Fabrication of a stable Ti/Pb-TiOxNWs/PbO2 anode and its application in benzoquinone degradation

Article abstract of DOI:10.1016/j.electacta.2020.137532

To delay passivation of a titanium (Ti) substrate as well as enhance adhesion between an electrodeposited PbO₂ coating and the Ti substrate, a titanium-lead composite oxide nanowire (Pb-TiO_xNWs) intermediate layer was formed in situ on the surface of porous Ti by alkali etching, ion substitution, and high-temperature calcination. At the same time, Ti/PbO₂ and Ti/TiO₂NWs/PbO₂ electrodes with porous Ti as a matrix were prepared for comparison. The surface structure and morphology of the prepared intermediate layer and the PbO₂ coating were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The influences of the composite oxide intermediate layer on the electrochemical performance of the PbO₂ electrode were analyzed by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and AC impedance spectroscopy (EIS). Accelerated lifetime tests were performed for electrodes with and without different intermediate layers. The results showed that PbO_x was incorporated into the titanium dioxide three-dimensional network structure, resulting in formation of Pb-TiO_xNWs. The surface of the Ti/Pb-TiO_xNWs/PbO₂ electrode was denser due to the smaller particle size of PbO₂. The preferred crystal orientation of β(110) was observed for PbO₂ deposited on Ti/Pb-TiO_xNWs. The oxygen evolution potential reached a maximum of 2.19 V for Ti/Pb-TiO_xNWs/PbO₂. Accelerated life tests showed that compared with Ti/PbO₂ and Ti/TiO₂NWs/PbO₂, the electrode life of Ti/Pb-TiO_xNWs/PbO₂ was increased by 91.7% and 35.3%, respectively. Therefore, it can be concluded that significantly improved morphology and electrochemical performance were achieved for titanium-based PbO₂ electrodes by the addition of a Pb-TiO_xNWs intermediate layer. In particular, the electrochemical stability of the PbO₂ coating electrodes was improved markedly by the Pb-TiO_xNWs intermediate layer. The electrodes were used for electrochemical oxidation of benzoquinone in wastewater (100 mg/L). It was found that chloride ions played a critical role in improving the current efficiency of electro-oxidative degradation. Under the same conditions, the COD removal rate in the presence of NaCl was 45% higher than in the presence of sulfate. The results of HPLC analysis of the intermediate products indicated that the oxidants electro-generated by chloride ions had stronger ring-opening and mineralization capabilities than those electro-generated by sulfate ions.

Full text of DOI:10.1016/j.electacta.2020.137532

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Energy Procedia 61 (2014) 643 – 647
The 6th International Conference on Applied Energy – ICAE2014
The experimental study of full-scale biomass-fired bubbling
fluidized bed boiler
Jan Skvarila,b,*, Anders Avelina, Jan Sandberga, Erik Dahlquista
aSchool of Bussiness Society and Engineering, Mälardalen University, Box 883, Högskoleplan 1, 721 23 Västerås, Sweden
bEnergy Institute, Faculty of Mechanical Engineering, Brno University of Technology, Technická 2, 616 69 Brno, Czech Republic
Abstract
This paper presents experimental data concerning combustion characteristics of full-scale biomass-fired bubbling
fluidized bed (BFB) steam boiler with a thermal output of 31 MW. The purpose of the experimental measurements is
to show how the values of selected combustion parameters vary in reality depending on measurement position.
Experimentation involves specifically a determination of combustion gas temperature and concentration of gas
species i.e. O2, CO2, CO and NOX at different positions in the furnace and the flue gas trains. Character of results
from the furnace indicates the intermediate stage of thermochemical reactions. Increased levels of CO close to the
wall have been found, this may be indicating reducing atmosphere and thereby increased corrosion risk. Results from
flue gas trains demonstrate that behavior there is related to the fluid dynamics and heat transfer, the temperature is too
low for further combustion reactions. Results show great variations among measured values of all measurands
depending on a distance along the line from the wall to the center of the boiler. The measurements from permanently
installed fixed sensors are not giving value representing average conditions, but overall profiles can be correlated to
online measurements from fixed sensors.
© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
© 2014 The Authors. Published by Elsevier Ltd.
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and/or peer-review under responsibility of ICAE
Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: Biomass combustion; steam boiler; fluidized bed; temperature measurements; flue gas concentration measurements.
1. Introduction
Fluidized bed combustion is considered as one of the most promising techniques because of its
flexibility, high combustion efficiency and low environmental impact [1]. It has been developed and
tested over many decades as presented by Koornneef et al. [2]. Nevertheless there are still some issues to
be studied in relation to the combustion of biomass as it is a fuel with certain specific properties that affect
* Jan Skvaril. Tel.: +46-21-151738; fax: +46-21-101480.
E-mail address: jan.skvaril@mdh.se.
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of ICAE2014
doi:10.1016/j.egypro.2014.11.1188

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Jan Skvaril et al. / Energy Procedia 61 (2014) 643 – 647
combustion process and equipment. The combustion diagnostics in industrial applications is uncertain
since the fixed sensors permanently installed in the boiler are placed in very few positions (Fig. 1.).
Therefore, it is necessary to carry out an experimental work in order to get more detailed insight of the
combustion process. Examination of the literature shows remarkable studies concerning full-scale steam
boilers (e.g. [3-5]), however the availability of the data from biomass fired boilers is very scarce. In
relation to the limited access to the data the research project was designed at Malardalen University in
order to obtain comprehensive experimental diagnostic data from different full-scale biomass-fired steam
boilers. The study presented here is a facet of comprehensive research project and the aim is to provide
data from bubbling fluidized bed (BFB) steam boiler with a thermal output of 31 MW. Experimentation
involves specifically a determination of combustion flue gas temperature and concentration of gas species
i.e. O2, CO2, CO, NOX at different positions in the furnace and flue gas trains. This data will be further
used for formation, development and verification of analytical and numerical mathematical models and
for controlling and optimization purposes.
2. Material and methods
2.1. Experimentation
The measurement procedure was designed so that the flue gases are extracted from the different points
lying on the line at certain positions in the furnace (Fig. 1. Ports SHE1, SHE2), and flue gas trains (Fig 1.
Ports SHE3, SHE4, SHE5). The distribution of individual measurement points on the line was proposed
in accordance with the assumption that there would be steeper gradients in positions near the wall. Flue
gases are drawn through the water-cooled suction pyrometer where the temperature is measured by
precalibrated type K thermocouple. After passing suction pyrometer, the flue gases are further dried and
purified and then pumped to the calibrated gas analyzers where the gas concentration is measured.
Analyzers used are Fuji Electric ZRJ Infrared GA for O2, CO2 and CO concentration analysis and Eco
Physics CLD 700 EL ht for NOx concentration analysis (both on dry gas stream). The signal from the
measuring instruments is processed in the PC logger and a recorded by PC at 10 second intervals. There
are approx. 90 values read for each measurand at each point.
Nominal operating parameters:
Thermal capacity (without
FGC)
30.8 MWt
Electrical output
10 MWe
Flue gas condenser (FGC)
output
7–8 MWt
Nominal steam parameters:
Flow rate of main steam
Temperature of main steam
Pressure of main steam
11.9 kg·s-1
480 °C
80 bar
Operating parameters during experimentation:
Total thermal output
Electrical output
25.8 MWt (± 5%)
7.7 MWe (± 5%)
0 MWt
FGC output
Flow rate of main steam
10.1 kg∙s-1 (± 5%)
Fuel properties during experimentation:
Net calorific value
8400 kJ∙kg-1
Moisture content (as received)
Ash content (dry matter)
54 wt%
1.4 wt%
Composition: Forest residue chips (73 wt%), Stem
wood chips (12 wt%), Sawdust (15 wt%)
Fig. 1. Bubbling fluidized bed boiler 30.8 MW

Jan Skvaril et al. / Energy Procedia 61 (2014) 643 – 647
645
2.2. Boiler design and operating parameters
The BFB boiler (Fig. 1.) is used for combined heat and power production and it was constructed by
Foster Wheeler AG. The furnace has evaporator membrane water walls on all four sides. The bottom
membrane panel and is covered with protective refractory lining, so that bubble caps protrude and lining
exceeds and covers membrane walls to a height of approx. 2.5 m from the furnace bottom. Primary air is
brought to the furnace through the bottom bubble caps. Secondary air is staged and brought in at several
levels along the furnace freeboard. The boiler consist of convective vertical pendant superheaters (SH I.,
SH II. and SH III.), economizer (ECO) and air preheaters (AP). The boiler is equipped with flue gas
recirculation and a flue gas condenser (FGC). Furthermore, there are sensors permanently installed for
temperature measurements (Fig. 1. T1 and T2) and O2 concentration measurements (Fig. 1. O2 1 and
O2 2). These sensors are placed approx. 0.3 m from the boiler wall and are used for online process
monitoring.
During experiments, the boiler was operated with an emphasis on steady operating conditions.
However some fluctuations were observed due to operational constrains such as variable fuel properties
and fluctuating demand in district heating (for details see, Fig. 1.) Fuel fired during experiments was a
mixture of solid biomass-based materials from broadleaves, conifers and shrubs (for details, see Fig. 1.).
3. Results and discussion
Note that average values are used when discussing results and the positive and negative error bars in
graphs represent the sample standard deviation both calculated from repeated measurements (approx. 90
values).
Character of the results from the points in the furnace (Ports SHE1, SHE2) indicates the intermediate
stage of thermochemical reactions. Results shows that temperature near port SHE1 increases uniformly
along the line (Fig. 2.). Temperature profile is correlates negatively with O2 concentration (Fig. 3.).
Whilst CO concentration is low near the wall and then abruptly rises and reaches the limit of the
instrument at the distance of 2.0 m (Fig. 4.). Moreover, CO concentration is characterized by a high
variation between the repetitions. NOX concentration rises evenly from point at 0.1 m to 2.0 m, and then
slightly decreases at 3.0 m from the wall (Fig. 5.). Thermochemical reactions near SHE1 take place most
intensively in the middle of the furnace, where the highest temperature occurs with relatively low level of
excess air. Combustion intensity decreases towards the wall. Abrupt change of CO concentration at 1.0 m
from the wall (Fig. 4.) suggests that secondary air flows along the boiler wall and does not fully
participate on combustion in the center of the furnace and thus creates the oxidizing atmosphere near the
wall.
Temperature in the vicinity of port SHE2 is lower than that near SHE1; it rapidly increases from 0.1 m
to 0.3 m and then rises moderately (Fig. 2.). O2 concentration is relatively high at 0.1 m then drops at 0.3
m, considerable peak is noticeable at 2.0 m from the boiler wall; at 3.0 m O2 concentration decreases (Fig.
3.). Unlike the measurements near SHE1, no significant correlation with temperature is observed. CO
concentration reaches higher values at the positions of 0.1 m and 0.3 m with a subsequent decline at 1.0 m
and at 2.0 m followed by an increase at 3.0 m from the boiler wall (Fig. 4.). High concentration of CO
close to the wall result in increased risk of corrosion and material deterioration of boiler walls, this may
reduce lifetime of equipment significantly [1]. Concentration of NOX follows “a parabolic profile” and
reaches the maximum at 3.0 m from the boiler wall (Fig. 5.). As near port SHE1, the most
thermochemical reactions take place in the center of the furnace and combustion intensity decrease
towards the wall. It is evident that a significant reduction in temperature at 0.1 m is accompanied by a
high concentration of O2 and CO. This indicates poor combustion efficiency, where the O2 is not fully
utilized and the carbon from the fuel is not completely converted into CO2. With increased CO we can

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Jan Skvaril et al. / Energy Procedia 61 (2014) 643 – 647
also expect a higher content of pyrolysis products especially on particle surfaces.
10
9
SHE1 partial load
SHE2 partial load
Centerline
1200
8
7
6
5
4
3
2
1100
1000
900
SHE1 partial load
SHE2 partial load
Centerline
800
700
0
1000
2000
3000
4000
0
1000
2000
3000
4000
Distance from the wall (mm)
Distance from the wall (mm)
Fig. 2. Temperature profile – side walls of the furnace
Fig. 3. O2 concentration profile – side walls of the furnace
2000
SHE1 partial load
110
90
SHE2 partial load
1500
Centerline
1000
70
SHE1 partial load
SHE2 partial load
Centerline
500
0
50
30
0
1000
2000
3000
4000
0
1000
2000
3000
4000
Distance from the wall (mm)
Distance from the wall (mm)
Fig. 4. CO concentration profile – side walls of the furnace
Fig. 5. NOX concentration profile – side walls of the furnace
Results show that behavior in flue gas trains is related to the fluid dynamics and heat transfer, the
temperature is too low for further combustion reactions [6]. The results of measurements in ports SHE3,
SHE4 and SHE5 show that the flue gas temperature is distributed unevenly over the line (Fig. 6.). The
highest temperature is reached at the distance of 0.1 m (SHE3, SHE4) and 0.35 m (SHE5). The cause of
uneven temperature distribution over the line (approximately 50 °C along the line) is asymmetrical air
supply due to the AP design. Profile measured next to the port SHE3 shows an abrupt increase of the
concentration of O2 at the distance of 0.35 m from the wall (Fig. 7.). It can be largely explained by the
leakage in the air preheater 3 for primary air. This hypothesis is also supported by diluted concentration
of CO and NOX in comparison with concentrations near SHE4 and SHE5. Great variations of presented
values near the port SHE3 are caused by changes in operating conditions in relation to the shutdown of
the turbine.
6
275
250
225
200
175
150
SHE3 partial load
SHE4 partial load
SHE5 partial load
Centerline
5,5
5
SHE3 partial load
SHE4 partial load
SHE5 partial load
Centerline
4,5
4
3,5
0
500
1000
1500
2000
0
500
1000
1500
2000
Distance from the wall (mm)
Distance from the wall (mm)
Fig. 6. Temperature profile – air preheaters
Fig. 7. O2 concentration profile – air preheaters
1000
75
SHE3 partial load
800
600
400
200
0
SHE4 partial load
SHE5 partial load
Centerline
55
SHE3 partial load
SHE4 partial load
SHE5 partial load
Centerline
35
15
0
500
1000
1500
2000
0
500
1000
1500
2000
Distance from the wall (mm)
Distance from the wall (mm)
Fig. 8. CO concentration profile – air preheaters
Fig. 9. NOX concentration profile – air preheaters

Jan Skvaril et al. / Energy Procedia 61 (2014) 643 – 647
647
In summary, results from the performed experimental measurements vary greatly along the line from
the wall to the center, therefore the results from fixed sensors permanently installed can’t be considered as
entirely representative (e.g. temperature at SHE1 is significantly higher at 3.0 m compare to the values at
0.3 m from the wall). If we assume that our experimental results show correct absolute values, profiles
such as these can be correlated with measurements from fixed sensors permanently installed in the boiler
to improve control in comparison to only using the data from fixed sensors.
4. Conclusions
Experimental measurements (i.e. combustion flue gas temperature and concentration of O2, CO2, CO,
and NOX) were carried out at different points in the full-scale biomass-fired bubbling fluidized bed boiler.
Results of the constructed profiles from the furnace indicates the intermediate stage of thermochemical
reactions and show that the thermochemical reactions peak at the center of the furnace and combustion
intensity decrease towards the wall. Furthermore, increased levels of CO concentration close to the wall
has been found in the vicinity of port SHE2. This maybe indicating reducing atmosphere and thereby
increased corrosion risk. Results of the constructed profiles from flue gas trains shows that behavior there
relates mainly to fluid dynamics and heat transfer. The temperature is too low for further combustion
reactions. Asymmetrical cooling of flue gases in flue gas duct was found and a leakage has been
identified in the air preheater 3. Moreover, it is evident that there are great variations among measured
values of all measurands depending on a distance along the line from the wall to the center of the boiler.
This indicates that the measurements from fixed sensors are not giving value representing the average
conditions, however overall profiles can be correlated to online measurements from fixed sensors.
References
[1] Khan AA, de Jong W, Jansen PJ, Spliethoff H. Biomass combustion in fluidized bed boilers: Potential and Remedies. Fuel
Processing Technology 2009;90(1): 21-50.
[2] Koornneef J, Junginger M, Faaij A. Development of fluidized bed combustion – An overview of trends, performance and
cost. Science PiEaC 2007; 19-55.
[3] Costa M, Azevedo JLT. Experimental Characterization of an Industrial Pulverized Coal-Fired Furnace Under Deep Staging
Conditions. Comb Sci. and Tech 2007;179(9):1923-35.
[4] Kuang M, Li Z, Zhang Y, Chen X, Jia J, Zhu Q. Asymmetric combustion characteristics and NOx emissions of a down-fired
300 MWe utility boiler at different boiler loads. Energy 2012;37(1):580-90.
[5] Li Z, Liu G, Zhu Q, Chen Z, Ren F. Combustion and NOx emission characteristics of a retrofitted down-fired 660 MWe
utility boiler at different loads. Applied Energy 2011;88(7):2400-6.
[6] Baukal CE. Industrial combustion testing. Taylor & Francis; 2011.
Biography
Jan Skvaril is a PhD student at Mälardalen University, Sweden. He holds a master’s
degree in Mechanical Engineering and master’s degree in Economics and Management,
both from Brno University of Technology. Currently he is studying fluid dynamics with
emphasis on how temperatures are distributed in different types of furnaces.