Palm Fruit
Palm Fruit
Palm Fruit
com
ScienceDirect
2021 8th International Conference on Power and Energy Systems Engineering (CPESE 2021),
10–12 September 2021, Fukuoka, Japan
Abstract
To achieve desired performance of steam boiler and low emission of hazardous air pollutants for generating electric power
from palm oil industry, combustion characteristics of grate firing should be optimal for difficult biomass fuels. This article aims
to present a design of step grate firing with high thermal efficiency for combustion of selected palm empty fruit bunch (EFB)
fuel with moisture content of 38.4% and ash content of 3.39%. The proposed grate firing is not only specially designed for
continuous 3-month operation but also particularly suited to construction of an industrial boiler rated at 80 tons/h, 45 bar, and
430 ◦ C. The reciprocating step grate as well as air distribution for optimized combustion characteristics is specified based on
computational fluid dynamics (CFD) analysis and practical experience. The required boiler can be successfully constructed and
smoothly operated by installing the designed step grate firing as part of a whole system at a biomass power plant in Thailand.
© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Peer-review under responsibility of the scientific committee of the 8th International Conference on Power and Energy Systems Engineering, CPESE,
2021.
Keywords: Biomass; Combustion characteristic; Grate firing; Palm empty fruit bunch; Reciprocating grate; Steam boiler; Step grate
1. Introduction
From a perspective of potential feedstock for steam and electricity generation, utilization of palm EFB as an
alternative fuel in direct combustion is one of interesting solutions to convert biomass wastes into clean energy
as well as to promote sustainability in palm oil production [1–3]. Nevertheless, there are possible risks affecting
efficiency of biomass steam boilers when utilizing the EFB as energy source to produce heat, steam, or electricity [4].
This is due to the fact that the high moisture content in the EFB greatly influences combustion process. In addition,
the potassium contain, one of alkali metals, in the EFB ash may cause slagging and fouling deposition problems
in the boiler [5,6]. Consequently, the poor combustion characteristics lead to uneven performance and unscheduled
∗ Corresponding author.
E-mail address: amphawan.ju@kmitl.ac.th (A. Julsereewong).
https://doi.org/10.1016/j.egyr.2021.11.142
2352-4847/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.
org/licenses/by/4.0/).
Peer-review under responsibility of the scientific committee of the 8th International Conference on Power and Energy Systems Engineering,
CPESE, 2021.
S. Srasri, N. Bhudsarakam, P. Limsutthiphong et al. Energy Reports 8 (2022) 275–282
Fig. 1. Calculation results of the CFD analysis. (a) bed volume; (b) solid phase bed combustion; (c) gas combustion coupling with radiation.
shutdown of the boiler. The pre-processing is therefore required to improve fuel properties of the EFB. However, the
EFB fuel can be either prepared by palm oil mills before selling to biomass power plants or handled by end-users in
according to their requirements. Alternatively, optimization of grate firing system can enhance biomass combustion
efficiency and reduce air pollutant emission [7].
A reciprocating step grate firing is one of commercialized combustion technologies suitable for burning various
types of biomass fuels with varying moisture contents and particle sizes such as wood pallets and refuse-derived
fuel [8,9]. In order to meet energy growing demands in sustainable manner, this article presents a design of step grate
firing suitably operating with palm EFB, one of byproducts from extracting palm oil, for constructing an industrial
boiler used to generate steam at high pressure and high temperature for electricity production at a biomass power
plant. The proposed grate firing design at the initial state utilized the CFD analysis as a tool for prediction of
achievement in combustion characteristics in an optimal way. After finishing the boiler construction, the on-site
testing and commissioning were then carried out to prove installation and operation of the designed step grate
firing.
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Fig. 2. Assembly drawings and combustion simulation results of the designed step grate firing (a) step grate and its stoker; (b) side view.
Fig. 3. Assembly drawing of the desired boiler for construction at the biomass power plant.
The inclined step grate is designed to convey the palm EFB fuel down from the feed point to the ash discharge
point by reciprocating movements of grate bars, which are arranged in alternate stationary and movable rows. It is
assumed that the fuel feeding distributes the fresh fuel evenly at constant rate over the entire grate bars. The fuel
combustion is controlled in four different zones, which are drying zone, devolatile zone, combustion zone, and char
burnout zone. The primary air is supplied to four combustion zones from under the step grate for biomass conversion
in the fuel bed, while the secondary air is supplied to the freeboard above the fuel bed for gas combustion [10].
Table 1 shows the properties of selected palm EFB, while Table 2 gives the parameters, descriptions, and units
used in the analysis. Table 3 summarizes the equations referred for modeling fixed bed combustion. The calculation
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S. Srasri, N. Bhudsarakam, P. Limsutthiphong et al. Energy Reports 8 (2022) 275–282
Fig. 4. Operator graphics on the DCS to operate the construed boiler (a) Page 1; (b) Page 2.
(FC%)
17.49 40.72 38.40 3.39 40.70 5.40 47.00 0.30 1.10 0.38 5.50 14.80
results of CFD combustion analysis are illustrated in Fig. 1. From the result of bed volume calculation in Fig. 1(a),
the estimated average temperature above the fuel bed is 760.96 ◦ C, and the temperatures after finished combustion
and radiation are 977.48 ◦ C and 828.23 ◦ C, respectively. The step grate should have the length of 13 m and speed
of 5 mm/s. From the result of solid phase bed combustion in Fig. 1(b), the average temperatures during drying,
devolatile and combustion, and char burnout zones are equal to 398.75 ◦ C, 730.2 ◦ C, and 1,063.8 ◦ C, respectively.
From the result for gas combustion coupling with radiation in Fig. 1(c), the high temperatures can be occurred over
the freeboard, it is implied that the gases are completely burned.
Based on the good CFD analysis results as well as relevant experiences, the assembly drawings and combustion
simulation results of the designed step grate firing are shown in the in Figs. 2(a) and 2(b). The reciprocating grate
stoker and ash discharger are driven by hydraulic cylinders. The palm EFB fuel is transported along the 18-step
grate through the furnace by the reciprocating action of two groups of the grate bars, which are fixed bars and
moving bars (see Fig. 2(a)). In addition, the grate bars are designed in three different specifications for installations
in different locations including drying zone, pyrolysis and char burnout zones, and left and right grate sides. Fig. 2(b)
illustrates the side view of the designed step grate firing. It is evident that temperature distribution of the designed
grate firing is optimal, because the high temperature regions are located over the freeboard. Fig. 3 displays the
assembly drawing for steam boiler construction at the biomass power plant by utilizing the designed grate firing as
a subsystem. After testing and commissioning, the constructed boiler can generate 80 tons of generated steam per
hour at maximum working pressure of 45 bar, feed water temperature of 120 ◦ C, and final steam temperature of
430 ◦ C. Figs. 4(a) and 4(b) show the operator graphics running on a distributed control system (DCS) at a central
control room of the biomass power plant to monitor and control the specified operating conditions of both the steam
boiler and the step grate firing. The significant information presented in representative illustration of process flow is
available for supporting the plant operators’ tasks. In addition, the designed step firing with high burning efficiency
can operate continuously for about 3 months without an unplanned shutdown.
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4. Conclusions
A design of reciprocating step grate firing for optimal combustion of difficult biomass fuel like palm EFB has
been presented in this article. In order to ensure the successful of employing this biomass fuel, the CFD analysis
for estimating optimal combustion characteristics has been described. Analysis results obtained from the helpful
CFD analysis can employ in conceptual design of the step grate firing at the initial stage. The continuous operation
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ρl X j
w j,l = ρ j (1−εl ) ; (8) [11]
Basic Combustion
L H V = H H V − 2, 442 × [8.9367X H + X MC ] (9) [12]
V̇g = 1.867X C + 11.19X H + 0.8X N + V̇N2 + 0.7X S + 0.3115X Cl + 1.244X MC (14) [12]
E 0 = 1.089 × 10−10 Tsr 2 + 1.881 × 10−7 Tsr + 2.1 × 10−5 (17) [11]
][ )]
0.1λg (Tg −Tsr ) 2+1.1Pr 1/3 Re0.6 +εsr σb d f Tenv 4 −Tsr 4
( ) (
6 (1 − ε A )
[
R2.2 = d f Hevap ; Tsr > 100 ◦ C (18) [11]
df
µg
Sc = (19) [11]
ρg Dig
ρg vg d p
Re = (20) [11]
µg
C p,g µg
Pr = (21) [11]
λg
Devolatile Combustion Step
( )
14, 192.1
R3 = 8.08 × 108 (v∞ − v) ex p − (22) [11]
Tsr
( )
d[C H4 ] −24, 373
= −4.4912 × 1013 ex p [C H4 ]−0.3 [O2 ]1.3 (23) [11]
dt Tg
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S. Srasri, N. Bhudsarakam, P. Limsutthiphong et al. Energy Reports 8 (2022) 275–282
Table 3 (continued).
Equation No. Ref.
( ) ( )
d [C O] −20, 143 −20, 143
= −1.1151 × 1019 exp [C O]1.0 [O2 ]0.25 [H2 O]0.5 + 1.4 × 1013 ex p [C O2 ] (24) [11]
dt Tg Tg
( )
d [H2 ] −41, 000
= −6.689 × 1016 ex p [H2 ]0.85 [O2 ]1.42 (25) [11]
dt 1.9858 × Tg
{ }
1.6 × 10−5 × (Tsr /393)1.5 (1 − ε B )2/3 vg (1 − ε B )1/3 {
C uel C O2
}
Rmi x = Cmi x ρgas 150 + 1.75 × min S ff uel , SO (26) [11]
d f εB
2 d f εB 2
( )n
Tg
α= × ε; n = 0.65 f orC O2 and0.45 f or H2 O (33) [14]
Tw
∑3 [
bi (log P)i + di (log pL)i
]
a + i=1
log CC O2 = ∑6 [ (34) [14]
1 + i=4 bi (log P)i−3 + di (log pL)i−3
]
∑3 [ (( ) )i ]
a + i=1 bi p H2 O + P /2 + di (log pL)i
C H2 O = ∑6 [ (( ) )i−3 ] (35) [14]
1 + i=4 bi p H2 O + P /2 + di (log pL)i−3
∑3 [ ( ))i ) ))i ]
bi pC O2 / pC O2 + p H2 O + di ln log pC O2 + p H2 O L
( ( ( (
a + i=1
CSO = ∑6 [ ( ))i−3 ) ))i−3 ] (36) [14]
bi pC O2 / pC O2 + p H2 O
( ( ( (
1 + i=4 + di ln log pC O2 + p H2 O L
ε = εC O2 CC O2 + ε H2 O C H2 O (1 − C S O ) + ε S O2
( )
(37) [14]
Q = σ A εT 4 − αTwall 4
( )
(38) [14]
−16σ n 2 T 3 ∆T
qr = (39) [15]
3 (α + σs ) − Z σs
results of the boiler to generate electric power verify that the designed grate firing can function effectively as per
the specified requirements. A numerical investigation for primary and secondary air distributions affecting on the
palm EFB combustion to enhance the boiler efficiency is the future study.
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S. Srasri, N. Bhudsarakam, P. Limsutthiphong et al. Energy Reports 8 (2022) 275–282
Table 3 (continued).
Equation No. Ref.
−σ Twall 4 − Tg 4
( )
qr,w = (40) [15]
ψ
k g (α + σs )
Nwall = (42) [15]
4σ Twall 4
r , s⃗)
d I (⃗ σT4 σs 4π
∫
+ (α + σs ) I (⃗
r , s⃗) = αn 2 I r⃗, s⃗′ Φ s⃗ · s⃗′ dΩ ′
( ) ( )
+ (44) [15]
ds π 4π 0
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