materials

Article Porous Structure Properties of gerardi Derived Carbon Materials

Natalia Howaniec 1,* ID and Adam Smoli ´nski 2 ID

1 Department of Energy Saving and Air Protection, Central Mining Institute, Pl. Gwarków 1, 40-166 Katowice, Poland 2 Central Mining Institute, Pl. Gwarków 1, 40-166 Katowice, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-32-259-2219

 Received: 30 April 2018; Accepted: 22 May 2018; Published: 24 May 2018 

Abstract: Various carbonaceous materials are valuable resources for thermochemical conversion processes and for production of materials of proven sorption properties, useful in environmental applications for gaseous and liquid media treatment. In both cases, the parameters of the porous structure of carbon materials are decisive in terms of their physical and mechanical properties, having direct effects on heat and mass transport as well as on sorption capacity and selectivity. The physical activation of carbon materials produced from various precursors is widely discussed in literature. In this respect, the effects of temperature and partial oxidation of carbonaceous materials with steam or carbon dioxide are mostly considered. The reports on the effects of pressure on the development of porous structures of carbon materials are, however, extremely limited, especially when as a precursor is concerned. In this paper, the results of an experimental study on the effects of pressure in the range of 1–4 MPa on the specific surface area, the total pore volume, average pore diameter, and microporosity of carbon materials prepared with the use of Andropogon gerardi biomass as a precursor are presented. The tested samples were prepared at the temperature of 1000 ◦C under an inert gas atmosphere in the high-pressure thermogravimetric analyzer. The most developed porous structure was reported for carbon materials produced under 3 MPa. The highest volume of narrow micropores was characteristic for materials carbonized under 2 MPa.

Keywords: biomass; char; porous structure; carbon material

1. Introduction Depletion of natural resources and increasing energy demand imply the need for implementation of more sustainable technologies in all aspects of economic development. In particular, this concerns recycling and utilization of waste materials but also the wider use of biomass as a raw material of low carbon footprint. Biomass valorization has been widely discussed recently in terms of its utilization in thermochemical processing technologies, including pyrolysis and gasification [1]. Gasification of biomass chars alone or in blends with fossil fuels or waste materials is also a subject of research, in particular when production of hydrogen as a clean energy carrier is considered [2–5]. Various biomass-derived materials and gasification residues are also valuable resources for the production of activated carbons of proven sorption properties, useful in environmental applications for gaseous and liquid media treatment [6–9]. In both cases, the parameters of porous structure of carbon materials determine their physical and mechanical properties, heat and mass transport processes, as well as sorption capacity and selectivity. The physical activation of carbon materials produced from various precursors is widely discussed in literature, where the effects of temperature, heating rate, activation time, and partial oxidation of carbonaceous materials with steam or carbon dioxide are mostly discussed [6,8,10,11]. Physical activation methods are often combined with chemical activation

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with various agents, e.g., KOH, NaOH, ZnCl2,H3PO4, (NH4)2S2O8 [7,9,12–14]. However, reports on the effects of pressure on the development of the porous structure of carbon materials are limited and concern mostly the characterization of coal chars’ behavior, including their reactivity, in gasification and combustion processes [15–18]. The studies presenting the effect of pressure on the development of biomass-derived carbon material porosity are even more scarce [19,20]. The pressures range applied in previous studies is also quite narrow and relatively low (0.5–2.5 MPa) [15,19], while the heating rates are high (of a few hundred ◦C per minute), representing the conditions of gasification reactors. Such conditions may not reflect the optimal terms for production of carbon materials of most developed or tailored porous structures. Therefore, in this paper, the results of the experimental study on the effects of physical activation with pressure in the range of 1–4 MPa, and at high temperature of 1000 ◦C on the development of porous structure of carbon materials prepared with the use of Andropogon gerardi biomass as a precursor are presented. The effect of pressure on the specific surface area, the total pore volume, average pore diameter, and microporosity of biomass-derived carbon materials is discussed. The results prove pressure to be a parameter of considerable potential in shaping the porous structure of carbon materials.

2. Materials and Methods The biomass sample was acquired from an experimental stand of M&D Farms Sp. z o. o. in Swierczow, Poland. Andropogon gerardi is a grass of 1–2.5 m long blades cultivated mainly as an and animal feed crop, but is also applicable in anti-erosion and reclamation activities in post-industrial areas [21,22]. Its cultivation potential in climate conditions of central Europe is considerable, producing up to 24 tons of dry mass per hectare on moderately moist soils of pH 5–8. This makes it also a potential precursor for carbon materials of enhanced sorption properties. The ultimate and proximate analyses of the biomass and its ash composition analysis were performed in the accredited laboratories of the Central Mining Institute with the application of relevant standards. The results of these analyses are given in Table1.

Table 1. Physical and chemical properties of Andropogon gerardi biomass.

Parameter, Unit Value Proximate Analysis Total moisture, % w/w 9.72 Ash, % w/w 3.87 Volatiles, % w/w 70.26 Fixed carbon, % w/w 16.15 Ultimate Analysis Sulfur, % w/w 0.06 Carbon, % w/w 53.3 Hydrogen, % w/w 7.57 Nitrogen, % w/w bdl Oxygen, % w/w 25.54 Heating Value Lower heating value, kJ/kg 14,242 Ash Composition

SiO2,% w/w 14.67 Al2O3,% w/w 3.18 Fe2O3,% w/w 0.93 CaO, % w/w 37.10 MgO, % w/w 3.46 Na2O, % w/w 0.53 K2O, % w/w 22.13 SO3,% w/w 4.12 TiO2,% w/w 0.15 P2O5,% w/w 13.17 ZnO, % w/w bdl Materials 2018, 11, 876 3 of 8 Materials 2018, 11, x FOR PEER REVIEW 3 of 8

TheThe biomassbiomass waswas carbonized carbonized with with the the use use of of a high-pressurea high-pressure thermogravimetric thermogravimetric analyzer analyzer with with the magneticthe magnetic suspension suspension balance balance mechanism mechanism (Rubotherm (Rubotherm GmbH, Bochum, GmbH, Germany),Bochum, atGermany), the temperature at the oftemperature 1000 ◦C in anof 1000 inert °C gas in (Ar) an inert atmosphere gas (Ar) and atmosphere under pressure and under of 1, 2,pressure 3, or 4 MPaof 1, for2, 3, 5 or h (see4 MPa Figure for 51 ).h A(see heating Figure rate 1). A of heating 20 ◦C/min rate of was 20 applied.°C/min was In eachapplied. experiment, In each experiment, 1 g of biomass 1 g of sample biomass was sample used. Thewas resultantused. The charsresultant were chars crushed were andcrushed sieved and below sieved 0.2 below mm, 0.2 outgassed mm, outgassed overnight overnight at 120 at◦ C120 and °C analyzedand analyzed with thewith use the of use gas sorptionof gas sorption analyzer analyzer Autosorb Autosorb iQ (Quantachrome iQ (Quantachrome Instruments, Instruments, Boynton Beach,Boynton FL, Beach, USA) FL, [23 ].USA)[23]. The specific The surface specific area surface was area quantified was quantifi on theed basis on ofthe the basis nitrogen of the isothermnitrogen dataisotherm at − 196data◦ Cat and−196 with °C and the with use ofthe the use multi-point of the multi-point BET method BET method [24]. Pore [24]. size Pore distribution size distribution of the micro-mesoporousof the micro-mesoporous material material was determined was determined with the with use the of use the Densityof the Density Functional Functional Theory Theory (DFT) method(DFT) method [25]. The [25]. total The pore total volume pore wasvolume determined was determined as the volume as the adsorbed volume atadsorbed the relative at the pressure relative of 0.99.pressure The volumeof 0.99. The and areavolume of micropores and area of were micr computedopores were with computed the use of with V-t-deBoer the use methodof V-t-deBoer based onmethod the nitrogen based on sorption the nitrogen isotherm sorption at −196 isotherm◦C data at [−26196] and °C withdata [26] the applicationand with the of application the Monte Carloof the (MC)Monte method Carlo and(MC) carbon method dioxide and isothermcarbon dioxide at 0 ◦C[ isot27].herm The images at 0 °C of [27]. the carbon The images materials of werethe carbon taken bymaterials scanning were electron taken microscopyby scanning (SEM)electron using microscopy a SU-3500N (SEM) microscope using a SU-3500N (Hitachi microscope High-Technologies (Hitachi Corporation,High-Technologies Tokyo, Corporation, Japan). Tokyo, Japan).

Figure 1. Schematic diagram of the installation forfor biomassbiomass carbonizationcarbonization atat highhigh pressure.pressure.

3.3. Results and Discussion TheThe resultsresults ofof thethe porousporous structurestructure characterizationcharacterization are givengiven inin TableTable2 2.. The The nitrogen nitrogen sorption sorption isothermsisotherms forfor allall charschars showshow thethe complexcomplex structurestructure ofof the micro- andand mesopores networknetwork reflectedreflected inin thethe highhigh uptake uptake at lowat low relative relative pressures, pressures, with microporouswith microporous structures structures and hysteresis and hysteresis loops distinctive loops fordistinctive mesoporous for mesoporous materials [23 materials]. The shape [23]. of The the hysteresisshape of the loop hysteresis is typical loop for irregular is typical structures for irregular and slit-likestructures pores and [ 23slit-like]. The pores exemplary [23]. The isotherm exemplary for sample isotherm no for 3 carbonized sample no under3 carbonized 3 MPa under is presented 3 MPa inis Figurepresented2a. Thisin Figure char 2a. was This proved char to was have proved the most to have developed the most porous developed structure porous in terms structure of the in specificterms of surface the specific area, surface total pore area, volume, total pore and vo microporelume, and volume micropore and areavolume (see and Table area2). The(see increaseTable 2). inThe carbonization increase in carbonization pressure in the pressure range ofin 1–3the range MPa resulted of 1–3 MPa in the resulted significant in the increase significant in the increase specific in surfacethe specific area surface and the area total and pore the volume. total pore With volume. the increase With the in pressureincrease fromin pressure 1 to 2 MPa,from the1 to specific2 MPa, surfacethe specific area andsurface the area total and pore the volume total doubled.pore volu Theme applicationdoubled. The of 3application MPa pressure of 3 resultedMPa pressure in the specificresulted surface in the areaspecific and surface the total area pore and volume the total almost pore three volume times almost higher three than fortimes chars higher produced than for at 1chars MPa produced (Table2). at Further 1 MPa increase (Table 2). in Further pressure increase to 4 MPa in pressure resulted to in 4 a MPa decrease resulted in the in specifica decrease surface in the areaspecific and surface the total area pore and volume the total of pore about volume 19 and of 23%, about respectively, 19 and 23%, when respectively, compared when to the compared maximum to valuesthe maximum reported values for 3 reported MPa chars. for 3 This MPa effect chars. is This in line effect with is in previous line with observations previous observations made for othermade precursors,for other precursors, including coalincluding [15–18 coal] and [15–18] biomass and [20 bi],omass though [20], the oppositethough the tendency opposite of thetendency surface of area the ofsurface biomass-derived area of biomass-derived carbon materials carbon to decrease materials with to decrease pressure haswith also pressure beenreported has also [been19]. reported [19].

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Table 2. Parameters of the porous structure of carbon materials determined on the basis of nitrogen sorptionTable 2. isothermParameters at −196 of the °C porousand carbon structure dioxide of carbonsorption materials isotherm determined at 0 °C. on the basis of nitrogen sorption isotherm at −196 ◦C and carbon dioxide sorption isotherm at 0 ◦C. Average Total Pore V-t-deBoer MC MC Sample Pressure, Multi-Point Pore Total Pore V-t-deBoer V-t-deBoer Sample Pressure, Multi-Point Average Pore Volume, Volume, V-t-deBoer MCVolume, Volume, MCArea, Area, No. MPa BET, m2/g Diameter, Volume, Volume, Area, m2/g No. MPa BET, m2/g Diameter, nm cm3/g cm3/g Area, m2/g cmcm3/g3/g m22/g nm cm3/g cm3/g 1 11 1 162.16 3.29 0.133 0.133 0.046 0.046 113.37113.37 0.1460.146 456.68 2 22 2 333.23 3.25 0.271 0.271 0.102 0.102 248.29248.29 0.1820.182 561.85 3 33 3 468.70 3.15 0.369 0.369 0.116 0.116 285.70285.70 0.1710.171 523.13 4 44 4 380.46 2.98 0.283 0.283 0.100 0.100 249.78249.78 0.1600.160 492.76

Figure 2. Nitrogen isotherm at −196 °C (a) and DFT volume histogram (b) for carbon material Figure 2. Nitrogen isotherm at −196 ◦C(a) and DFT volume histogram (b) for carbon material produced under 3 MPa and 1000 °C. produced under 3 MPa and 1000 ◦C. Micropore volume and area determined with the application of the V-t-deBoer method (pores width Microporerange 0.6–2.0) volume almost andtripled area with determined the increase with in carbonization the application pressure of the from V-t-deBoer 1 to 3 MPa method and slightly(pores decreased width range from 0.6–2.0) the maximum almost tripledvalues (of with abou thet increase14%) with in further carbonization rise in pressure pressure to from 4 MPa. 1 to The3 MPa data andfor micropore slightly decreased volume and from area the determined maximum valueson the (ofbasis about of the 14%) carbon with dioxide further isotherm rise in pressure (pore widthto 4 MPa.range The0.4–1.5 data nm) for showed micropore a 25% volume increase and in area micropore determined volume on and the 23% basis increase of the carbon in micropore dioxide areaisotherm with the (pore pressure width rangeincrease 0.4–1.5 from nm) 1 MPa showed to 2 a MPa 25% increaseand a slight in micropore decrease volume of about and 6% 23% and increase 7%, respectively,in micropore with area increase with the of pressure pressure increase to 3 MPa, from and 1 MPaof 6% to with 2 MPa further and a rise slight in decreasepressure ofto about4 MPa 6% (Tableand 7%,2). The respectively, variations with in the increase average of pressurepore diameter to 3 MPa, of carbon and of materials 6% with produced further rise under in pressure various to pressures4 MPa (Table were 2not). Thesignificant variations and inwithin the average the experime pore diameterntal error of(Table carbon 2). materialsThe pores producedof the diameter under ofvarious 0.57 nm pressures contributed were the notlargest significant share of and the withintotal pore the volume experimental according error to (Tablethe DFT2). model. The pores of the diameterThe DFT of 0.57pore nm size contributed distribution the results largest showed share of thethe totaldominant pore volumerole of accordingmicroporosity to the (pores DFT model.of a diameterThe below DFT 2 pore nm), size especially distribution in the resultsrange of showed up to 1 thenm, dominant in shaping role the ofpore microporosity volume of the (pores carbon of a materialsdiameter tested below (see 2 nm), Figure especially 2b). Micropores in the range cont ofributed up to 1 nm,to approximately in shaping the 49–56% pore volume of the of total the carbonpore volumematerials of chars. tested The (see next Figure distinct2b). Microporespore diameter contributed range visible to approximately in the volume 49–56%histogram of thewas total 2–5 porenm withvolume the share of chars. of about The 20% next in distinct the total pore pore diameter volume. range The remaining visible in the part volume of pores, histogram approximately was 2–5 27– nm 34%with of the sharetotal pore of about volume, 20% comprised in the total macrop poreores volume. of diameter The remaining over 5 nm part (Figure of pores, 2b). approximately 27–34%Pore ofsize the distribution total pore volume, within comprisedthe entire macroporespore width ofrange diameter measured over 5 nmbased (Figure on the2b). nitrogen sorptionPore isotherm size distribution as well as within narrow the entiremicropore pore widthdistribution range measuredmeasuredbased on th one basis the nitrogen of the carbon sorption dioxideisotherm isotherm as well data as narrowshowed microporecomparable distribution shares of pores measured of particular on the diameter basis of theranges carbon in carbon dioxide materialsisotherm volume, data showed irrespective comparable of pressure shares applied of pores (Figure of particular 3). Howeve diameterr, the ranges values in of carbon pore volumes materials andvolume, areas differentiated irrespective of the pressure carbon applied materials (Figure produc3).ed However, under various the values pressure of pore conditions volumes (Table and areas 2). differentiatedThe disintegration the carbon of materialscarbon materials produced produced under various under pressure pressure conditions and at (Tablehigh 2temperature). resultingThe mainly disintegration from moisture of carbon and volatiles materials release produced under underlow heating pressure rates and can atbe highseen in temperature Figure 4. Theresulting differences mainly in fromparticle moisture shape andbetween volatiles carbon release materials under produced low heating under rates various can be seenpressures in Figure are4 . slight.The differencesAll the samples in particle maintained shape between the parent carbon mate materialsrial cell produced shape, underwhile variousdemonstrating pressures slits are slight.and openings,All the samples which seem maintained to become the more parent complex material with cell the shape, pressures while demonstratingapplied from 1 slitsto 3 andMPaopenings, (Figure 4a–c).which It can seem be to also become noticed more that complex increasing with the the pressure pressures in the applied range from of 1–3 1 toMPa 3 MPa resulted (Figure in 4raisinga–c). It the can numberbe also of noticed smaller that particles increasing visibl thee in pressure cavities increated the range by the of 1–3devol MPaatilization. resulted The in raising carbon thematerials number producedof smaller under particles 4 MPa visible showed in cavities a smoother created surface by the th devolatilization.an the particles create The carbond under materials lower pressures produced (Figure 4d). This may be the effect of fusion of the previously melted particles under the final external

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underMaterials 4 MPa 2018, showed 11, x FOR PEER a smoother REVIEW surface than the particles created under lower pressures (Figure5 of 8 4d). This may be the effect of fusion of the previously melted particles under the final external stress of the stress of the high pressure applied. The surface of the cavities is also smoother and poor in smaller high pressureMaterials 2018 applied., 11, x FOR The PEER surface REVIEW of the cavities is also smoother and poor in smaller particles.5 of 8 particles.

stress of100 the high pressure applied. The surface of the cavities100 is also smoother and poor in smaller particles.90 90 % 80 80 70 100 70100 90 6090 1.3-1.5 nm 60 > 5 nm 50 % 80 5080 1.0-1.3 nm 2-5 nm 40 70 4070 1.3-1.50.9-1.0 nm nm 60 1-2 nm 60

-MC method, % method, -MC 30 30 > 5 nm 0.7-0.9 nm 50 50 1.0-1.3 nm

20 < 1.0 nm per microporevolume 20

volume - DFT method, 2-5 nm

40 size Micropore distribution 40 0.4-0.7 nm 10 10 0.9-1.0 nm Pore size distribution pore per 1-2 nm

30 % method, -MC 30 0 0 0.7-0.9 nm

20 < 1.0 nm per microporevolume 20

volume - DFT method, 1234 1234

Micropore size Micropore distribution 0.4-0.7 nm 10 10 Pore size distribution pore per Pressure, MPa Pressure, MPa 0 0 1234(a)1234 (b) Pressure, MPa Pressure, MPa Figure 3. Pore size distribution per pore volume (a) and narrow micropore size distribution per (a) (b) Figurenarrow 3. Pore micropore size distribution volume (b) for per carbon pore volumematerials (a tested.) and narrow micropore size distribution per narrow microporeFigure volume 3. Pore (b size) for distribution carbon materials per pore tested. volume (a) and narrow micropore size distribution per narrow micropore volume (b) for carbon materials tested.

Figure 4. Cont. Materials 2018, 11, 876 6 of 8

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Figure 4. SEM images of carbon materials produced at 1000 °C and under the pressure of (a) 1 MPa, Figure 4. SEM images of carbon materials produced at 1000 ◦C and under the pressure of (a) 1 MPa, (b) 2 MPa, (c) 3 MP, and (d) 4 MPa. (b) 2 MPa, (c) 3 MP, and (d) 4 MPa. The dominant role of micropores in the porous structure of chars generated under high Thepressure dominant might role imply of microporesthat relatively in low the heating porous rates structure applied of in chars this study generated enabled under free swelling high pressure behavior of chars, which enhanced the development of microporosity. Intensive melting behavior, might implypreviously that observed relatively to be low more heating pronounced rates with applied high inheating this studyrates applied enabled in carbonization free swelling step, behavior of chars,would which lead enhanced to the development the development of macroporou of microporosity.s materials Intensive [18]. The meltingslight decrease behavior, in previouslythe observeddevelopment to be more of pronounced porous structure with under high the heating pressure rates of applied4 MPa reflected in carbonization in the decreased step, specific would lead to the developmentsurface area ofand macroporous the total pore volume, materials as well [18 as]. micropore The slight area decrease and volume, in the may development also be the effect of porous structureof undermerging the of pressurepreviously of melted 4 MPa particles reflected under in the increased decreased external specific stress. surface The pressure area and of the3 MPa total pore may, therefore, be considered as the threshold value under the experimental conditions applied, volume, as well as micropore area and volume, may also be the effect of merging of previously melted exceeding which would result in the counteracting effects of the pressure on the porous structure particlesdevelopment under the with increased the release external of moisture stress. and The volatile pressures (Table of 1) 3 MPaas well may, as swelling therefore, behavior be considered of the as the thresholdcarbonized value material under [18–20]. the experimental If the target is conditions to receive a applied,carbon material exceeding with the which largest would volume result of in the counteractingnarrow effectsmicropores, of the then pressure the value on of the 2 MPa porous can structurebe considered development optimal on withthe basis the of release the carbon of moisture and volatilesdioxide (Table isotherm1) as data well (Table as swelling2). behavior of the carbonized material [ 18–20]. If the target is to receive a carbonThe results material observed with proved the largest that volumethe pressure of narrowmay be micropores,considered as thenan effective the value parameter of 2 MPa can shaping the porous structure of carbon materials, especially with lower heating rates. Similar values be considered optimal on the basis of the carbon dioxide isotherm data (Table2). of the specific surface area as well as micropore area and volume were reported for the more Thecomplex results and observed costly process proved of physical that the and pressurechemical activation may be of considered peat with carbon as an dioxide effective at 750 parameter °C shapingand the acid porous washing structure [8]. Comparable of carbon values materials, of the especially specific surface with area, lower micropore heating volume, rates. Similar and total values of the specificpore surfacevolume to area presented as well here as we microporere also obtained area and with volume chemical were activation reported by impregnation for the more with complex and costlyH3PO process4 of cotton of physicalstalks (ratio and of chemical0.3:1) followed activation by thermal of peat activation with carbonat 500 °C dioxide for 2 h [14]. at 750 Carbon◦C and acid washingmaterials [8]. Comparable produced from values hazelnut of the shells specific by mixing surface with area, citric micropore acid, autoclaved volume, at and250 °C total for pore7.5 h, volume and then thermally treated at 600 °C for 2 h also showed similar values of the specific surface area to presentedand the here total were pore volume also obtained to the ones with reported chemical here activation[7]. by impregnation with H3PO4 of cotton ◦ stalks (ratioThis of 0.3:1) proves followed that pressure by thermal may be activation considered at as 500 a practicalC for 2 tool h [14 for]. Carbonproduction materials of porous produced ◦ from hazelnutmaterials shells from byvarious mixing precursors, with citric including acid, autoclavednon-lignocellulosic at 250 biomass,C for 7.5 known h, and to thenbe less thermally treatedapplicable at 600 ◦C in for activated 2 h also showedcarbon production. similar values It may of thereplace, specific at least surface to some area andextent, the the total pore volumetime-consuming to the ones reported and labor-intensive here [7]. processes of chemical and physical activation. Clearly, the application of pressure itself is not sufficient to provide materials of the specific surface area and This proves that pressure may be considered as a practical tool for production of porous materials total pore volume comparable with commercial activated carbons or the materials after a complex from variouscycle of precursors,physical and chemical including activation non-lignocellulosic of most suitable biomass, precursors known[6,8,12,14]. to Nevertheless, be less applicable its in activatedapplication carbon was production. proved to produce It may wood- replace, and gra atss-derived least to somematerials extent, with porous the time-consuming structures that and labor-intensiveare more processesdeveloped than of chemical those obtained and physical by the chemic activation.al activation Clearly, of waste the applicationbiomass, e.g., of pressure husk itself is not sufficient[9,20].The to relatively provide low materials heating of rates the applied specific in surface this study area also and resulted total pore in higher volume values comparable of the with commercialspecific activated surface carbonsarea (of or 40%) the materialsand micropore after aarea complex (of 60%) cycle than of physical previously and observed chemical for activation pine-derived carbon materials produced under 2 MPa with high heating rates of 500 °C/min [19]. of most suitable precursors [6,8,12,14]. Nevertheless, its application was proved to produce wood- and grass-derived materials with porous structures that are more developed than those obtained by the chemical activation of waste biomass, e.g., rice husk [9,20].The relatively low heating rates applied in this study also resulted in higher values of the specific surface area (of 40%) and micropore area (of 60%) than previously observed for pine-derived carbon materials produced under 2 MPa with high Materials 2018, 11, 876 7 of 8 heating rates of 500 ◦C/min [19]. This implies that low heating rates are more applicable in tailoring the porous structure of carbon materials for high microporosity.

4. Conclusions In this paper, the results of an experimental investigation of the effects of pressure on high temperature carbonization of Andropogon geraradi biomass were presented, enabling the following conclusions to be drawn:

1. Carbonization of the selected biomass under 3 MPa gives carbon materials with the most developed porous structure in terms of the specific surface area, the total pore volume, and microporosity, as determined by nitrogen sorption isotherm; 2. The pressure of 2 MPa was proved to enhance the development of narrow microporosity, as quantified by carbon dioxide isotherm; 3. Application of pressure during carbonization enables tailoring of the porous structure of carbon materials.

Author Contributions: Conceptualization, N.H. and A.S.; Methodology, N.H.; Investigation, N.H.; Writing-Original Draft Preparation, N.H. and A.S.; Writing-Review & Editing, N.H. and A.S.; Visualization, N.H.; Funding Acquisition, N.H. Funding: This research was funded by the Ministry of Science and Higher Education, Poland, grant number [11157018]. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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