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Biofuels and Biomass
ASH DEPOSITION IN AIR-BLOWN GASIFICATION OF PEAT AND WOODY BIOMASS IN A FLUIDZED-BED GASIFIER Yuanyuan Shao, Da Meng, Chunbao (Charles) Xu, Fernando Preto, and Jesse Zhu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00158 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018
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ASH DEPOSITION IN AIR-BLOWN GASIFICATION OF PEAT
2
AND WOODY BIOMASS IN A FLUIDZED-BED GASIFIER
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c
Yuanyuan Shaoa,b, Da Meng b, Chunbao (Charles) Xu*a, Fernando Preto , Jesse Zhu a,b
5 6
a
Dept of Chemical & Biochemical Eng., University of Western Ontario, London, ON, N6A 5B9
7
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of
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Chemical Engineering and Technology, Tianjin University, Tianjin, 300072
9
c
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CanmetENERGY, Natural Resources Canada, 1 Haanel Dr., Ottawa, Ontario K1A 1M1
Canada
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*Author to whom correspondence should be addressed. E-mail: Yuanyuan Shao :
13
[email protected], Chunbao (Charles) Xu:
[email protected] 14 15
ABSTRACT
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The ash deposition behaviors in air-blown gasification of a woody biomass and a
17
Canadian peat were studied using a pilot scale bubbling fluidized bed gasifier operating
18
at an equivalence ratio (ER) of 0.2-0.35. An air-cooled probe was installed in the furnace
19
of the gasifier to collect ash deposits on its surface. Four different bed materials
20
including olivine, dolomite, limestone and iron ore were used in this study to compare
21
their effects on ash deposition rate during the gasification. The experimental results
22
demonstrated that among the four bed materials, the use of olivine resulted in the lowest
23
ash deposition rate < 1.0 gm-2h-1, compared with ~ 16 gm-2h-1 for limestone in the
24
gasification of pine sawdust at an equivalence ratio (ER) of 0.35. The superb
25
performance of olivine in retarding ash deposition could be accounted for its outstanding 1
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thermal stability and mechanical strength. The other three bed materials, in particular
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limestone, were fragile during the fluidized bed gasification, and the fractured fines from
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the bed materials were found to deposit along with the fuel-ash on the heat transfer
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surface, leading on higher ash deposition rates. The ash deposition rates in the air-blown
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gasification process also strongly depended on the ER and the fuel type. The ash
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deposition rates for the peat gasification were much higher than those for the pine
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sawdust gasification.
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Keywords: Ash deposition, air-blown gasification, bubbling fluidized bed, woody biomass, peat, bed materials.
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1. Introduction
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Biomass as a renewable and carbon-neutral fuel with abundant resources [1-3] has
14
been widely utilized for heat and power generation via kinds of conversion processes
15
such as direct combustion/co-combustion, bio-conversions [4,5] and chemicals
16
transformation by gasification, pyrolysis and aqueous-phase catalytic processing [6-8].
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Among all biomass conversion technologies, gasification is attractive because of its
18
higher energy efficiency, larger biomass loading compared to combustion, reduced CO2
19
emissions and compact equipment with a smaller footprint [8,9]. Biomass gasification is
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a thermo-chemical conversion technology using gasification agents including air/oxygen,
21
steam or CO2 for converting biomass into low medium heating value fuel gases (5-15
22
MJ/Nm3). The gases produced during biomass gasification such as H2, CO, CO2, CH4
23
and C2+ can be utilized directly as fuels for heat and electricity generation, or as
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feedstocks for productions of liquid fuels and chemicals (methanol, ethanol, dimethyl
25
ether and Fischer-Tropsch oils, etc.) [10]. 2
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However, biomass gasification technology still has some challenges, in particular the
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tar formation and the low quality of the gas products. In biomass gasification, a highly
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variable mixture of condensable aromatic hydrocarbons (single ring to 5-ring aromatic
4
compounds) along with other oxygen-containing hydrocarbons and complex
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poly-aromatic-hydrocarbons, so called tar, are produced [11]. In an air-blown fluidized
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bed gasifier, typical tar contents in producer gas were reported between 0.5 and 100 g/m3
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[11-14]. The production of tar instead of combustible gases decreases the gasification
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efficiency and the condensation and deposition of tar at temperatures below 350°C can
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lead to fouling and potential blockage of downstream equipment and piping [11].
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On the other hand, biomass fuels, particularly some agricultural residues, usually
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contain high concentrations of inherent inorganic elements such as potassium, sodium,
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calcium, silicon and phosphorus etc. [15], which lead to an increased tendency of ash
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deposition, leading to fouling, slagging and corrosion problems [16,17]. Moreover, high
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chlorine contents were also found in some biomass to increase the alkali metal emission
15
from the ash in high temperature [18-20]. The alkali/alkaline-containing vapors thus may
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react with other elements and partially condense onto the reactor internal wall or some
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heat transfer surfaces [21]. Some fly ash may also enter downstream equipment and
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damage gas turbine hardware as a result of alkali corrosion and/or deposition [22].
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Many researches have been carried to eliminate the tar in biomass gasification
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progresses and the removal methods can be classified in two types: primary method in
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the gasifier itself and secondary method outside the gasifier. The usage of reactive bed
22
materials in fluidized-bed gasifiers has been proven to be an effective and economical
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primary approach to reduce/remove tar during gasification [11,23]. Reactive bed
24
materials such as natural olivine ((FexMg1-x) 2SiO4), calcined dolomite (CaO-MgO) and
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calcined limestone (calcite) were investigated and have been commonly used for in-bed 3
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tar control during biomass gasification [7,24-28]. A number of studies have been
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published concerning the effects of bed materials on the reduction of tar formation.
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Besides, several researches concerned about ash deposition behavior in different
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positions in the furnace of coals combustion process and established simple models with
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CFD [29-31]. However, there is almost no reported study concerning the effects of bed
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materials and other operating parameters for fluidized bed gasification on ash deposition.
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Attrition and thermal instability of the bed materials are the major issues for the usage of
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such active bed materials in a fluidized bed reactor [32,33], which would affect the ash
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deposition behaviors during the gasification process.
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In this study, air-blown gasification tests were performed on a pilot-scale air-blown
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bubbling fluidized bed gasifier using four different bed materials (olivine, limestone,
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dolomite, and iron ore) at varying equivalence ratios (ER) within the range 0.20-0.35. In
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oxygen/air-blown gasification, ER has been commonly used as an important operating
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parameter, defined as the ratio of oxygen content of air supply to oxygen required for
15
complete combustion [11]. An air-cooled probe was installed in the freeboard of the
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gasifier to simulate the surface of reactor wall, heat transfer surface or the surface of the
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downstream equipment/pipes. A typical woody biomass (i.e., white pine sawdust) and a
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Canadian peat were used for the gasification tests.
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2. Materials, Apparatus and Methods
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2.1. Materials and Preparation
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A woody biomass, white pine sawdust and a Canadian peat fuel were used in this study.
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The white pine sawdust was supplied by a local sawmill in southern Ontario and the
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Canadian peat fuel was obtained from Peat Resources Limited. The detailed proximate
25
and ultimate analyses of these two fuels and their ash compositions are given in Table 1. 4
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The white pine sawdust has a very low ash content, i.e., 0.4 wt% on a dry basis (db) and
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is characterized by a remarkedly high ratio of volatile matter/fixed carbon (VM/FC≈5.6).
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The peat contains strikingly high chlorine content of 2008 µg/g and is characterized by
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its low VM/FC ratio (≈2.3). With regard to ash composition, both fuels contain a high
5
concentrations of alkaline-earth metals (Ca, Mg) in their ashes, but the white pine has
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higher alkali metals (K+Na) than the peat. The ash from the white pine is enriched with
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basic oxides (CaO, MgO, K2O, Na2O and Fe2O3) and P2O5, while the peat is balanced
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between acidic oxides (SiO2, Al2O3, and TiO2) and basic oxides.
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Olivine, limestone, dolomite and iron ore were used as bed materials in the gasification
10
tests. The olivine, limestone and dolomite investigated as received from various
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commercial sources. The olivine sand used mainly consists of MgO (42.5 wt%), SiO2
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(42 wt%), Fe2O3 (8.5 wt%), CaO (0.9 wt%), and Al2O3 (0.8 wt%). The major
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components of the dolomite are CaO (30.4 wt%) and MgO (21.7 wt%). The limestone
14
after calcination contains mainly CaO. In addition, limonite iron ore was obtained from
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the former Steep Rock Mine site in Atikokan, Ontario. An analysis of the material by
16
X-ray diffraction (XRD) showed that the iron ore is mainly composed of iron oxides, in
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the form of goethite (FeOOH) and hematite (Fe2O3). From inductively coupled
18
plasma-atomic emission spectrometry (ICP-AES) analysis of the iron oxide, the Fe
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content of material was measured at 42.2 wt%.
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As for the feedstock preparation for the tests, the white pine sawdust as received
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contained a relatively high moisture content (38 wt%). This material was sieved to
22
remove large particles (>10 mm) and dried down to a 15-20% moisture content using a
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rotary dryer with a screening facility at CanmetENERGY. The peat was received in
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pellets with a moisture content of approximately 36 wt%. Due to poor fluidizability of 5
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the peat pellets and, in order to obtain more representative results comparable to those
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from the pine sawdust, these pellets were crushed and sieved to 1-4 mm particle size
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using an electrical grinder. Because of the partial loss of the moisture of the feedstock
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during the crushing and further air drying, the crushed peat had a moisture content of
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around 25 wt% when they were applied to the gasification tests. The actual moisture
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content of the fuel used was measured prior to each test to determine the fuel feeding
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rate for the gasification test. The bed materials (olivine, limestone and dolomite) were
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crushed and sieved to ensure a uniform particle diameter (about 1 mm diameter), while
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the iron ore was crushed and sieved to particles of a size of about 0.85 mm instead
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because of its higher particle density. All bed materials were in-situ calcined in air
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at >750 °C within the fluidized bed reactor during the warm-up combustion phase of
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each test using propane gas.
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2.2. Gasification Facility
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The gasification tests were conducted on a pilot-scale, air-blown fluidized bed gasifier,
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as schematically illustrated in Figure 1. The core part of the system is a stainless steel
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cylindrical riser with a 127 mm inner diameter and a 4.55 m height. The facility was a
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bubbling fluidized bed reactor with a maximum feeding capacity of 25 kg/h, equipped
18
with a belt feeder combined with a rotary airlock valve and a cyclone for fly ash
19
collection. A water-cooled condenser was also equipped to the facility for tar removal.
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Moreover, multiple temperature sampling ports (T1-T9 in Figure 1) were located at
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different heights on the riser column and a flue gas sampling system was situated at the
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flue gas pipe. The unit was coupled with a primary air inlet and four secondary air inlets.
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The major secondary air supply was introduced through line IV to the fluidized bed
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reactor at 560 mm above the center of the fuel feeding ports. Measurements of flue gas 6
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compositions (H2, CO, CO2, O2, CH4, SO2 and N2) and flue gas temperature were
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performed. A custom-designed air-cooled probe was installed vertically in the freeboard
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region of the fluidized bed combustor using a flange at the top. The probe was made of
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SS 316L according to ASTM A511 standard and its geometry is shown in detail in
5
Figure 2. The total effective length of the probe (for ash deposition inside the freeboard
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zone) was 838.2 mm, and the total external surface area was calculated to be 0.134 m2.
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During the steady operation in all the tests, the surface temperature of the probe was
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maintained in 450±10 ºC by carefully controlling the flow rate of the cooling air.
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2.3. Testing Methodologies and Parameters
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The reactor was filled with 12 to 17 kg bed material in each test. The bed was fluidized
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by the primary air (around 185 L/min) and was heated up to above 750ºC for 1-2 hours
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using propane gas before introducing the solid fuel. A combustion mode was initially
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performed at an air-to-fuel ratio of 1.4 with a feed rate of about 8 kg/h of the fuel to
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warm up the whole system before the unit was switched to gasification mode. When a
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relatively stable bed temperature profile was reached, the gasification mode was started
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by increasing the fuel feed rate to 10-25 kg/h and decreasing the total air flow rate to
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around 300 L/min to match the desired equivalence ratio (ER). The value of the ER has
18
been observed to strongly influence the gas product compositions and gasification
19
efficiency for air-blown biomass gasification [34,35]. Usually, the ER is in a range from
20
0.2 to 0.4 for biomass gasification to avoid incomplete gasification and excessive char
21
formation at an excessively low ER (0.4) [36]. The specific
23
operational conditions for each gasification test can be found in Table 2. The gasification
24
tests were performed with the four aforementioned bed materials (dolomite, olivine, 7
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limestone and iron ore) at varying ER from 0.20 to 0.35. By adjusting both fuel feeding
2
rate and total air flow rate, the desired ER can be maintained. For instance, as shown in
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Table 2, an higher ER was usually achieved by decreasing the fuel feeding rate while
4
keeping the total air flow rate at around 290-300 L/min for most runs, However, in the
5
tests with a same ER for different bed materials, the fuel feeding rate was kept
6
approximately the same on a thermal input basis.
7
At the steady state, the gasification temperatures (in the dense phase of the fluidized
8
bed) were maintained in the range of 700- 900°C (depending on the applied ER) by the
9
partial combustion of the fuel without external heating. A stable temperature profile
10
along the bed height could be obtained through adjusting the secondary air and primary
11
air ratios (while maintaining a constant total air flow rate). In this study, because the
12
secondary air was introduced above the dense phase of the fluidized bed, inevitable heat
13
losses occurred along the reactor column. The average flue gas temperature in the
14
freeboard and in the vicinity of the ash deposition probe varied thus within a relatively
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wide range of 500-700ºC, a temperature that still prevented tar condensation of tar.
16
Additionally, a significant loss of bed material due to physical attrition and thermal
17
fragmentation was observed during some tests, so additional bed materials had to be
18
added to keep a constant bed level, which was monitored via three pressure sensors
19
located at different heights of the bed as shown in Figure 1. In a typical run, at least 3-4
20
hours of stable operation were performed before cooling down the reactor to room
21
temperature using nitrogen.
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A cleaned probe was installed vertically in the reactor, right before the whole unit
23
attained a steady state of operation of the gasification mode. When the whole unit was
24
cooled to room temperature, the ash deposition probe was carefully removed from the
8
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furnace and the deposited ash was completely recovered for weighing to calculate the
2
ash deposition rate (DA, g m-2 h-1), as defined below:
3
DA =
Mass of collected ash deposit (g) Surface area of the probe (m 2 ) × Duration (h)
(1)
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The collected deposited ashes were analyzed for various characterizations by using
5
X-ray fluorescence (XRF) for chemical compositions in accordance to the ASTM D4326
6
standard, and Scanning Electron Microscope (SEM), in order to have a clear view
7
regarding the effects of the different operations (i.e. bed materials and ER) on the
8
morphology.
9
Additionally, a non-iso-kinetic tar sampling system was used in this study, using a train
10
of impingers containing an isopropanol solvent, as illustrated in Figure 3. The produced
11
gas was drawn using a vacuum pump through a particulate filter into electrically heated
12
lines (maintained at > 350°C to prevent the condensation of tar). The tar was condensed
13
at the impinge train, and the incondensable product gas flowed through a wet-gas meter
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and vacuum pump before it was finally vented. The impinger system was composed of
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six solvent-containing vessels, four in a water bath (20°C) and two in an ice bath at 0°C,
16
plus a final droplet trap. The tar sampling started after reaching steady state operation
17
and continued for 45-90 minutes. The total gas volume and smpling time were recorded
18
with a wet gas meter. Following gasification, the solvent/tar mixture was collected. The
19
impinger system and any piping below 350°C were washed with isopropanol, and the
20
solvent/tar mixture filtered to remove any residual particulate matters. The isopropanol
21
was evaporated at 50°C under reduced pressure with a rotary evaporator, and the tar was
22
weighed for calculation of the tar concentration in the produced gas.
23
Because of the relatively large operating scale and the complexity of the fluidized bed
24
facilities, it normally took 2-3 days to complete a successful gasification test (including 9
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fuel/facility preparation, operation, after-run cleaning/maintenance). Hence in this study
2
duplicate tests were carried out only for the reference test (gasification of pine sawdust
3
using limestone as the bed material at ER=0.3) to examine the reproducibility of the ash
4
deposition rates. The relative standard deviations of the ash deposition rates were within
5
15.0%.
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3. Results and Discussion
7
3.1. Effects of bed materials on fly ash deposition
8
Figure 4 shows the ash deposition rates during the pine sawdust gasification with
9
different bed materials at various ERs. The ash deposition rates generally remained
10
nearly constant for all the bed materials with the ER ranging from 0.2 to 0.3. For
11
example, when using olivine sand as the bed material, ash deposition rates did not
12
significantly change with ER and were considerably low (< 1.0 g m-2 h-1) with a small
13
peak deposition rate (DA=0.81 g m-2 h-1) at ER=0.3. In the tests using iron ore as the bed
14
material, the ash deposition rates fluctuated depending on the ER. In contrast, DA in the
15
tests with limestone bed material constantly increased with increasing ER, in particular
16
as the ER increased from 0.3 to 0.35 (as clearly indicated in Figure 4). Since the excess
17
air could react with CO and make the gasifier hotter at high ER, which can check in
18
Fig.6, the significant DA could be resulted as the limestone’s fragile characteristic in high
19
temperature. For the whole range of ER tested (0.2-0.35), the overall ash deposition rates
20
of different bed materials had the following sequence of order: iron ore > dolomite ≈
21
limestone > olivine. With the use of olivine, the lowest DA values (0.3.
14
It is thus of a particular interest to discuss the possible causes for the superb
15
performance of olivine, as well as the poor performance of limestone, in the fluidized
16
bed biomass gasification with respect to fly ash deposition. Olivine showed outstanding
17
mechanical strength with negligible formation of fines during the tests, which is
18
consistent with observations made by other researchers [28,38]. The attrition resistance
19
of different bed materials during the fluidized bed pine sawdust gasification at ER=0.30
20
is compared in Figure 7. Negligible loss of the olivine bed material was observed during
21
the test. In contrast, the calcined limestone was found to be very fragile, producing a
22
substantial amount of fines and then leading to a loss of more than 50 % of the bed
23
materials during the test (as shown in Figure 7). A visual comparison of deposits
24
collected from the tests at the same ER (0.3) with these four bed materials is shown in
25
Figure 8. Different from the grey color deposits (originated from the fuel ash) collected 11
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from the test with olivine sand, fines originated from the respective bed materials were
2
observable in the deposits collected from the tests using the other three bed materials.
3
For example, as shown in Figure 8, particles of the burgundy color were present in the
4
ash deposits from the test with iron ore, and white particles were observed in the
5
deposits from the tests using dolomite and limestone. The losses of these three bed
6
materials (i.e., limestone, or dolomite, or iron ore) were further confirmed by the XRF
7
analyses of the collected cyclone bottom ashes. Moreover, comparing the SEM image of
8
the ash deposit collected from olivine test at ER=0.35 with those from tests of limestone
9
at ER=0.20 and limestone at ER=0.35, respectively, as shown in Fig.9, it is easily
10
visually observed that countless fine particle covered/formed the limestone deposit at
11
ER=0.35. However, those deposits got from olivine test and limestone test at low ER
12
presented relatively smooth surface. Figure 10 shows the chemical compositions
13
(determined by XRF analysis) of the ash deposits collected from the pine sawdust
14
gasification using olivine sand (Fig. 10A) and limestone (Fig. 10B) as the bed materials.
15
It should be noted that significant losses on fusion were observed in the XRF analyses of
16
the ash deposits (as shown in Fig. 10A), which were likely due to the presence of the
17
carbonaceous matters such as the unburned carbon or low boiling point alkali salts in the
18
deposits. Compared to the olivine tests, the deposits from the tests using limestone
19
contained an extraordinary high content of CaO (>50 wt% of the deposits), originated in
20
the limestone bed materials. The CaO in calcined limestone is known to be able to
21
improve the formation of alkali carbonates, sulphates and chlorides [39], which have a
22
relatively lower melting point (≈600 °C), and hence might favor ash deposition as
23
observed in Figure 4.
24
Although fast ash deposition might also related to the chlorine deposition as discussed 12
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in previous works [21,40] and other literature works [41,42], the ash deposit collected
2
from each run in this study was unfortunately not enough for an analysis of chlorine
3
contents in the deposits. Therefore, it can be concluded that the ash deposition behaviors
4
during the pine gasification in a fluidized bed with various bed materials do not seem to
5
be related to tar formation and their activities for biomass gasification and tar reduction,
6
but are more likely related to their resistance to attrition during the fluidized bed tests.
7
3.2. Effects of different fuels on ash deposition
8
Figure 11 presents the comparison of ash deposition rates during the gasification of the
9
pine sawdust and crushed peat at various ERs but with the same bed material (olivine
10
sand). From Figure 11, one may find that at all ERs tested (0.20 through 0.35), the ash
11
deposition rates for peat gasification were consistently faster than those for the pine
12
sawdust. For instance, the maximum deposition rate attained 5.5 g m-2 h-1t for the peat
13
gasification, compared to only 0.3 g m-2 h-1 for the pine sawdust gasification at an ER of
14
0.2. The compositions of the ash deposits from each fuel are
15
comparatively shown in Figure 12. Generally the deposited ashes from the peat
16
gasification tests contained higher concentrations of MgO and SO3, but lower
17
concentrations of K2O and CaO. These composition distributions are actually in a good
18
agreement with the fuel ash compositions as shown in Table 1. At a lower ER (0.2 or
19
0.25), the SiO2 was founded to be at a significantly higher concentration in the ash
20
deposits from the peat gasification than those from the pine sawdust gasification, which
21
was also expected as the peat fuel ash contains a markedly higher SiO2 (28.1 wt% for the
22
crushed peat and 6.7 wt% for the pine sawdust). However, at a higher ER (0.3 or 0.35),
23
the SiO2 concentration in the ash deposits from the pine sawdust gasification increased
24
greatly, higher than that from the peat gasification. The enrichment of SiO2 in the ash
25
deposits from the pine sawdust might be due to the contamination from the olivine bed 13
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1
materials (containing a 42 wt% of SiO2). Even though the olivine sand has superb
2
thermal and mechanical stability, a slight degradation of the bed materials to form fines
3
did occur in the pine sawdust gasification process at a higher ER, as evidenced
4
previously in Figure 7. In term of the Fe2O3, perhaps the high Fe content is from the steel
5
deposition tube due to chemical interactions and the deposition extraction procedure.
6
As described in the previous section, the ash deposition from the pine sawdust
7
gasification using olivine was negligible due to the thermal and mechanical stability of
8
the olivine sand, as well as the very low ash content of the pine sawdust (0.4 wt% db).
9
With the same gasification conditions, and especially using the same bed material, the
10
difference in the ash deposition rates between the peat and pine sawdust fuel must result
11
from the differences in fuel properties, in particular the volatile matters and fuel ash
12
properties. As mentioned previously, the pine sawdust has more volatile matters than the
13
peat, which would make the woody biomass more reactive in gasification, releasing
14
more volatile vapour. The larger volatile vapour formation that entrained the fly ash
15
species led to a shorter contact time between the fly ash and the ash deposition probe
16
[43], which could hence result in a lower ash deposition rate as shown in Figure 11. The
17
formation of a larger volume of volatile matters from the pine sawdust gasification
18
correlates to a greater tar generation as clearly shown in Figure 13.
19
More importantly, the difference in the ash deposition rates between the peat and pine
20
sawdust fuel might be explained by the big difference in the fuel ash content and
21
compositions. As given previously in Table 1, the white pine sawdust has a very low ash
22
content, i.e., 0.4 wt% on a dry basis, while the peat contains a 2 wt% ash and a strikingly
23
high chlorine content of 2008 µg/g. The high ash content of the peat fuel might account
24
for its faster ash deposition rates in the gasification process. Furthermore, the high
25
chlorine content of the peat fuel was believed to play an important role in promoting the 14
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Energy & Fuels
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ash deposition. As similarly observed in our previous studies on co-firing biomass and/or
2
peat with lignite coal in the same fluidized-bed facilities [21,41,43], peat fuel with a high
3
chlorine content exhibited a much higher ash deposition tendency than other fuels with a
4
lower Cl content.
5 6
4. Conclusions
7
In this study, the four common bed materials including olivine, dolomite, limestone and
8
iron ore were investigated for the ash deposition behaviors in the gasification of pine
9
sawdust and peat. The criterion about the bed material applicability is deposition rate,
10
and it can conclude that the olivine lead to the lowest ash deposition rate which less than
11
1.0 gm-2h-1 in the gasification of pine sawdust at ER of 0.35, with a starkly comparison
12
of ~16gm-2h-1 for limestone. Additionally, the kind of fuel also has great impact on the
13
deposition rate in gasification process. The crushed peat inclined to generate greater ash
14
production than the pine sawdust. The remarked conclusion can be stated as follows.
15
(1) Among the four bed materials, the use of limestone led to the highest gasification
16
efficiency (measured by the lowest tar formation), but also the highest ash
17
deposition rate, being ~ 16 gm-2h-1 in the gasification of pine sawdust at an
18
equivalence ratio (ER) of 0.35.
19
(2) The use of olivine resulted in the lowest ash deposition rate < 1.0 gm-2h-1, and the
20
reasonably good performance of olivine in retarding ash deposition could be
21
attributed to its outstanding thermal stability and mechanical strength.
22
(3) The other three bed materials, in particular limestone, were fragile during the
23
fluidized bed gasification, and the fractured fines from the bed materials were
24
found to deposit along with the fuel-ash on the heat transfer surface, leading to
25
higher ash deposition rates. 15
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Page 16 of 31
1
(4) The ash deposition rates for the peat gasification were much higher than those for
2
the pine sawdust gasification. This might be explained by the big difference in
3
the fuel ash content and compositions. Compared with the pine sawdust, the peat
4
fuel used in this work contains much higher ash content (2 wt% ash) and
5
strikingly higher chlorine content (2008 µg/g).
6 7
ACKNOWLEDGEMENTS
8
The authors are grateful for the financial support from Natural Sciences and
9
Engineering Research Council of Canada (NSERC) through the Discovery Grant
10
Program. Special thanks are also extended to Guy Tourigny, Dave Wambolt, Dan Walsh
11
and Bart Young for their participation and great assistance on the tests.
12 13
The authors would also like to acknowledge the funding from the National Natural Science Foundation of China (Grant No. 21606170).
14 15
REFERENCES
16
[1] Fernholz K. Energy from woody biomass: a review of harvesting guidelines and a
17
discussion
18
http://www.dovetailinc.org/files/DovetailBioGuides0709.pdf, 2009.
19 20 21 22 23 24 25
of
related
challenges.
Divetail
Partners,
Inc.,
[2] Demirbas A. Global renewable energy resources. Energy Sources Part A-Recovery Util Environ Eff. 2006;28(8):779-92. [3] Zhou ZQ, Yin XL, Xu J, Ma LL. The development situation of biomass gasification power generation in China. Energy Policy. 2012;51:52-7. [4] Cuellar A D, Herzog H. A path forward for low carbon power from biomass.[J]. Energies, 2015, 8(3):1701-1715. [5] Menon V, Rao M. Trends in bioconversion of lignocellulose: biofuels, platform 16
ACS Paragon Plus Environment
Page 17 of 31 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
chemicals & biorefinery concept.[J]. Progress in Energy & Combustion Science,
2
2012, 38(4):522-550.
3
[6] Serrano-Ruiz JC, Dumesic JA. Catalytic routes for the conversion of biomass into
4
liquid hydrocarbon transportation fuels. Energy & Environmental Science.
5
2011;4(1):83-99.
6
[7] Shen Y, Yoshikawa K. Recent progresses in catalytic tar elimination during biomass
7
gasification or pyrolysis-A review. Renewable & Sustainable Energy Reviews.
8
2013;21:371-92.
9 10
[8] Zhang LH, Xu CB, Champagne P. Overview of recent advances in thermo-chemical conversion of biomass. Energy Conv Manag. 2010;51(5):969-82.
11
[9] Morehouse JH, Detwiler KW. Assessment of a biomass gasification co-generation
12
plant based on the UCS's "principles for bioenergy development. The 2nd
13
International Conference on Energy Sustainability, Jacksonvill, USA. vol.
14
1:399-404, 2008.
15 16 17
[10] McKendry P. Energy production from biomass (part 3): gasification technologies. Bioresource Technology 2002; 83:55–63. [11] Lopamudra D, Ptasinski KJ, Janssen FJJG. A review of the primary measures for tar
18
elimination
19
2003;24:125–140.
20 21
in
biomass
gasification
processes.
Biomass
and
Bioenergy
[12] McKendry P. Energy production from biomass (part 1): overview of biomass. Bioresource Technology 2002; 83:37–46.
22
[13] Han J, Kim H. The reduction and control technology of tar during biomass
23
gasification/pyrolysis: An overview. Renewable and Sustainable Energy Reviews
24
2008; 12: 397-416.
25
[14] Asadullah M, Miyazawa T, Kunimori K. Catalyst development for the gasification 17
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
of biomass in the dual-bed gasifier. Applied Catalysis A: General 2003; 255:169–
2
80.
3
[15] Tchoffor PA, Davidsson KO, Thunman H. Transformation and Release of Potassium,
4
Chlorine, and Sulfur from Wheat Straw under Conditions Relevant to Dual
5
Fluidized Bed Gasification. Energy & Fuels. 2013;27(12):7510-20.
6
[16] Bryers RW. Fireside slagging, fouling, and high-temperature corrosion of
7
heat-transfer surface due to impurities in steam-raising fuels. Progress in Energy
8
and Combustion Science 1996; 22: 29-120.
9
[17] Lane DJ, van Eyk PJ, Ashman PJ, Kwong CW, de Nys R, Roberts DA, et al.
10
Release of Cl, S, P, K, and Na during Thermal Conversion of Algal Biomass.
11
Energy & Fuels. 2015;29(4):2542-54.
12 13
[18] Wei XL, Schnell U, Hein KRG. Behaviour of gaseous chlorine and alkali metals during biomass thermal utilisation. Fuel. 2005;84(7-8):841-8.
14
[19] Zhang Z, Liu J, Shen FH, Yang YJ, Liu F. On-line measurement and kinetic studies
15
of sodium release during biomass gasification and pyrolysis. Fuel. 2016;178:202-8.
16
[20] Shao, Yuanyuan, Wang, et al. Energies, Vol. 5, Pages 5171-5189: Ash Deposition in
17
Biomass Combustion or Co-Firing for Power/Heat Generation[J]. 2012.
18
[21] Shao Y, Xu C, Zhu J, Preto F, Wang J, Tourigny G, et al. Ash and chlorine
19
deposition during co-combustion of lignite and a chlorine-rich Canadian peat in a
20
fluidized bed - Effects of blending ratio, moisture content and sulfur addition. Fuel.
21
2012;95(1):25-34.
22 23
[22] Salo K., Mojtahedi W. Fate of alkali and trace metals in biomass gasification. Biomass and Bioenergy 1998; 15:263-7.
24
[23] Anis S, Zainal ZA. Tar reduction in biomass producer gas via mechanical, catalytic
25
and thermal methods: A review. Renewable & Sustainable Energy Reviews. 18
ACS Paragon Plus Environment
Page 18 of 31
Page 19 of 31 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
2011;15(5):2355-77.
2
[24] Pencho J., Schildhauer T, Sturzenegger M, Biollaz S, Wokaun A. Reactive bed
3
materials for improved biomass gasification in a circulating fluidized bed reactor.
4
Chemical Engineering Science 2008; 63:2465-76.
5 6
[25] Gusta E, Dalai A, Uddin M, Sasaoka E. Catalytic decomposition of biomass tars with dolomites. Energy Fuels 2009; 23:2264-72.
7
[26] Weimer T., Berger R, Hawthorne C, Abanades JC. Lime enhanced gasification of
8
solid fuels: Examination of a process for simultaneous hydrogen production and
9
CO2 capture. Fuel 2008; 87: 1678-86.
10
[27] Weerachanchai P, Horio M, Tangsathitkulchai C. Effects of gasifying conditions and
11
bed materials on fluidized bed steam gasification of wood biomass. Bioresource
12
Technology. 2009;100(3):1419-27.
13 14
[28] Rapagnà S, Jand N, Kiennemann A, Foscolo PU. Steam-gasification of biomass in a fluidised-bed of olivine particles. Biomass and Bioenergy 2000; 19:187-97.
15
[29] Pérez, M. G., Fry, A. R., Vakkilainen, E., & Whitty, K. J. (2016). Ash deposit
16
analysis of the convective section of a pilot-scale combustor firing two different
17
subbituminous coals. Energy & Fuels, 30(10).
18
[30] Beckmann, A. M., Mancini, M., Weber, R., Seebold, S., & Müller, M. (2016).
19
Measurements and CFD modeling of a pulverized coal flame with emphasis on ash
20
deposition. Fuel, 167, 168-179.
21
[31] Baxter, L. L. (2000). Ash deposit formation and deposit properties. a comprehensive
22
summary of research conducted at sandia's combustion research facility. Office of
23
Scientific & Technical Information Technical Reports.
24
[32] Bartels M, Lin W, Nijenhuis J, Kapteijn F, van Ommen JR. Agglomeration in
25
fluidized beds at high temperatures: Mechanisms, detection and prevention. 19
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Progress in Energy and Combustion Science. 2008;34(5):633-66.
2
[33] Kaknics J, Michel R, Richard A, Poirier J. High-Temperature Interactions between
3
Molten Miscanthus Ashes and Bed Materials in a Fluidized-Bed Gasifier. Energy
4
& Fuels. 2015;29(3):1785-92.
5
[34] Lv PM, Xiong ZH, Chang J, Wu CZ, Chen Y, Zhu JX. An experimental study on
6
biomass air-steam gasification in a fluidized bed. Bioresource Technology.
7
2004;95(1):95-101.
8
[35] Kumar A, Eskridge K, Jones DD, Hanna MA. Steam-air fluidized bed gasification
9
of distillers grains: Effects of steam to biomass ratio, equivalence ratio and
10
gasification temperature. Bioresource Technology. 2009;100(6):2062-8.
11
[36] Narváez I, Orío A, Aznar MP, Corella J. Biomass gasification with air in an
12
atmospheric bubbling fluidized bed. Effect of six operational variables on the
13
quality of produced raw gas. Industrial and Engineering Chemistry Research 1996;
14
35:2110-20.
15
[37] Hurley S, Shao Y, Preto F, Tourigny G, Li H, Xu C. Catalytic gasification of
16
biomass in a pilot-scale fluidized bed reactor. Proceeding of the 2nd annual particle
17
technology research centre conference, London, Ontario, Canada, 2009.
18
[38] Devi L, Craje M, Thüne P, et al. Olivine as tar removal catalyst for biomass
19
gasifiers: Catalyst characterization[J]. Applied Catalysis A General, 2005,
20
294(1):68-79.
21
[39] Florin NH, Harris AT. Enhanced hydrogen production from biomass with in situ
22
carbon dioxide capture using calcium oxide sorbents. Chem Eng Sci.
23
2008;63(2):287-316.
24
[40] Shao Y, Wang J, Xu C, Zhu J, Preto F, Tourigny G, et al. An experimental and
25
modeling study of ash deposition behaviour for co-firing peat with lignite. Applied 20
ACS Paragon Plus Environment
Page 20 of 31
Page 21 of 31 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
Energy. 2011;88(8):2635-40.
2
[41] Theis M, Skrifvars BJ, Zevenhoven M, Hupa M, Tran H. Fouling tendency of ash
3
resulting from burning mixtures of biofuels. Part 3: Influence of probe surface
4
temperature.
Fuel 2006; 85:2002-11.
5
[42] Aho M, Silvennoinen J. Preventing chlorine deposition on heat transfer surfaces
6
with aluminium–silicon rich biomass residue and additive. Fuel 2004;
7
83:1299-305.
8
[43] Shao Y, Xu C, Zhu J, Preto F, Wang J, Tourigny G, Badour C, Li H. Ash deposition
9
during co-firing biomass and coal in a fluidized-bed combustor. Energy Fuels 2010;
10
24:4681-8.
11
21
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1 2 3
Tables and Figures Table 1. Proximate and ultimate analyses of the fuels and their ash compositions
Pine Sawdust
Crushed Peat
Proximate analysis, wt% dry base 1
Ash
0.4
2.0
Volatile matters (VM)
84.5
68.6
Fixed carbon (FC)
15.1
29.4
Moisture, wt% as received
38.0
35.8
Moisture, wt% after air dried
15-20
25
20.6
21.4
Carbon
52.5
56.1
Hydrogen
6.3
5.7
Nitrogen
0.1
0.8
Sulphur
