Study of thermal energy of alternative solid fuels Текст научной статьи по специальности «Промышленные биотехнологии»

ИЗВЕСТИЯ ВУЗОВ. ПРИКЛАДНАЯ ХИМИЯ И БИОТЕХНОЛОГИЯ Том 8 №4 2018

ХИМИЧЕСКАЯ ТЕХНОЛОГИЯ / CHEMICAL TECHNOLOGY Оригинальная статья / Original article УДК 62-664.2:662.312.4

DOI: http://dx.doi.org/10.21285/2227-2925-2018-8-4-117-124

STUDY OF THERMAL ENERGY OF ALTERNATIVE SOLID FUELS

© M.J. Ioelovich

Celdesigner Ltd

2, Bergman St., Rehovot 7670504, Israel

ABSTRACT. In this paper, solid fuels made of plant biomass or its blends with plastic additive were studied as an alternative to fossil coals. For this purpose, experimental and calculation methods were applied to determine the higher (HHV) and lower (LHV) heating values of individual components of plant biomass (lignin, cellulose, hemicelluloses, extractives, etc.), as well as of some components of plastics. The experiments were carried out using an oxygen bomb calorimeter, whereas calculations were performed by the equations: HHV, (kJ/g) = Eo M1(x + 0,295y - 0,42z) and LHV, (kJ/g) = Eo M1(x + 0,242y - 0,42z); where Eo = 413 kJ/mol, x, y and z is number of atoms C, H and O, respectively, in molecule of organic substance or in repeat unit of polymer having molecular mass M. Using the results obtained for individual components, the calorific values of various biomasses and their blends with plastic additives were found according to additivity rule, as follows: HHV = I(Wj HHV) and LHV = I(w, LVHj), where w, is weight part of the component in the biomass sample. The results revealed that calculated calorific values for the solid fuels were close to experimentally obtained values. The obtained data evidence on adequacy of the additivity rule to evaluate the thermal energy of solid fuels based on biomass. It has been also found that fuel pellets consisting of plant biomass and plastic additive are the most promising solid fuels, since they provide a higher calorific value and increased energy density than the biomass only.

Keywords: plant biomass, plastic, fuel pellets, cellulose, hemicelluloses, lignin, extractives, calorific values, calorimetry, calculation

Information about the article. Received May 19, 2018; accepted for publication November 25, 2018; available online December 29, 2018.

For citation: Ioelovich M.J. Study of thermal energy of alternative solid fuels Izvestiya Vuzov. Prikladnaya Khimiya i Biotekhnologiya [Proceedings of Universities. Applied Chemistry and Biotechnology]. 2018, vol. 8, no. 4, pp. 117-124. (In Russian). DOI: 10.21285/2227-2925-2018-8-4-117-124

ИЗУЧЕНИЕ ТЕПЛОВОЙ ЭНЕРГИИ АЛЬТЕРНАТИВНЫХ ТВЕРДЫХ ТОПЛИВ

© М.Я. Иоелович

ООО «Celdesigner»

7670504, Израиль, г. Реховот, ул. Бергмана, 2

РЕЗЮМЕ. В данной работе проведено изучение твердых топлив на основе растительной биомассы и ее смесей с полимерными добавками в качестве альтернативы ископаемым углям. С этой целью была определена высшая (HHVi) и низшая (LHVi) энергии сгорания отдельных компонентов растительной биомассы (лигнин, целлюлоза, гемицеллюлоза, экстрактивные вещества и др.), а также некоторых синтетических полимеров, входящих в состав пластиков. Эксперименты проводились с использованием кислородной калориметрической бомбы, а расчеты выполнялись с помощью уравнений: HHVi (kJ/g) = Eo M1 (x + 0,295y - 0,42z) и LHVi (kJ/g) = Eo M (x + 0,242y - 0,42z), где Eo = 413 kJ/mol; x, y и z - число атомов C, H и O в молекуле органического вещества или в повторяющемся звене полимера, c молекулярной массой M. Используя результаты, полученные для отдельных компонентов, была определена теплотворная способность различных биомасс и их смесей с полимерами в соответствии с правилом аддитивности: HHV = I(wi HHVi) и LHV = I(wi LVHi), где wi - массовая доля компонента в топливе. Результаты показали, что найденные значения теплотворной способности близки к экспериментальным, что свидетельствует об адекватности правила аддитивности для оценки удельной тепловой энергии сгорания твердого топлива на основе биомассы. Было также обнаружено, что топливные гранулы, состоящие из растительной биомассы и добавок пластиков, являются наиболее перспективным твердым топливом, поскольку они обеспечивают более высокую теплотворную способность и повышенную плотность тепловой энергии, чем топливо, состоящее лишь из биомассы.

Ключевые слова: растительная биомасса, пластик, топливные гранулы, целлюлоза, гемицеллю-лоза, лигнин, экстрактивные вещества, теплотворная способность, калориметрия, расчеты.

Информация о статье. Дата поступления 19 мая 2018 г.; дата принятия к печати 25 ноября 2018 г.; дата онлайн-размещения 29 декабря 2018 г.

Для цитирования: Иоелович М.Я. Изучение тепловой энергии альтернативных твердых топлив // Известия вузов. Прикладная химия и биотехнология. 2018. Т. 8, N 4. С. 117-124. Р01: 10.21285/2227-29252018-8-4-117-124

INTRODUCTION

Currently, the main solid fuels are fossil coals, which provides 28-30% of annual energy consumption in the world, about 160-180 EJ [1]. To generate such energy, more than 6 billion tons of coal are burned each year. However, this fossil source of energy is not reproduced in nature, and therefore its reserves are permanently depleted. Besides, the burning of coals is accompanied by emission of greenhouse gas - carbon dioxide, in the huge volume of 1700-1800 m3 from each ton, which can exacerbate the problem of global warming [2].

An alternative to coal can be a solid fuel based on plant biomass, which in contrast to this fossil fuel, is reproduced in nature. The term "biomass" means here a variety of plant materials, as well as their residues and wastes [3]. Diverse types of biomass can be used to produce of solid fuels such as soft- and hardwood; herbaceous plants (e.g. Miscanthus, Switchgrass, Bermuda grass, etc.); forest residues (e.g. sawdust, twigs, shrubs, etc.); residues of agricultural plants (e.g. stalks, husks, cobs, etc.); residues, waste and trash of textile, pulp, paper, cardboard and plants, as well as algae biomass, etc. Total amount of available biomass aimed especially for energy production is estimated at 8-10 million tons [4].

As is known, all constituents of biomass are photosynthesized in chlorophyll pigment of plant leaves from carbon dioxide and water, absorbing red and blue-violet sunlight [5, 6]. Photosynthesis is a complex, multistage process, which can be summarized as follows: XCO2 + 0,5YH2O + light ^ CxHyOz + (X + 0,25Y - 0,5Z)O2.

In fact, plant biomass can be considered an accumulator of solar energy captured during photosynthesis. To generate the heat energy, the biomass is burned, resulting in the release of accumulated solar energy. A specific feature of the biomass that it is neutral for emission of carbon dioxide, since its combustion produces the same amount of this greenhouse gas as it was absorbed from the atmosphere during photosynthesis.

However, the initial biomass is a heterogeneous material having low bulk density, which consists of pieces of different shapes, sizes and compositions. These negative features leads to deterioration in fuel properties - unstable calorific value, low density of thermal energy and insufficient combustion efficiency [7]. To increase the low energetic density, the loose biomass should be converted into dense pellets. Nevertheless, even after

compaction, other negative biomass characteristics remain, such as low calorific value and sensitivity of the solid fuel to water absorption. A promising way to improve the fuel features of biomass is the use of plastic binders in the pelletization process [8]. As is known, large amounts of plastic waste about 250-300 million tons are thrown out annually and pollute the environment [9]. After separation of PET bottles from plastic debris, the main fraction of the plastic waste consists of poly-olefins, which can be used as additive to biomass for cofiring.

The use of biomass and its mixtures with polymer binders for production of solid fuels requires knowledge of the specific energy of combustion, which can be expressed by lower (LHV) and higher (HHV) heating value. Numerous attempts have been performed to determine the HHV for certain types of biomass and synthetic polymers using the precise experimental method - combustion of sample in a bomb calorimeter [10-13], as well as various calculation methods [13-16]. However, the calorimetric measurements are lengthy and requires multiple repetitions to obtain a reliable result; besides bomb calorimeter is a complex, expensive and not always available device. On the other hand, to calculate the HHV of each biomass type, its own equation has been proposed [13-15]. Since an unlimited number of different types of biomass and its based compositions exists, an unlimited number of experiments or calculations are required to determine the HHV of various solid fuels, which is impossible to realize.

For this reason, another approach should be used. As is known, any plant biomass contains cellulose, hemicelluloses, lignin, extractives and small amounts of some other substances. Besides, mixed solid fuel can contain also plastic additive. Thus, to find the HHV of solid fuels made of biomass or its mixture with plastics, it is sufficient to determine the HHV for limited number of individual components and their content in the sample and then use the additivity rule.

Although the HHV of individual components of plant biomass has been studied by various researchers, the obtained results were insufficiently reliable since they were varied over a wide range. For example, a considerable variation in HHV for lignin samples, from 17 to 29 kJ/g was observed [15, 17, 18], whereas the variation in HHV for cellulose and hemicellulose samples was from 16,9 to 18,6 kJ/g [10, 15, 17-19].

The purpose of this study was an accurate determination of calorific values for individual components of biomass, as well as for some synthetic polymers, in order to use the obtained results to evaluate the potential of thermal energy for solid fuels based on biomass and its composites by means of additivity rule.

EXPERIMENTAL

Biomass

Various types of biomass were used such as chips of spruce and pine (Institute of wood chemistry, Latvia), chips of poplar (ZeaChem Inc., USA), corn stalks (Iowa Corn, USA), wheat straw (Atlantic Pine Straw, USA), switchgrass (Nott Farms, Canada) and bagasse of sugarcane (Cosan, Brazil), as well as waste paper and cardboard (Amnir Recycling, Israel). The biomass samples were cut, knife-milled and screened through a sieve to obtain the fraction of 1-2 mm.

Cellulose

Refined and bleached chemical Kraft pulp (KRP) and cotton cellulose (COC) were supplied from Buckeye Technologies, Inc. Samples of Avi-cel MCC PH-101 (MCC-1) and PH-301 (MCC-2) were supplied from FMC. Besides cellulose was isolated from bagasse (BAC), wheat straw (WSC), corn stalks (CSC) and switchgrass (SGC) by a two-stage pulping using dilute nitric acid and alkali with additional bleaching [20]. The cellulose samples were additionally purified by extraction with organic solvents, chelating agent (EDTA), boiling 2% NaOH and boiling water to neutral pH; then samples were rinsed with deionized water, ethanol and dried at 380 K to constant weight. The main characteristics of the samples (crystallinity index CrI, degree of polymerization DP, content of ash and a-cellulose) are shown in the Table 1.

Lignin

Samples of acid-insoluble Klason lignin were isolated from various biomasses using standard TAPPI method T222 om-02.

Hemicelluloses

Sample of birch xylan was supplied from Sig-ma-Aldrich, while sample of mannan was extracted from spruce wood [21].

Extractives

Organic extractives were isolated from biomass samples using NREL standard procedure LAP-010.

Other components

Samples of starch, abietic resin acid (ABA) and triglycerides of stearic (TGS), palmitic (TGP), lauric (TGL) and oleic (TGO) acids were supplied from Sigma-Aldrich, and carnauba wax (CWax) was supplied from Bruchem Inc.

Polyolefins (POL)

Samples of powdered polyolefins were supplied from Ineos Olefins and Sigma-Aldrich.

Pelletization

The ground biomass was blended with powdered polymer, and the mixture was compacted under pressure 50 MPa at temperature of 445450 K for 1-2 min. For comparison, the pelletized biomass was also prepared under the same conditions.

Characterization of samples

Chemical composition of biomass samples was studied by standard NREL methods [22]: LAP-001 (total solids), LAP-002 (cellulose and hemicellu-loses), LAP-003 (lignin), LAP-005 (ash) and LAP-010 (extractives). Bulk density was tested by standard method ASTM E-873. Average degree of polymerization (DP) of cellulose was measured by the viscosity method using diluted cellulose solutions in Ca-doxen [23]. Index of cellulose crystallinity (CrI) was estimated using Jayme and Knolle method [24].

Combustion calorimetry

Combustion of the samples in pelletized form (cca. 1 g) was carried out in a bomb calorimeter Parr-1341 at oxygen pressure of 3 MPa with 1 ml of added deionized water. The temperature was measured with accurac y ± 0,001 K. The value of energy equivalent of the calorimetric system was determined by combustion of standard benzoic acid. The corrections for ignition, as well as for formation and dissolution of acids were done. For each sample three experiments were performed to obtain a reliable higher heating value (HHV).

RESULTS AND DISCUSSION

The specific combustion energy of organic matters can be expressed by higher heating (calorific) value (HHV) and lower heating (calorific) value (LHV). The HHV is determined directly by combustion of sample in a bomb calorimeter, whereas LHV is found from relationships:

LHV = HHV - 0,22H eq. 1

LHV = HHV - 22hM1 eq. 2

where H is percentage of hydrogen in the sample and h is number of hydrogen atoms in substance or in repeat unit of polymer having molecular mass M.

The results of experimental determination of calorific values of main components of plant biomass are shown in Tables 2-5.

As can be seen from Tables 2 and 3, when determine the calorific values of polysaccharides (cellulose, hemicelluloses and starch) the relative divergence between values obtained for cellulose samples was small, about ± 0,3%, and for non-cellulose carbohydrates about ± 1,3%.The obtained average calorific values were in the range of the values given in literature [10, 15, 17-19].

Table 1

Characteristics of cellulose samples

Таблица 1

Характеристика образцов целлюлозы

Sample CrI DP Ash,% a-cellulose, %

MCC-1 0,70 220 <0,1 >99

MCC-2 0,72 170 <0,1 >99

COC 0,67 2700 0,1 >99

KRP 0,62 1200 0,2 98

BGC 0,54 720 0,2 97

WSC 0,53 650 0,2 97

CSC 0,52 630 0,3 97

SGC 0,52 610 0,3 97

Table 2

HHV and LHV of cellulose samples

Таблица 2

Значения HHV и LHV образцов целлюлозы

Sample HHV, kJ/g LHV, kJ/g

MCC-1 17,41 16,05

MCC-2 17,40 16,00

COC 17,42 16,06

KRP 17,43 16,07

BGC 17,47 16,11

WSC 17,51 16,15

CSC 17,48 16,12

SGC 17,52 16,16

Average: 17,46 ± 0,05 16,09 ± 0,05

Table 3

HHV and LHV of non-cellulose carbohydrates

Таблица 3

Значения HHV и LHV нецеллюлозных карбогидратов

Sample HHV, kJ/g LHV, kJ/g

Starch 17,40 16,00

Mannan 17,53 16,17

Xylan 17,78 16,45

Average: 17,57 ± 0,19 16,21 ± 0,22

Table 4

HHV and LHV of lignin samples

Таблица 4

Значения HHV и LHV для образцов лигнина

Sample HHV, kJ/g LHV, kJ/g

Lignin of softwood (Lignin-S)

Lignin of spruce 26,73 25,53

Lignin of pine 27,00 25,84

Average: 26,86 ± 0,14 25,68 ± 0,15

Lignin of hardwood herbaceous plants (Lignin-H)

Lignin of poplar 25,45 24,28

Lignin of bagasse 25,21 24,05

Lignin of switchgrass 25,27 24,11

Lignin of straw 25,16 24,00

Lignin of corn stalks 25,20 24,04

Average: 25,26 ± 0,11 24,10 ± 0,11

Studies of lignin samples have shown that HHV and LHV for lignins of softwood were higher than the corresponding values for lignins isolated from hardwood and herbaceous plants (Table 4).

For different lipids and extractives the close calorific values were determined within the relative divergence of 2,5% (Table 5).

The determined calorific values of some poly-olefins were shown in Table 6. The obtained values are confirmed by literature data [13, 25].

In addition to experimental, there are also quite accurate calculating methods to determine the calori-

fic values of individual components of biomass, as well as plastics. For this purpose, the following equations can be used [26]:

HHV, (kJ/g) = Eo M(x + 0.295y - 0.42z eq. 3

LHV, (kJ/g) = Eo M (x + 0.242y - 0.42z) eq. 4

where Eo = 413 kJ/mol; x, y and z is number of atoms C, H and O respectively in molecule of organic substance or in repeat unit of polymer, having molecular mass M.

Table 5

HHV and LHV of lipids and extractives (extr)

Таблица 5

Значения HHV и LHV для липидов и экстрактивных веществ (extr)

Sample HHV, kJ/g LHV, kJ/g

TGS 40,34 37,62

TGP 39,66 37,01

TGL 37,80 35,23

TGO 39,04 36,51

ABA 38,37 36,16

CWax 38,25 36,11

Extr of spruce 38,10 36,00

Extr of poplar 37,78 35,21

Average: 38,66 ± 0,99 36,23 ± 0,88

Table 6

HHV and LHV of polyolefins

Таблица 6

Значения HHV и LHV полиолефинов

Sample HHV, kJ/g LHV, kJ/g

PEHD 47,11 44,05

PELD 47,12 44,06

PP 47,15 44,10

PS 41,14 39,45

Average: 45,63 ± 2,60 42,92 ± 2,00

Table 7

Experimental and calculated calorific values in kJ/g

Таблица 7

Экспериментальные и расчётные значения теплотворной способности (кДж/г)

Sample Formula M HHV exp (calc) LHV exp (calc)

Cellulose -C6H10O5- 162 17,5 (17,5) 16,1 (16,1)

Starch -C6H10O5- 162 17,4 (17,5) 16,0 (16,1)

Hexosans -C6H10O5- 162 17,5 (17,5) 16,1 (16,1)

Pentosans -C5H8O4- 132 17,8 (17,8) 16,5 (16,4)

Lignin-S -C11H10O3.2- 193 26,9 (27,0) 25,7 (25,8)

Lignin-H -C108H10O36- 197 25,3 (25,6) 24,1 (24,5)

Extr (ABA) C20H30O2 302 38,4 (38,3) 36,2 (36,1)

PE -CH2- 14 47,1 (46,9) 44,1 (43,8)

PP -C3H6- 42 47,1 (46,9) 44,1 (43,8)

PS -C8H8- 104 41,1 (41,1) 39,5 (39,4)

Table 8

Chemical composition of biomass (wt. part)

Таблица 8

Химическая композиция образцов биомассы (массовые доли)

Biomass Abbr. Cellulose *Hemi Lignin Extr. Ash

Pine wood PIW 0,48 0,20 0,27 0,04 0,01

Spruce wood SPW 0,47 0,22 0,27 0,03 0,01

Poplar wood POW 0,46 0,26 0,24 0,02 0,02

Bagasse BAG 0,38 0,33 0,20 0,03 0,06

Corn stalks COS 0,37 0,35 0,19 0,03 0,06

Switchgrass SWG 0,37 0,36 0,18 0,03 0,06

Wheat straw WHS 0,36 0,37 0,19 0,02 0,06

Cardboard CAB 0,60 0,12 0,18 0,03 0,07

Wrapping paper WRP 0,73 0,07 0,05 0,03 0,12

*Hemi denotes Hemicelluloses

Table 9

Calorific values of biomass samples in kJ/g

Таблица 9

Теплотворная способность образцов биомассы (кДж/г)

Biomass HHV adr (exp) LHV adr (exp)

PIW 20,74 (20,68) 19,38 (19,43)

SPW 20,54 (20,48) 19,18 (19,20)

POW 19,51 (19,55) 18,20 (18,17)

BAG 18,71 (18,82) 17,44 (17,51)

COS 18,62 (18,60) 17,36 (17,30)

SWG 18,55 (18,46) 17,28 (17,21)

WHS 18,42 (18,44) 17,16 (17,18)

CAB 18,35 (18,40) 17,08 (17,00)

WRP 16,43 (16,37) 15,20 (15,16)

Table 10

Energetic features of pelletized solid fuels*

Таблица 10

Энергетические характеристики гранулированного топлива*

Biomass HHV adr (exp) LHV adr (exp) HED LED

Initial SPW 20,54 (20,48) 19,18 (19,20) 5,12 4,80

Pellets of SPW 20,54 (20,62) 19,18 (19,31) 12,35 11,54

Pellets of SPW & 20% POL 25,55 (25,50) 23,94 (23,97) 16,58 15,56

Pellets of SPW & 30% POL 28,05 (28,11) 26,30 (26,42) 19,67 18,42

Initial BAG 18,71 (18,82) 17,44 (17,51) 3,75 3,50

Pellets of BAG 18,71 (18,77) 17,44 (17,54) 11,25 10,51

Pellets of BAG & 20% POL 24,10 (24,16) 22,54 (22,60) 15,68 14,67

Pellets of BAG & 30% POL 26,78 (26,86) 25,08 (25,16) 18,10 16,83

*HHV and LHV are expressed in kJ/g; HED and LED - higher and lower density of thermal energy, respectively, expressed in GJ/m3

The results of compositional analysis of biomass samples by standard NREL methods are shown in Table 8.

The obtained HHVi and LHVi of individual components (Table 7) and their content (w^ in the biomass (Table 8) were used to evaluate the calorific values of biomass samples according to additivity rule:

HHV = Zwi HHV eq. 5

LHV = Zwi LVHi eq. 6

Besides, the pelletized fuels containing of biomass or composite of biomass and mixed poly-olefins (POL) were prepared. The calorific values

Изучение тепловой энергии альтернативных твердых топлив

of these fuels were determined experimentally (exp) or calculated using the additivity rule (adr). The results revealed that calculated calorific values of solid fuels are confirmed by experimentally obtained values (Tables 9, 10). These data evidence on adequacy of the additivity rule to evaluate the specific thermal energy of solid fuels based on biomass.

Studies of pelletized fuels also showed that the densification of initial biomass does not change the calorific value, but increases the density of thermal energy. Additive of polyolefins to biomass samples increases both calorific value and energy density of the pelletized fuels (Table 10). As a result, energetic features of biomass based pellets become comparable with those of fossil coal.

As can be seen from Table 7, the calorific values calculated by equations 3 and 4 are close to experimentally values determined using a calo-rimetric bomb. Thus, the calculation method can be successfully used for determination of calorific values of organic components of biomass, as well as of some plastics.

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CONCLUSSION

Solid fuels made of plant biomass and its compositions with plastic additive were studied as an alternative to solid fossil fuels such as coals. For this purpose, experimental and calculation methods were used to determine the higher and lower heating values of individual components of plant biomass and plastics. Using the results determined for individual components, the calorific values of various biomass samples and biomass-plastic blends were calculated by means of additivity rule. The results revealed that calculated calorific values of solid fuels were close to experimentally obtained values. Thus, the obtained data evidence the adequacy of the additivity rule to evaluate the specific thermal energy of solid fuels based on biomass. It has been also found that fuel pellets consisting of plant biomass and additive of plastics are the most promising solid fuels, since they provide a higher calorific value and increased density of thermal energy than biomass only. Moreover, energetic features of pelletized fuels become comparable with those of fossil coals.

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Contribution

Ioelovich M.J. carried out the experimental work, on the basis of the results summarized the material and wrote the manuscript. Ioelovich M.J. has exclusive author's rights and bear exclusive responsibility for plagiarism.

Conflict of interests

The author declares no conflict of interests regarding the publication of this article.

AUTHORS' INDEX Affiliations

Michael J. Ioelovich

Dr. Sci. (Chemistry), Professor Head of Chemical Department Celdesigner Ltd e-mail: [email protected]

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Критерии авторства

Иоелович М.Я. выполнил экспериментальную работу, на основании полученных результатов провел обобщение и написал рукопись. Иоелович М.Я. имеет на статью эксклюзивные авторские права и несёт исключительную ответственность за плагиат.

Конфликт интересов

Автор заявляет об отсутствии конфликта интересов.

СВЕДЕНИЯ ОБ АВТОРАХ Принадлежность к организации

Михаил Я. Иоелович

Д.х.н., профессор, заведующий химическим отделом Celdesigner Ltd e-mail: [email protected]