to 250ºC. This step is related to the release of volatiles resulting from the degradation of hemicellulose. The third stage, occurring at 300–550ºC, is characterised by the decomposition of cellulose and lignin. The maximum rate of weight loss occurred in the third section with weight loss of around 45 wt%. Nevertheless, no significant weight loss was observed above 650ºC, indicating that a temperature at this level or above could be preferred for preparation of activated biochar. The total weight loss recorded was approximately 85 wt%.

Biochar extraction was obtained after uniform ligneous biomass gasification. Since it has high fixed carbon content and is suitable for active carbon production, oak woodchip feedstock was chosen. The supplied biomass was utilised directly in gasification without any prior treatment. The optimum temperatures determined in these gasification tests were in the range of 800–1000ºC, while the air:fuel ratio was determined as 1.6. Gasification system operations were carried out accordingly. The applied gasification conditions are shown in the mass and energy diagram given in Figure 6. The energy value of the feedstock was calculated as 20 MJ kg−1 on a dry basis. Additionally, about 80 kg h−1 of ambient air was used for the gasification agent. According to the conditions in this operational tests, 5.5 kg of biochar is obtained per hour from 50 kg h−1 fuel supply to the gasifier. In addition, a total of 850 MJ syngas thermal energy was determined in the mass energy balance which was utilised as an energy source for the activation unit integrated into the system to apply process intensification. These tests were a fine example of intensification since two processes, namely gasification and activation, retrofitted to each other to preserve and use heat in the process as compared to conventional activation.

Fig. 6

Mass and energy balance for gasifier during the production of biochar. Hot gas efficiency = 85%

Mass and energy balance for gasifier during the production of biochar. Hot gas efficiency = 85%

One critical aspect in the assessment of the potential for activated biochar production from oak woodchip by using the updraft gasifier is the energy and mass balance of the process. The mass and energy equilibrium on the reactor provide a quantitative measure of the efficiency for conversion of feedstock to produced gas and biochar using this particular type of gasifier. Mass and energy balances for a specific type of feedstock will vary from one type of gasifier to another as the thermodynamic equilibria and reaction kinetics of the three head reactions in gasification vary depending on the gasifier operating conditions.

The mass balance analysis on the gasifier requires an evaluation of the inputs to and outputs from the gasifier. From the calculations, achieving 100% closure is not easy and the results illustrate the complications of acquiring this data. However, the mass balance closure for the initial run was found to average 85% which represents a reasonable figure for the initial proof of concept assessment of oak woodchip gasification study trials. The mass and energy balance diagram for the initial run is given in Figure 6.

Considering the mass and energy balance data, the reactor was operated and syngas and biochar production were obtained at high rates close to theoretical calculations. During operations, approximately 5.5 kg of biochar were obtained per hour, and this produced biochar were transferred to the biochar activation unit shown in Figure 1. Also, syngas produced by gasification was burned in a specially designed syngas burner and thermal heat obtained was transferred through the thermal heater to the biochar activation unit. During the experimental study, nine different activation operations were performed at different temperatures (700ºC, 750ºC and 850ºC) each with a different residence time (30 min, 60 min and 90 min). Due to the reactor design, a heating rate of approximately 20ºC min−1 was conducted in all studies, which is a favourable condition for activating biochar. The steam flow rate was 1.3 kg min−1 in accordance with this heating rate and reactor design.

Gasifier byproduct of approximately 5.5 kg biochar was transferred to the activation reactor. The first experiment was carried out at 700ºC for 30 min residence time. The same procedure was applied at 700ºC for 60 min and 90 min residence time, after which the samples were cooled with nitrogen gas. Subsequently, experiments were carried out at 750ºC and 850ºC under the same conditions. Burn-off values of the samples formed after activation were extracted from the biochar obtained by weight from the first value and were determined as a percentage. Effects of activation time and temperature on burn-off of activated biochar are given in Figure 7. In addition, BET surface area and total pore volume analyses were performed in each sample obtained and given in Figure 8 and Figure 9.

Fig. 7

Effect of activation time and temperature on burn-off of activated biochar

Effect of activation time and temperature on burn-off of activated biochar

Fig. 8

Progress of BET surface area and total pore volume with burn off of activated biochar

Progress of BET surface area and total pore volume with burn off of activated biochar

Fig. 9

Effect of activation time and temperature on BET surface area of activated biochar

Effect of activation time and temperature on BET surface area of activated biochar

According to Figure 7, during the activation of the biochar with steam method, there is a mass loss between 40–70%. It is evident from the figure that the biochar regularly loses mass due to an increase in the activation temperature and residence time. It is clearly seen from the data that the highest mass loss is observed at 850ºC when activation period reaches 90 min. The possible reason for that is some carbon in the biochar structure reacts with water at high temperatures and forms carbon monoxide and hydrogen (syngas). In this thermochemical process, carbon leaves the structure and causes mass loss but creates more pores in the structure.

Physical adsorption and BET surface area measurement were performed. According to BET surface area and total pore volume analysis shown in Figure 8 and Figure 9, it is clearly seen that temperature and residence times have a significant effect on the formation of the biochar surface morphology. It can be understood from these figures that the most appropriate temperature and residence time interval applied for activation for oak biochar was 60 min at 750ºC.

The resultant biochar produced from the gasification as a byproduct is compared with partially activated biochar utilising steam in Table IV. It was determined that the volume and area of the surface pores of the steam treated biochar increased significantly (total pore volume 0.022 cm3 g−1 and 0.231 cm3 g−1, BET surface area 21.35 m2 g−1 and 458.28 m2 g−1, respectively). It was compared with the commercial activated carbon used in water filters produced from coconut shell and physical adsorption and surface area measurement are presented in detail (0.326 cm3 g−1 to 648.96 m2 g−1).

Table IV

BET Results of Biochar and Activated Biochar with Steam Activation

BET results (m2 g−1)
Biochar 21.35
Activated biochar (750ºC, 60 min) 458.28

BET surface area analyses were carried out on biochar and by activating the biochar obtained as a result of gasification in the steam activation unit shown in Figure 1 at the specified temperatures. The results are given in Table IV. The most suitable temperature for activation was found at 750ºC. Since the 1 h residence time activated biochar at 750ºC gave the highest result (458.28 m2 g−1), other analyses such as FTIR, XRD and SEM of the selected activated biochar obtained were conducted. These results were also compared with the biochar produced from the gasifier without any activation.

Finally, during the steam activation process, the porosity increased apparently when the temperature rose to about 750ºC. On the other hand, the micrographs obtained from the biochar and activated biochar showed no discrepancies in terms of morphological properties. As both samples yielded different porosity values, this result can be said to be coherent with the physisorption results obtained from surface analyses.

The discussion section, first of all, deals with the properties of the carbonisation process. The produced activated biochar was found to yield an increase in the surface area (from 21.35 m2 g−1 to 458.28 m2 g−1) and total volume (from 0.022 cm3 g−1 to 0.231 cm3 g−1) as a result of the transition from limited air to steam in the gasification system. Generally, the rise in the temperature of the gasification process causes a higher discharge of volatile matter. This, in turn, leads to an increase in the original porous structure that will be further developed during the activation phase. On the other hand, high temperature in the gasification process results in the softening and sintering of the high molecular weight volatiles, which leads to the depolymerisation and shrinkage of the particles. This also causes a reduction in the micropore surface area and volume. Nevertheless, a temperature below 700ºC in the gasification phase impedes the complete devolatilisation of the low molecular weight volatiles, and as a result, prevents the initial porosity from being further developed.

An increase in the temperature from 800ºC to 1000ºC leads to a decrease in the yield efficiency of biochar from gasified oak woodchips. This is caused by the oak woodchips being partially decomposed to gaseous products. Therefore, the ideal gasification and activation temperature was 750ºC, which was used to activate the samples in the following.

The ratio of the mass of activated biochar to the mass of the raw material was calculated to determine the yield of activated biochar. The oak biochar activated at 750ºC was examined to evaluate the effects of the activation temperature and the hold time on the yield of activated biochar. The sample underwent activation with steam at predetermined activation temperatures and constant hold time (1 h), different hold times and constant activation temperature (750ºC).

An activation temperature of 750ºC at constant hold time caused a change in the yield of activated biochar. This is linked to the elimination of volatile matter arising from the decomposition of the main oak woodchips compounds, i.e., cellulose (a long glucose polymer without branches) and hemicellulose (composed of a variety of branched saccharides). Due to the decomposition of all cellulose and hemicellulose, the yield becomes stable at a temperature above 700ºC. As a result, lignin, the third component of oak woodchips that is more challenging to decompose, remains. In fact, lignin is known to decompose slowly at a temperature ranging from room temperature to 900ºC (38). Yet, the decomposition of cellulose and hemicellulose generates a porosity in oak woodchips that enables more efficient diffusion of oxygen to the particles. Hence, an increase is obtained in the kinetic reaction of lignin with oxygen. According to these results, the more cellulose and hemicellulose the raw material contains, the faster the decomposition of lignin takes place. The decomposition of the biomass actually occurs in two steps during activation: the decomposition of the cellulose and hemicellulose takes place first. This first step leads to an increase in the porosity of the activated biochar, and, as a result, the oxidising agent can easily diffuse into the particles. Secondly, the lignin reacts with the oxidising agent.

The endothermic reaction of carbon with water to produce carbon dioxide and hydrogen is thermodynamically more desired and is quicker at 750ºC. Longer hold time generates an increase in the amount of discharged volatile matter.

Surface energies of oak biochar produced after gasification were measured by SEM and energy dispersive X-ray spectroscopy (EDS) analysis was performed at GTU Laboratories (Figure 10 and Table V). Surface pores were clearly seen in SEM analyses and images were found to be consistent. In addition, biochar produced after the oak gasification process has a carbon content of about 80–85%. These findings are confirmed by EDS measured at different areas of the sample since silicon, oxygen, potassium, calcium and magnesium were observed on the surface (see the spectrum in Table V), which mainly contributed to the formation of low melting point eutectics.

Fig. 10

SEM analysis of: (a)–(c) biochar and (d)–(f) activated biochar

SEM analysis of: (a)–(c) biochar and (d)–(f) activated biochar

Table V

Energy Dispersive Spectroscopy Analysis of Oak Woodchips Biochar and Activated Biochar

Material Element Weight, % Atom, % Error, % Net intensity, %
Activated biochar (750°C, 60 min) C 84.28 87.72 0 11607.58
O 15.72 12.28 0.01 827.78
Biochar without any activation C 79.66 84.32 0 30208.26
O 19.14 15.21 0 3085.12
Mg 0.22 0.12 0.02 221.97
Al 0.17 0.08 0.02 183.87
Si 0.13 0.06 0.02 153.36
K 0.36 0.12 0.03 225.72
Ca 0.32 0.1 0.03 166.7

XRD is an analysis method to prove whether the structure is crystalline. This method is widely used in the synthesis of activated carbon. Although activated carbons are generally amorphous structures, crystals can be found in the activated carbon structure depending on the synthesis methods.

Considering the XRD results showed in Figure 11 of the synthesised activated biochar, it is seen that there is no crystalline phase in the structure and the structure is completely amorphous. In addition, it was understandable that no peak was seen in previous studies since the synthesis conditions were considered to be quite amorphous. It is seen that all activated carbon samples have three different amorphous phases. A horizontal baseline can be seen in the diffractogram of biochar and activated biochar, which indicates that a significant proportion of the matter is amorphous. It can be inferred from the XDR pattern of the biochar that the gasification process had a significant impact. Owing to the decomposition of cellulose and hemicellulose during the thermal treatment, the diffraction peaks achieved at 2θ = 16.3º and 20.60º disappear. Subsequent to the pyrolysis, only two broad diffraction peaks at around 23.58º and 43º remain, which could be related to the presence of carbon and graphite (39).

Fig. 11

Activated biochar and biochar XRD analyses

Activated biochar and biochar XRD analyses

Surface functional groups of the biochar at high temperature (750ºC) with steam were measured. Surface functional groups of the biochar were also analysed by FTIR in the laboratories at GTU. Considering the formation of surface functional groups of the biochar, these analyses revealed that gasification is a correct method in the production of activated carbon. As seen in Figure 12, as expected for activated carbon, C–H stretching vibration was observed at 2973 cm−1, C=O and C=N at ~1591 cm−1 and C–O stretching at ~1045 cm−1. Also, stretching was found in the alcohols, phenols, ether and ester groups. As in the case of commercial activated carbon, a strong band is seen at about 799 cm−1 that is described as COOH vibrations in carboxylic groups; biochar and partially activated biochar were also observed.

Fig. 12

FTIR surface functional group analysis of biochar, activated biochar and commercial activated carbon

FTIR surface functional group analysis of biochar, activated biochar and commercial activated carbon

4. Conclusion

This paper investigated the possibility of production of activated carbon from biochar generated as byproduct from gasification of oak woodchips by means of a physical activation method with an activating agent: pure steam. According to the SEM examination of raw biochar obtained from oak woodchips and activated biochar, the gasification process led to a gradual increase in the porosity, and the steam process caused further activation. Carbons with increased porosity and surface area were produced by steam activation of biochar from oak woodchips. Substoichiometric air gasification resulted in a maximum BET surface area and micropore volume of 21.35 m2 g−1 and 0.022 cm3 g−1, respectively. On the other hand, steam activation of biochar further resulted in a maximum BET surface area and micropore volume of 458.28 m2 g−1 and 0.231 cm3 g−1, respectively. Hence, steam activation yielded a more than ten-fold increase in this study. Also, oxygen functional groups (carboxylates, lactones and phenols) on the surface led to a high degree of aromaticity in the activated biochar.

One of the major contributions of this study for the state-of-the-art is that the production of activated carbon will not require external thermal energy for activation, which causes a considerable increase in the cost of the production. The process is intensified by eliminating the thermal activation step and biochar produced as a byproduct as a result of gasification has a high carbon content, highly porous structure and is a semi-activated substance. Previous studies have also shown that specific processes can be employed to convert biochar into activated carbon. Furthermore, less costly and more sustainable production of activated carbon can be obtained by using byproduct biochar produced from biomass gasification systems.

To sum up, if prepared under optimal settings, the activated biochar produced from oak woodchips can have a large surface area and microporous volume. These highly porous carbon substances with low production costs can be effectively used to control environmental pollution through, for example, adsorption of gaseous pollutants and heavy metals.

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Acknowledgement

This study was financially supported by the Scientific and Technological Research Council of Turkey – TÜBİTAK (Project Number: 119Y029).

The Authors


Ahmet Erdem received his Doctorate degree (2016–2021) in Environmental Engineering from Gebze Technical University, Turkey. He has been a member of academic staff at Gebze Technical University since 2015. He worked as a commissioning engineer at a waste gasification research and development project in South Korea in 2014. The project was sponsored by the Korean Environmental Ministry and managed by Beltran Korea as subcontractor. His PhD thesis is on activated carbon production from biochar produced from gasification.


Murat Dogru received his Doctorate degree (1996–2000) from the Chemical and Process Engineering department at Newcastle University, UK. He researched and lectured for eight years as a member of academic staff at Newcastle University from 2000 until 2008. He then worked as an associate consultant in the Adapt Low Carbon Group at the University of East Anglia, UK, from 2008. He obtained his associate professorship in 2003 and followed by professor title in 2020 in Environmental Engineering at Gebze Technical University in Istanbul, Turkey. Dogru is also adviser at Gasification Consultancy Ltd, Newcastle, UK and R&D consultant at Beltran Technologies, Inc, New York, USA, for the industrial development of gasification systems using a variety of biomass and wastes in order to develop renewable energy generation systems for local governments and industries. He has published more than 100 international publications in journals and international symposiums which all have direct relevance to renewable energy generation technologies, in particular biomass and waste gasification.

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