A Review of Full Biomass-Fired Conversion in a LargeScale Circulating Fluidized Bed Boiler using Palm Kernel
Shells and Woodchips
Danny M. Urian1* and Aldrin D. Calderon1
1 School of Mechanical, Manufacturing, and Energy Engineering, Mapúa University
658 Muralla Street, Intramuros, Manila 1002, Philippines
Danny M. Urian* (dmurian@mymail.mapua.edu.ph)
Aldrin D. Calderon (adcalderon@mapua.edu.ph)
Abstract. Many studies have been conducted on the co-firing of coal and biomass for carbon reduction. However, only some have examined using 100% biomass as the primary fuel in large-scale Circulating Fluidized Bed (CFB) boilers. This review examines the feasibility and challenges of transitioning largescale CFB boilers from coal-biomass co-firing to utilizing 100% palm kernel
shells (PKS) and woodchips. CFB technology offers inherent advantages for biomass combustion, including fuel flexibility and lower operating temperatures,
but several technical barriers must be overcome to ensure efficient and reliable
operation. The review analyzes key challenges, including managing high ash
content, mitigating emissions, agglomeration, fouling, slagging, corrosion, optimizing combustion efficiency, and ensuring long-term operational reliability. The review examines potential solutions, such as fuel pre-treatment techniques, biomass co-firing, combustion optimization control, advanced ash management systems, heat recovery, and the application of computational modeling
tools like Computational Particle Fluid Dynamics (CPFD) to simulate and optimize boiler design and operation. Though challenges remain, transitioning largescale CFB boilers to 100% PKS and woodchip firing presents a significant opportunity to advance sustainable energy production. Further research and development of innovative solutions tailored to the unique properties of PKS and
woodchips are crucial to unlocking the full potential of these biomass resources.
Keywords: PKS, Woodchips, CFB Boiler, Boiler Efficiency, and Emissions.
1
Introduction
The world faces a pressing energy challenge. Nearly a billion new energy consumers
are expected to emerge more or less than a decade from now [1]; this and rising living
standards will significantly increase global energy consumption. The world's growing
energy demands require sustainable solutions that provide reliable power while minimizing environmental impact. Simply producing more energy is not enough; we must
2
transition away from environmentally damaging fossil fuels and towards renewable energy sources. Biomass offers a promising pathway for this transition. As a readily
available resource, biomass can help fill energy gaps, offering a carbon-neutral alternative to fossil fuels. The carbon released during biomass combustion is balanced by the
carbon absorbed during its growth, making it a more sustainable energy source [2].
Among the various technologies for biomass power generation, circulating fluidized
bed (CFB) boilers have emerged as a promising option. CFB boilers offer several advantages for biomass combustion, including their ability to efficiently burn various biomass fuels, including agricultural residues, woody biomass, and energy crops
[3]. This adaptability of CFB boilers reassures us that they can effectively utilize the
diverse range of biomass resources available. High combustion efficiency, turbulent
mixing, and long residence times in the fluidized bed promote complete combustion,
leading to high energy conversion efficiencies [4]. CFB boilers are well-suited for
emissions control due to their lower combustion temperatures and the ability to integrate in-situ emissions control technologies [5].
The conversion of existing large-scale CFB boilers from fossil fuels to 100% biomass firing is a growing interest, as it can significantly reduce GHGs and contribute to
a sustainable green energy mix [6]. Palm kernel shells and woodchips are abundant
and readily available biomass fuels with significant potential for large-scale power generation in CFB boilers. PKS, a by-product of palm oil production, is common in many
tropical regions, like Malaysia, Indonesia, and West Africa. Woodchips, derived from
forestry residues, offer a versatile and sustainable fuel source [7] typically available in
South-East Asian Regions.
This review paper investigates the transition of large-scale CFB boilers from coalbiomass co-firing to 100% biomass firing, specifically focusing on palm kernel shells
and woodchips as primary fuels. Analyze this conversion's technical challenges and
opportunities, assessing its impact on combustion and boiler efficiency, emissions, and
overall feasibility. Key aspects explored include fuel handling and pre-treatment, combustion optimization strategies, co-firing potential, ash management techniques, and
applying computational modeling tools to enhance performance. By providing an overview of these critical factors, this review aims to give important information surrounding the full biomass conversion in CFB boilers, ultimately promoting sustainable, efficient, and cleaner energy.
2
Technical Feasibility
2.1
Fuel Feeding Systems for PKS and Woodchips
Transitioning large-scale CFB boilers from fossil fuels to 100% PKS or woodchip firing presents unique challenges and opportunities in fuel handling and feeding systems
[6]. Unlike coal, whose bulk density is 700 ~ 1100 kg/m3, as shown in Table 1 [8],
PKS and woodchips have distinct physical characteristics that necessitate specific design considerations. PKS, with its typical bulk density of 740 kg/m3 (wet) and 650
kg/m3 (dry) [9] and size distribution, can be efficiently transported and fed into the
boiler using conventional mechanical conveying systems, such as screw and belt
3
feeders [10]. This contrasts with lighter and less dense biomass fuels, such as woodchips, which may require alternative feeding mechanisms [11].
Woodchips have relatively low bulk densities, typically 200 ~ 400 kg/m3 [12]. This
low density can lead to consistent and controlled fuel feeding issues, which is key for
maintaining stable boiler operation. Additionally, the moisture content of these biomass fuels can vary significantly, from around 10% to 50% on a wet basis, depending
on the source and pre-treatment [13]. Advanced fuel feeding systems, such as pneumatic conveying or rotary valves, may be employed to address these challenges. Pneumatic conveying systems use air to transport the fuel, ensuring reliable and uniform fuel
delivery to the boiler. Rotary valves, on the other hand, are used to control the flow of
fuel into the boiler [14].
The varying characteristics of palm kernel shells and woodchips, even within the
same batch, pose significant difficulties for their utilization in large-scale CFB boilers.
These biomass fuels may display substantial differences in moisture content, particle
size distribution, and bulk density [15]. As a result, the boiler output may experience
undesirable fluctuations, making it difficult to maintain stable and efficient operation.
Addressing these fuel property variations through effective pre-treatment and fuel handling strategies is important to utilize the full potential of PKS and woodchips in largescale CFB boiler conversions. Effective pre-treatment methods include drying the biomass to reduce moisture content and screening to ensure a consistent particle size distribution. In terms of fuel handling, as previously mentioned, using specialized equipment designed for biomass fuels can help prevent issues like bridging and reduced flowability [14][16][17].
PKS and woodchips typically have higher moisture content than coal, which can lead
to operational issues like clogging and corrosion in feeding systems [18]. The irregular
shapes and often large sizes of PKS and woodchips, as shown in Table 1, can pose
challenges for conveying and feeding. They are prone to interlocking and jamming,
particularly in conventional feeding systems designed for granular fuels [19].
Table 1. Bulk density and particle size comparison of coal, PKS, and woodchips.
Units
Coal
PKS
Woodchips
kg/m³
700 ∼ 1100
650 ∼ 740
200 ∼ 400
mm
1∼5
3 ∼ 15
5 ∼ 50
2.2
Boiler Efficiency with PKS and Woodchips
Switching from coal to PKS or woodchips in a CFB boiler can significantly impact the
overall boiler efficiency due to the fuel properties and combustion behavior differences
between biomass fuels and conventional coal [20][21]. As shown in Table 2, PKS and
woodchips typically have a lower heating value than coal, meaning they contain less
4
energy per unit mass [21][22]. Also, biomass fuels' higher moisture content can reduce
their effective heating value. It can negatively impact combustion efficiency as more
energy evaporates the moisture, leaving less for steam generation. Understanding these
fuel property differences and their impact on combustion behavior is essential for optimizing the boiler operation and maximizing the energy output when transitioning to
full biomass firing.
Despite the high combustion efficiency of CFB boilers, variations in the physical
characteristics of PKS and woodchips, such as particle size and distribution, can influence their burning behavior and effectiveness within the boiler [23]. Incomplete or
inefficient combustion of these biomass fuels can result in energy losses and elevated
emissions.
CFB boilers can handle increased ash levels, and the composition of the ash can
influence slagging and fouling propensities [24]; this could impair heat transfer efficiency and require frequent boiler cleaning and maintenance.
The performance of the fuel delivery system, encompassing storage, handling, and
transportation, can significantly impact the overall boiler efficiency. Energy losses may
arise from friction, material deterioration, and operational inefficiencies.
Table 2. Typical ultimate analysis for each fuel type & mixed fuel properties (AR basis) [21].
Item
3 Types of Fuel
Units
Coal
PKS
WC
Mixed
Fuel
%-wt.
58.74
8.25
33.01
Total Moisture
%
30.94
17.94
31.06
29.907
HHV
MJ/kg
17.93
16.75
17.64
17.736
LHV
MJ/kg
16.40
14.99
15.63
16.027
C
%
48.30
42.40
32.40
42.565
H
%
5.02
5.21
5.96
5.346
N
%
0.98
0.33
1.33
1.042
O
%
10.87
31.62
25.20
17.312
Total S
%
0.0900
0.0300
0.040
0.069
Total Cl
%
0.0040
0.0961
0.010
0.014
Ash
%
3.80
2.37
4.00
3.75
Total Moisture
%
30.94
17.94
31.06
29.91
(Total)
%
100.0
100.0
100.0
100.00
Ultimate Analysis
Co-firing ratio
(wt. basis)
5
3
Environmental Impact
3.1
Emissions in PKS and Woodchip-Fired CFB Boilers
Biomass is considered a carbon-neutral fuel source, but its combustion produces emissions that must be effectively controlled to meet environmental regulations and minimize air pollution. CFB boilers offer several advantages in this regard, as they inherently operate at lower combustion temperatures, typically below 850°C, which can suppress the formation of nitrogen oxides [25].
Burning palm kernel shells in an oxy-fuel environment reduced NOx emissions by
16.5% and significantly reduced unburnt carbon in fly ash. Due to biomass's lower
nitrogen and sulfur content, PKS combustion produced lower NOx and SO2 emissions
than coal combustion. This study highlighted the benefits of small-scale oxy-combustion of PKS. However, further research is needed to investigate the impact of flue gas
recycling, heat transfer, ash deposition, and scalability on larger systems [26].
Burning wood chips in a CFB combustor can create oxygen-depleted regions, leading to unburned gases like CO and hydrocarbons, even reaching the cyclone. These
gases typically burn off due to high temperatures, but reducing excess air to decrease
NOx emissions can significantly increase CO [27]. Compared to coal, PKS and woodchip combustion generally result in lower SO2 and Hg emissions [28]. However, they
can produce higher particulate matter, NOx, and CO levels if combustion is not properly
managed [29].
4
Operational Impact
4.1
Common Operational Issues on Biomass Fuels
Alkali metals, abundant in biomass fuels, contribute significantly to this problem by
forming low-melting-point silicates that create a sticky surface, promoting further deposition. Complex chemical interactions involving chlorine, sulfur, silicon, and alkali
metals can cause severe corrosion, especially in high-alkali biomass fuel systems [25].
Deposits like ash can accumulate on heat transfer surfaces (fouling) or surfaces exposed
to radiant heat (slagging). These deposits, primarily composed of inorganic materials
from the fuel, hinder heat transfer and can lead to corrosion, ultimately reducing the
equipment's lifespan [30].
A study on the co-combustion of woodchips and with rapeseed cake pellets in a CFB
boiler showed that the high phosphorus content in rapeseed cake significantly contributed to agglomeration and fouling ions [31]; this is important to consider as the industry
explores new fuel combinations, even though the co-firing of PKS and woodchips still
requires further investigation.
In fluidized bed combustion, temperatures between 800 ~ 950°C allow for both the
breakdown of Polycyclic aromatic hydrocarbons (PAHs) by oxidants and the formation
of PAHs from the breakdown of larger molecules during pyrolysis [32]. Above 850°C,
PAH formation increases due to endothermic reactions, especially in areas like the
dense fluidized zone and freeboard region, with metals like iron and copper potentially
6
accelerating this process [25]. The buildup of PAHs and their by-products inside the
boiler system can lead to fouling, which reduces the boiler's heat transfer efficiency and
increases the risk of corrosion. This fouling and corrosion, in turn, requires more frequent cleaning and maintenance, leading to higher operational costs and more downtime for the CFB boiler [25].
Introducing limestone [31] to the boiler during co-combustion can prevent the formation of potassium silicates, which contribute to fouling. Limestone also enhances
phosphorus retention in the bed by forming high-melting calcium phosphates, reducing
slagging potential. Torrefaction of PKS before combustion can significantly reduce
fouling [33]; this process involves heating the biomass to a specific temperature range
of 200 ~ 300°C in a controlled environment, resulting in a more energy-dense and less
problematic fuel. Implementing an optimized soot-blowing system [34] can help remove deposits from heat transfer surfaces; this system should be tailored to the specific
boiler design and fuel characteristics to ensure efficient cleaning without compromising
boiler performance. It is important to understand that the best approach typically includes a blend of these strategies customized to the particular boiler design, fuel properties, and operational objectives.
5
Potential Optimization Methods and Research Opportunities
5.1
Optimization Methods
Fig. 1. Conceptual map of full biomass firing optimization methods.
7
The transition to full biomass-firing in large-scale CFB boilers presents several opportunities for optimization, as shown in Fig. 1; this flowchart outlines combustion control
optimization through thermal and fuel-air ratio, fuel feed ratio, and fuel pre-treatment.
It also includes heat recovery, ash management, and the possibility of co-firing PKS
and woodchips (WC) for more efficient combustion.
Fuel Pre-Treatment. Drying and size reduction can significantly improve combustion
efficiency and reduce energy losses associated with moisture evaporation. Moisture
content in biomass fuels directly impacts combustion efficiency. PKS and woodchips
often contain significant moisture, which requires energy to evaporate during combustion, reducing the net energy output [13].
Drying removes excess moisture from the fuel, directly correlating to lower emissions and improved combustion efficiency; this is critical as high moisture content can
lead to increased formation of sulfuric acid, a major contributor to fouling. Drier fuels
require less energy to ignite and combust, leading to lower operating costs and improved overall boiler efficiency [35]. Dry fuels are easier to grind, allowing for better
size reduction and more homogenous fuel particles. This homogeneity promotes more
efficient combustion and reduces the likelihood of unburnt particles contributing to
slagging and fouling [13].
The irregular shapes and frequently large sizes of PKS and woodchips can pose challenges for conveying and feeding [36]. As mentioned, they are prone to interlocking
and jamming, particularly in conventional feeding systems designed for granular fuels.
Size reduction through chipping, grinding, or milling can improve flowability, enhance
mixing with combustion air, and promote complete combustion [19]; this leads to
higher heat release rates and reduces the potential for unburnt particles to contribute to
deposit formation. Consistent particle size distribution ensures uniform air-fuel mixing, leading to more efficient combustion and reduced formation of ash particles that
can contribute to slagging. Smaller, uniformly sized particles are easier to transport
pneumatically, improving fuel feeding consistency and reducing the risk of blockages
or uneven distribution in the boiler.
CFB Boiler Co-firing. Investigating the efficiency of CFB boilers co-firing biomass,
particularly PKS, and woodchips, is critical. By simulating different co-firing scenarios, researchers can gain insights into the impact of biomass blend ratios on overall
boiler efficiency, helping to optimize fuel blending strategies for both performance and
emissions reduction [21].
Combustion Control Optimization. Precise control over combustion parameters like
thermal, air-to-fuel ratio, bed temperature, and fuel distribution is vital for maximizing
combustion efficiency and minimizing emissions [37]; this may involve using advanced combustion monitoring and control systems and careful tuning of the CFB boiler's operational parameters. Maintaining optimal conditions can minimize the release
of alkali metals and phosphorus, reduce their interaction with other elements, and mitigate deposit formation, reducing the risk of potential operational issues.
8
Ash Management. Effective ash handling and disposal systems are essential for maintaining boiler efficiency and minimizing downtime for cleaning. Bed ash recirculation
and efficient ash removal systems can help mitigate ash-related operational issues [21].
Optimizing bed pressure drop can significantly impact both combustion efficiency and
ash characteristics. A study found an optimal bed pressure drop of around 5.7 kPa,
which minimized carbon content in the fly ash while improving overall combustion
efficiency. This finding underscores the interconnectedness of boiler operation parameters and the importance of a holistic approach to ash management [38].
Heat Recovery. Implementing effective heat recovery systems to capture waste heat
from flue gases can significantly improve boiler efficiency [39]. Effective operation of
soot-blowing in the superheaters and air-preheaters plays a vital role in heat recovery
from flue gas. Important parameter indicators must be monitored, and the differential
temperature and flue gas pressure between the upstream and downstream must be controlled.
5.2
Research Opportunities
Despite the growing interest in biomass utilization for power generation, there needs to
be more publicly available data on the efficiency of large-scale CFB boilers firing 100%
PKS or woodchips. This gap in knowledge stems from several factors: Full conversion
of large-scale CFB boilers from coal to 100% PKS or woodchips is still relatively uncommon. Many existing facilities operate in co-firing mode, blending biomass with
coal, limiting the availability of real-world data on dedicated biomass-fired systems. Factors influencing boiler efficiency include fuel characteristics, design, operating conditions, and maintenance practices. This site-specific variability makes it difficult to generalize findings from one installation to another. Agglomeration also poses
a significant challenge when operating CFB combustors and gasifiers. The fluidization
process can influence agglomerates' formation, disrupting heat distribution within the
system. In severe cases, agglomeration can lead to de-fluidization, forcing an unplanned boiler shutdown [40].
Addressing these knowledge gaps is vital; further research is needed for rigorous
testing and monitoring of dedicated PKS/woodchip-fired CFB boilers, essential for
quantifying their efficiency under various operating conditions. Advanced modeling
tools, e.g., Computational Particle Fluid Dynamics (CPFD), can help simulate the 3phase flow involving bed material, solid fuels, and gas flow. Study the fluidization
behavior in the furnace bed to identify the uneven heat distribution and predict efficiency under various scenarios. By simulating key parameters like particle size distribution, critical fluidization velocity, and pressure drop [41], CPFD can provide valuable insights into agglomeration processes, including the formation and distribution of
pollutants during combustion. CPFD has proven valuable in optimizing CFB systems,
but it is important to acknowledge the underlying limitations of CPFD modeling. Accurately representing complex flow regimes remains a challenge, which can impact the
applicability of CPFD in certain scenarios. Despite these limitations, CPFD has
9
succeeded in areas like optimizing cyclone separator arrangement and analyzing the
dynamic behavior of CFB boilers during thermal startup [42].
6
Conclusion
The flexibility of CFB technology makes the transition to 100% biomass fuels technically possible. However, widespread adoption will hinge on overcoming several key
challenges. Specifically, advancements in fuel pre-treatment are crucial to optimize
combustion efficiency, including drying and size reduction. They can also play a vital
role in improving the flowability, air mixing, and overall combustion performance of
PKS and woodchips in CFB boilers. Addressing the challenges posed by these biomass
feedstocks' irregular shapes and large sizes will ensure reliable fuel handling and feeding to minimize fouling, slagging, and corrosion and ensure long-term operational reliability. Effective ash handling and disposal systems are essential for maintaining boiler
efficiency and minimizing downtime for cleaning. Strategies like bed ash recirculation
and efficient ash removal systems can help mitigate ash-related operational issues. Furthermore, optimizing bed pressure drop can significantly impact both combustion efficiency and ash characteristics, underscoring the interconnectedness of boiler operation
parameters and the importance of a holistic approach to ash management.
Further research in these areas is paramount, and we must develop innovative solutions tailored to the unique properties of PKS and wood chips. Advanced modeling
tools, such as CPFD, offer promising avenues for predicting agglomeration, simulating,
and optimizing the complex fluidization behavior and combustion dynamics within
CFB boilers firing at 100% biomass fuels.
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