Subscriber access provided by University of Massachusetts Amherst Libraries
Biofuels and Biomass
SPONTANEOUS EMISSION MEASUREMENTS OF SELECTED ALKALI RADICALS DURING THE COMBUSTION OF A SINGLE BIOMASS PELLET Marius Sadeckas, Nerijus Stri#gas, Paulius Andri#nas, Robertas Navakas, Marius Praspaliauskas, Miriam Rabaçal, and Mario Costa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01002 • Publication Date (Web): 02 Jun 2018 Downloaded from http://pubs.acs.org on June 3, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
SPONTANEOUS EMISSION MEASUREMENTS
2
OF SELECTED ALKALI RADICALS DURING
3
THE COMBUSTION OF A SINGLE BIOMASS
4
PELLET
5
M. Sadeckas1,*, N. Striūgas1, P. Andriūnas1, R. Navakas1, M. Praspaliauskas2, M. Rabaçal3#, M.
6
Costa3
7 8 9 10 11 12 13
1. Lithuanian Energy Institute, Laboratory of Combustion Processes, Breslaujos g. 3, 44403 Kaunas, Lithuania 2. Lithuanian Energy Institute, Laboratory of Heat Equipment Research and Testing, Breslaujos g. 3, 44403 Kaunas, Lithuania 3. IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal
14 15 16
*Corresponding author: M. Sadeckas,
[email protected] 17
# Now at: Aerothermochemistry and Combustion Systems Laboratory, ETH Zurich, Switzerland
18 19
Submitted to Energy & Fuels
20
May, 2018
21
ACS Paragon Plus Environment
1
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 47
22
ABSTRACT
23
Spontaneous emission intensities of Na*, Ca* and K* during the combustion of single wood and
24
straw pellets doped with known concentrations of Na, Ca and K were measured using optical
25
bandpass filters mounted on an ICCD camera. The impregnated biomass pellets were suspended
26
in a natural gas flat flame at 750 ºC and 1000 ºC. Before making the pellets, the biomass samples
27
were washed and soaked in order to demineralize and dope the wood and the straw with different
28
concentrations of Na, Ca and K (0.5, 2 and 5 wt.%). During the experiments, the temperature at
29
the center of the pellets was measured with a thermocouple and the combustions stages were
30
identified with the help of the temperature derivative. The results reveal that at the lower gas
31
temperature the emission of the selected alkalis is marginal, in agreement with previous studies.
32
At the higher gas temperature, the emission profiles reveal that the release of K and Na are
33
released distinctively in the volatile combustion and char combustion stages. The presence of
34
large amounts of silica and alumina may trap the alkalis in the solid-phase, leading to a reduced
35
emission of these species during the char combustion. Calcium does not evaporate at the tested
36
temperature conditions, but the temperature is high enough to promote the decomposition of
37
calcium oxalate in the outer layer of the pellet, leading to a flat emission profile during the
38
combustion stages. Finally, the total integrated emission increased proportionally with the
39
increase of the doping concentration of all species.
40 41
KEYWORDS: Potassium, Calcium, Sodium, Chemiluminescence, Biomass, Combustion
42
ACS Paragon Plus Environment
2
Page 3 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
43
Energy & Fuels
1. Introduction
44
Solid biomass, derived mostly from wood, is by far the first source of fuel used for heat
45
production. Due to both environmental and economic reasons, woody biomass is obtained from
46
by-products of forest management operations and wood industry, such as sawmills. More than
47
half of the solid biomass consumed by the power industry is in the form of woodchips and
48
pellets. With the increase of the biomass usage, the quality of the feedstock for the energy
49
production became an important issue1. During the thermal degradation of biomass, numerous
50
chemical and physical transformations occur, and it is difficult to forecast the combustion
51
process when fuel composition fluctuates. Furthermore, sometimes biomass is a mixture of
52
straw, sawdust and other agricultural residues2. High content of alkali-metals hampers utilization
53
of biomass as fuel. The moisture and ash contents and particle size are some of the main biomass
54
characteristics that determine the quality of combustion3. The characteristics of the biomass
55
influence directly the combustion process; for instance, slagging due to accumulation of ash
56
might occur when low-quality biomass is used4. The combustion of low quality biomass poses a
57
significant challenge due to the typically high amounts of ash, in particular alkali-metals5. Large
58
amounts of alkali-metals in biomass may lead to corrosion of the boiler’s surfaces6. The release
59
and transformation of potassium (K), chlorine (Cl), and sulphur (S) from biomass during
60
thermochemical conversion processes may lead to agglomeration of bed material and poisoning
61
of catalysts used in the downstream processes of gasifiers7. Moreover, fouling, slagging, and
62
corrosion damage the heat exchange tubes and reduces the efficiency of the boiler8. As an
63
example, the condensation of potassium species is the origin of the slag layer formation on
64
superheater surfaces during biomass combustion9,10. Slag deposits and corrosion on the heating
65
surfaces reduce the operational time of the power plant and increases downtime11. The
ACS Paragon Plus Environment
3
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 47
66
knowledge of particle formation and deposition mechanisms is important to derive measures for
67
lowering particulate emissions and harmful deposits12. The effects of the latter may differ
68
depending on phase transformations of potassium species in the gas and condensed phases13.
69
The release of gas-phase alkali species and K, Na and Ca ion’s has been mainly studied
70
with focus on sodium chloride (NaCl) and potassium chloride (KCl)14. Johansen et al.15
71
identified a convenient relation between K and Cl release; chlorine is the main facilitator for
72
potassium release through sublimation of KCl in a temperature range of 700-800 °C. Knudsen et
73
al.16 determined that potassium release increases at higher temperatures; in particular, between
74
50% and 90% of the total potassium was released to the gas-phase at a temperature of 1150 °C.
75
In addition, during the devolatilization stage of biomass the release of potassium reaches the
76
maximum yield17. The catalytic effect of potassium on the devolatilization of biomass was
77
shown experimentally. Cai. et al.18 showed that potassium has catalytical effect on the
78
devolatilization of biomass. Jones et al.19 detected potassium catalysis in the char burn-out rates
79
of stationary particles. Furthermore, the authors showed that the remaining K content in ash
80
depends strongly on the temperature of combustion. With the rise of the temperature, the K
81
emission and the possibility of slag formation increase. Kim et al.3 determined that an increase in
82
the combustion temperature reduces the emission of NaCl, while the emission of KCl increases
83
at higher temperatures as well. Despite previous studies on the release of alkali-metals through
84
the doping of biomass being mainly based on chlorides, Ca-K-Na oxalates are reported to be
85
presented in the biomass, specifically in the organic matter20, and their release also requires more
86
studies.
87
Relations between the biomass characteristics, such as size, mass, and density, were
88
identified and the combustion kinetics of biomass particles were examined21. The experimental
ACS Paragon Plus Environment
4
Page 5 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
89
explanation for the correlation between the sample mass and ignition delay, volatile flame
90
duration and char burn duration were also evaluated. From the emission profile of potassium, it
91
was learned that the highest intensity of potassium release occurs at the beginning of the
92
combustion process, while it gradually decreases at the later stages13. The emission of potassium
93
can be directly associated with the combustion stages, namely devolatilization and char
94
oxidation13. Besides potassium, sodium and calcium have large impact on the combustion
95
reactions as well3.
96
In general, alkali-metals are detected using spectroscopic methods22, but a number of other
97
methods are used as well. Fatehi et al.23 used laser induced breakdown spectroscopy (LIBS) to
98
study the alkali-metals release during the combustion and gasification of biomass. Precision is
99
the main advantage of the LIBS technique; however, it requires expensive equipments and
100
experienced operators. Glazer et al.24 used excimer laser induced fluorescence (ELIF) to measure
101
gas-phase concentrations of alkali-metals (K and Na) in a circulating fluidized bed (CFB)
102
furnace fired with biomass containing high amounts of K and Na. Erbel et al.25 also used ELIF to
103
detect K and Na species in the gaseous products during the gasification of biomass. Finally,
104
alkali species were measured online using a surface ionization (SI) technique in Idbäcken and
105
Nässjö power plants in Sweden, which showed the possibility to detect alkali-metals in the gas or
106
condensed phases26.
107
The above-mentioned techniques for detection of alkali species during combustion have
108
advantages and disadvantages. The clearest disadvantages are the high cost and the complex
109
equipment handling and operation. The detection of alkali species though spontaneous emission
110
during combustion might be a simpler alternative for the observation of the release of the alkali
111
species in flames. Currently, this method is mostly used in combustion diagnostics to predict
ACS Paragon Plus Environment
5
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 47
112
NOx formation and other pollutants by detecting OH*, CH* and C2* flame radicals27, or to
113
determine correlations between CH* and OH* flame chemiluminescence ratios and air-fuel
114
equivalence ratios in industrial flames28. The aim of the present work is to propose a low-cost yet
115
effective technique to analyze the spontaneous emission from Na*, K*, and Ca* during the
116
combustion of single wood and straw pellets containing different concentrations of alkali
117
minerals. This is achieved by using optical bandpass filters mounted on an intensified charge
118
coupled device (ICCD) camera to measure emission intensity at the corresponding wavelengths
119
associated with Na*, K* and Ca*. The emission characteristics of alkali materials at different
120
combustion temperatures of the fuel were examined. The practical furnaces typically operate at
121
high temperatures11 despite various fuels containing different amounts of alkali-metals being
122
fired29. If biomass fuels of very low quality or high humidity (up to 65 wt.%) are combusted, the
123
temperature over the fuel bed is quite low. Therefore, this study examines the combustion of
124
pellets at two different temperatures, namely 750 ºC and 1000 ºC. The alkali emission
125
characteristics were analyzed in terms of the relative intensity emission and integrated emission
126
intensity during combustion. The internal pellet temperature during combustion was measured,
127
and subsequently the morphology of the char samples was examined.
128 129
2. Material and Methods
130
2.1.
Biomass fuels
131
In this work, two biomass fuels, wood and straw, were used. Table 1 presents the main
132
properties of the two biomass fuels. The samples were analyzed in accordance with the standards
133
using an IKA C5000 calorimeter, a Flash 2000 CHNS analyzer, an ICP-OES Optima 8000 and a
134
NETZSCH STA 449 F3 Jupiter thermogravimeter.
ACS Paragon Plus Environment
6
Page 7 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
135
Table 1. Main properties of the raw wood and straw. Proximate analysis (wt.%) Moisture Volatile matter
Fixed carbon
Ash
Higher heating
Lower heating
value (MJ/kg)
value (MJ/kg)
Wood
2.94
78.56
15.5
3.5
19.12
18.05
Straw
4.04
73.13
18.8
4.15
17.53
16.52
Ultimate analysis (wt.%) C
H
O
N
S
Cl
Wood
51.38
5.58
39.48
