\documentclass[10pt]{article}
\usepackage{fullpage}
\usepackage{setspace}
\usepackage{parskip}
\usepackage{titlesec}
\usepackage[section]{placeins}
\usepackage{xcolor}
\usepackage{breakcites}
\usepackage{lineno}
\usepackage{hyphenat}
\PassOptionsToPackage{hyphens}{url}
\usepackage[colorlinks = true,
linkcolor = blue,
urlcolor = blue,
citecolor = blue,
anchorcolor = blue]{hyperref}
\usepackage{etoolbox}
\makeatletter
\patchcmd\@combinedblfloats{\box\@outputbox}{\unvbox\@outputbox}{}{%
\errmessage{\noexpand\@combinedblfloats could not be patched}%
}%
\makeatother
\usepackage{natbib}
\renewenvironment{abstract}
{{\bfseries\noindent{\abstractname}\par\nobreak}\footnotesize}
{\bigskip}
\titlespacing{\section}{0pt}{*3}{*1}
\titlespacing{\subsection}{0pt}{*2}{*0.5}
\titlespacing{\subsubsection}{0pt}{*1.5}{0pt}
\usepackage{authblk}
\usepackage{graphicx}
\usepackage[space]{grffile}
\usepackage{latexsym}
\usepackage{textcomp}
\usepackage{longtable}
\usepackage{tabulary}
\usepackage{booktabs,array,multirow}
\usepackage{amsfonts,amsmath,amssymb}
\providecommand\citet{\cite}
\providecommand\citep{\cite}
\providecommand\citealt{\cite}
% You can conditionalize code for latexml or normal latex using this.
\newif\iflatexml\latexmlfalse
\providecommand{\tightlist}{\setlength{\itemsep}{0pt}\setlength{\parskip}{0pt}}%
\AtBeginDocument{\DeclareGraphicsExtensions{.pdf,.PDF,.eps,.EPS,.png,.PNG,.tif,.TIF,.jpg,.JPG,.jpeg,.JPEG}}
\usepackage[utf8]{inputenc}
\usepackage[ngerman,english]{babel}
\begin{document}
\title{Energy Template}
\author[1]{hjc9384}%
\affil[1]{Affiliation not available}%
\vspace{-1em}
\date{\today}
\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup
\selectlanguage{english}
\begin{abstract}
EFB is a byproduct of the palm oil production process and has recently
been used as a power plant fuel. In general, the water content of
un-dried EFB is very high, 60-70\%, for use as fuel for power plants.
The use of high water content EFB as a fuel for the plant reduces the
efficiency of the boiler and therefore the drying process is essential.
Drying increases the heating value and boiler efficiency, but it is a
trade-off relationship that consumes both cost and energy. It is
therefore important to dry properly. The purpose of this study is to
model an EFB 10 MW power plant that integrates economic feasibility
studies to find optimal drying conditions. A hot air dryer was used to
utilize steam from the power plant. The water content of the dried EFB
could be the same under different conditions. (Optimum drying condition
is more suitable for describing trade-off relationship than optimal
term.) Optimum drying condition was when steam reuse ratio was 25\%,
drying time was 22 minutes and water content was 9.79\% at optimum point
. The cost was reduced by 5.75\% compared to non drying.%
\end{abstract}%
\sloppy
~\textbf{1. Introduction}
Using biomass as fuel for power plants is well accepted by many
countries due to fossil fuel depletion and global warming. Indonesia,
one of them, plans to reduce the proportion of fossil fuel power plants
by 2025 and increase the proportion of biomass power plants.
\textbf{{[}References{]}} As a fuel for power plants, various types of
biomass such as wood, grain, and MSW(Municipal Solid Wastes) will be
used. EFB(Empty Fruit Bunch), a kind of biomass, which occurs in palm
oil production in Indonesia, seems to be the most suitable fuel. Because
Indonesia is the largest producer of palm oil, it produces a large
amount of EFB. Therefore, fuel prices are lower than other biomass, and
it is easy to supply and secure fuel continuously. However, not all the
EFB is suitable as fuel for power plants except when properly dried.
Moisture content of biomass like EFB is typically 60\%-70\%.
\textbf{{[}Reference{]}} this highly moisture content causes many
problem such as lowering combustion temperature and stability of
burning, higher CO and VOC emissions, difficulty of boiler operation. In
addition the boiler efficiency is reduced by increasing the heat loss of
the boiler such as flue gas loss, chemical unburned carbon loss, and
mechanical unburned carbon loss. \textbf{{[}Reference{]}}
Having multiple disadvantages using raw-biomass as fuel for power plant
can be solved by removing moisture from biomass. Drying method is
broadly divided into mechanical drying and thermal drying. Mechanical
drying methods can reduce moisture by up to 50 wt\% through shredding,
grinding, pressing and filtering. Thermal drying with direct or indirect
dryer is used to lower the moisture to less than 50 wt\%. Thermal drying
requires large energy and cost because moisture have high specific heat.
Recently drying processes have been integrated with power plants to
increase energy efficiency and reduce costs. To dry the feedstock
directly or indirectly, the waste heat and steam discarded from the
plant are used appropriately.
The lower the moisture content of the biomass, the higher heating value
and the boiler efficiency, but the energy and cost required for drying
also increase. So it is important that determination of optimum drying
level by trade-off among higher heating value of biomass, boiler
efficiency, energy input and dryer cost. In many literature, the optimum
moisture content of biomass is 10-20\%. \textbf{{[}References{]}}
Gebreegziabher et al. Studied the trade-off between drying level, dryer
cost, energy consumption and boiler efficiency, When the heating value
is limited to 15 MJ / kG (moisture content 17\%), the operating
conditions are mainly focused on drying temperature and particle size.
Few discussions on how to obtain the optimum moisture content in a true
sense. In this study, we have modeled a 10-MW EFB power plant
incorporating economic evaluation. Through the process model, optimal
drying conditions and optimal moisture content were determined by
considering operating conditions such as drying temperature, drying
time, and steam recirculation ratio. Even if the moisture content of the
dried EFB was the same, the efficiencies could be different if they were
reached under different drying conditions.
\textbf{2 Process flow diagram description}
A 10-MW small-scale biomass power plant under construction in Indonesia
was simulated. The overall process consists of a shredding process, a
drying process, a boiler, and a steam cycle (Figure 1). The amount of
dried EFB to generate 10-MW will be less than raw-EFB, since the dried
EFB increase the heating value and boiler efficiency. In other words,
simulation was designed to vary the amount of EFB to 10-MW power
generation. The process flow proceeds as follows.
1. EFB of 60\% moisture content is finely crushed by shredder to 5mm
size, Moisture content is lowered to 48\%. EFB at 48\% moisture content
enters the hot air rotary dryer.
2. The air entering the rotary dryer rises the temperature by heat
exchange with part of the steam coming out of the turbine. The heated
air comes into direct contact with the EFB and the inside of the dryer,
and moisture is evaporated through the material and heat exchange,
reducing the moisture content of the EFB to 20\%. Air and EFB are
supposed to ideally mix well. The steam(191 \selectlanguage{ngerman}° C, 12atm) used for drying
is fully condensed (188 ° C, 12atm) after preheating the air and the
condensed water enters the Boiled Feed Water Tank (BFWT) to preheat the
water.
3. The dried EFB enters the boiler and burns and the flue gas is
discharged at 200 degrees Celsius. The heat generated by the burning of
the EFB results in a steam of 433 degrees Celsius 60 atm.
4. The high-temperature and high-pressure steam is discharged at 0.107
atm after turning the turbine and produces 10 MW of electricity. A
portion of the steam coming out of the turbine extraction valve is used
for drying. The VLP discharged at 0.107atm recirculates the steam cycle
when it is fully condensed by the condenser.
2.1 properties of EFB
As the results of industrial and elemental analyzes are required to
model the biomass combustion reaction in Aspen Plus, industrial and
elemental analyzes were conducted on EFB. (Fig. 2) If we know the water
content of EFB and the high calorific value at that time, The high
calorific value according to the water content can be estimated by
AspenPlus (Figure 3) Estimation of the high calorific value according to
the remaining water content using AspenPlus is very similar to the
experimental value. Also, when the constant water content is exceeded,
black out area where EFB does not burn occurs. {[}Consider the
quotation{]}
~
3. \textbf{Process model}
The drying process consists of a direct dryer and an air heater. The
material and energy balance for the following units are given below.
3.1.1 Dryer model
The dryer can be classified into co-current and counter-current types
depending on the direction in which the solid and air flow. A
counter-current dryer can achieve a solid with a lower moisture content.
However, when the dryer operates at high temperatures, the driest solids
contact the hottest air, which can cause a fire if the solids are
flammable. So, co-current dryer was selected in this study because
biomass generally has a risk of fire. In the dryer, the solid and hot
air are in direct contact with each other, and the water of the solid
moves to the air due to the transfer phenomenon. The evaporation process
requires a large amount of energy because moisture has a high specific
heat. The material and energy balance were calculated assuming that the
dryer was in a steady state. Eq. (4) represents the material balance of
water with respect to the solid, and Eq. (5) represents the material
balance for air. Eq. (4) and Eq. (5) gives a material balance equation
for moisture. {[}Reference{]} Eqs. (6) to (9) show the energy balance.
By integrating the following Eqs (1) \textasciitilde{} (12), the
moisture content and temperature can be calculated as the air and solids
leave the dryer. If there is a difference from the conventional
equation, we have to arbitrarily specify the difference between the
outlet temperature of the solid and the air to calculate the material
and energy balance. However, if you know Q and the drying rate, the
equation below calculates the solid and air outlet temperatures, energy
and material balance by Aspen Plus. {[}Picture presentation{]}
~
3.2 Drying kinetics
By modeling the drying curve, the limiting moisture content, the
equilibrium moisture content and the heat and mass transfer coefficient
for EFB in the drying kinetics, Q and the drying rate can be known and
the energy consumption required for EFB drying can be predicted.
3.2.1 Drying curve
The drying process goes through several stages of drying. First, the
solid is heated by a heat source like hot air. Then the moisture on the
surface of the solid is evaporated, which is called the constant rate
drying period. The drying rate is also the same regardless of the
material if the drying conditions are the same because the moisture on
the surface of the material evaporates. When the moisture on the surface
of the material is removed, the moisture inside the solid is evaporated.
This period is called the falling rate drying period, and the point at
which the rate changes from the constant rate drying period to the
falling rate drying period is called the critical moisture content. When
drying continues, the moisture of the dry air equilibrates with the
moisture of the solid and no more moisture evaporates. This point is
called the equilibrium moisture content. Since the particle structure
inside the material affects the drying rate during the falling rate
drying period, the material has different drying curves in the falling
rate drying period. In order to more accurately predict the evaporative
behavior of EFB, we reflected a falling rate drying curve for the EFB on
Aspen Plus.
The critical moisture content depends not only on the material and shape
but also on velocity and temperature of the drying air. Since many
factors affect the critical moisture content, we used the mean value of
the critical moisture content shown in Table. The equilibrium moisture
content is influenced by relative humidity and temperature of
surrounding air. The equilibrium moisture content is usually close to
zero at high temperatures and relative humidity.
3.2.2 Convective heat and mass transfer coefficient
Heat and mass transfer coefficient is calculated by using Eqs. (14) and
(15).
to calculate Lewis, Schmidt and Reynolds number are obtained through
stream analysis of Aspen Plus. The diameter L in the original mass
transfer coefficient and Reynolds number equation was replaced by the
sauter mean diameter and the median particle diameter of EFB,
respectively. As shown in Eqs. (14) \textasciitilde{} (21), it can be
seen that the heat, mass transfer coefficient and evaporation are better
at higher drying temperature, velocity and smaller particle size. The
air velocity in the dryer was typically 0.5 m / s to 1.5 m / s, and the
mean value was used to calculate the Reynolds number.
3.6 Net power generation
Net power generation is to subtract work consumed by pump, blower from
work produced by turbine.
~
3.7 Integration of economic evaluation
Since the equipment cost is influenced by various factors such as usage,
capacity, and materials, it is necessary to roughly estimate the
equipment cost using equation. Total capital investment in the power
plant neglected labor costs, installation costs, transportation costs,
and pipeline installation costs except for some equipment costs. The
equipment cost only considers devices such as Dryer, Shredder, and
Furnace that are affected by the amount of EFB. Turbines are not
considered because they are the same at 10 MW power generation, and
detailed things like a pump blowers are ignored. The year in which the
equipment cost estimate for dryers and shredders was developed was 2003
and the furnace was developed in 2002. Eqs 28 \textasciitilde{} 30
reflect the cost index in 2017.
3.7.1 Dryer cost
The cost of the dryer is affected by the area. Knowing the mass flow
rate and drying time of the EFB, before estimating the dryer area, can
estimate the volume of the dryer. It is assumed that the volume occupied
by the EFB in the dryer is 20\% and the density of the EFB is 500 kg /
m3. If the EFB does not shrink during the drying process, the volume
occupied by the EFB in the dryer is Eq. 24.
~
When the dryer operates at a load of 20\%, the volume of the dryer is
calculated by Eq. 25. The L / D ratio of the dryer should be known to
determine the area of the dryer. Typical rotary dryers have an L / D
ratio of 4-10. The L / D ratio was selected as the average value of 7.
The diameter of the dryer can be expressed in equation of volume like
Eq. (26), and then the area is calculated by Eq. (27) {[}dryer cost{]}
3.7.2 Shredder cost
The shredder which can be crushed by 5mm size is hammer mill. The hammer
mill is related to the mass flow rate of the EFB and the equipment cost
is estimated Eq. 28
3.7.3 Furnace cost
Ignoring detailed cost such as piping, steam drum, soot blower, fan,
deaerator, and pump included in the boiler, only combustion was
considered. The furnace cost is varied by the heat transfer rate and can
be estimated by Eq. (30).
~
In this study, we only considered the fuel cost of EFB including the
transportation cost of EFB as the operating cost except maintenance,
labor, drug, and reprocessing costs necessary to operate the remaining
power plants. EFB fuel cost is \$ 14.08 / ton.
\textbf{4. Objective function}
Considering depreciation as 20 years of life for a typical power plant,
annual equipment cost is estimated as Eq. (31). Annual operating costs
are calculated by Eq. (32) that multiplies the EFB fuel cost by the
annual usage. The objective function was to minimize the annual total
cost (AC) that annual equipment costs plus annual operating costs.
Depreciation cost =
Annual working expenditure = 14.08\$/ton*(annual usage)
AC = Depreciation cost + Annual working expenditure
\textbf{5. Results}
A case study was conducted by changing the steam recirculation ratio and
drying time to find the optimum point. In all cases, EFB of 60\%
moisture content was applied by shredder to the condition that the
particle size was 5mm and moisture content was 48\%. When the EFB from
the shredder is not dried, the base case is set to produce 10 MW of
power.
5.1 Case study result except economic evaluation
Case study, without consideration of capital cost and operating cost,
only compared fuel consumption for the same 10MW power generation. The
results for the case study are shown in Table. 3. Figure 4 shows a
three-dimensional graph of fuel consumption for the same power
generation of 10 MW when the steam recirculation ratio is varied from 0
to 0.9 and the drying time is varied from 1 to 60 minutes. In the base
case, the hot air mass flow rate is zero. This is because the steam
recirculation ratio used for drying is zero. And to produce 10MW
electric power, EFB of 60\% moisture content was demanded 19,851kg / hr.
The amount of EFB from the shredder was 15,270 kg / hr. Even without the
energy used in the drying process, the amount of work required to
operate the steam cycle was 95 kW. In case 1, 2 and 3, the change of the
parameter value according to the steam recirculation ratio was compared
at the same drying time, and in case 4, 5 and 6, the parameter values
according to the drying time change were compared at the same ratio.
Table. 3, the relative efficiency is the difference between the fuel
consumption of the base and the other case. In cases 1, 2 and 3, the
efficiency increases gradually as the ratio increases. In case 2, the
efficiency becomes maximum at 8.15\%. After Case 2, the efficiency
decreases gradually and becomes -0.63\% lower than Case 3 without
drying. In other words, Case 3 uses too much steam for drying. Comparing
Case 4, 5, and 6, it can be seen that efficiency increases as the drying
time becomes longer. Because it can evaporate moisture from the EFB
without additional energy consumption until the relative humidity of the
air reaches 99\%. Therefore, in the three-dimensional graph of figure 4,
the point with the lowest fuel consumption was at the point where the
drying time was 60 minutes and the steam recirculation ratio was 0.26.
However, since the increase in drying time is much greater than the
amount of EFB fuel reduction after a certain point, the cost of the
dryer will increase as a result. The other change is that the longer the
drying time, the smaller the difference between the EFB and the outlet
temperature of the air.
5.2 Case study result including economic evaluation
We have conducted a case study to reflect the equipment and operation
costs and compared it with the drying process which power plant
currently being constructed in Indonesia. This drying process reduce
moisture content of EFB from 48\% to 20\%. figure. 5 is the objective
function graph for ratio and time. The point at which the objective
function represents the smallest value is the optimal point. In the
optimal point, the steam recirculation ratio, drying time, and moisture
content of the dried EFB were 0.25, 22 min, and 9.79\%, respectively.
Although there were a little differences according to the drying time,
most of the cases with steam recirculation ratio above 0.38 were not
feasible to dry. The yellow dot in figure 5 shows the points where the
moisture content of the dried EFB is 20\%. Among them, the red and green
dots represent the worst and the best conditions, respectively, which
can reach a moisture content of 20\%. The black dot represents the best
condition among the conditions that can reach the moisture content of
15, 25, 30, and 35\%, and the blue dot is the minimum value of objective
function. Table 4 shows the data for each point. The moisture content of
Case 1,6 was almost the same moisture content under different
conditions, but showed a large difference in the cost savings. It does
not matter how much moisture is dried, and it is important to know how
to dry it. Cases 1, 2, 3, 4, and 5 select the best drying conditions at
each moisture content. In terms of water efficiency, 9.79\% EFB is the
most cost-effective, but it does not show any significant difference in
cost savings even when using EFB, which is 10-20\%, as fuel for the
power plant. From 20\% or more, it can be seen that the difference in
the cost saving rate becomes so large that it can not be ignored.
\textbf{6. Conclusion}
The EFB 10MW power plant process model created by Aspen Plus was used to
optimize drying process operation. The model also considers the drying
kinetics and material balance and energy balance of the dryer depending
on the material properties of EFB. The model balanced the balance
between steam, drying time, capital investment and operating costs used
for drying. The optimal drying conditions were found to be 25, 22min,
and 9.79\%, respectively. Also the objective function graph shows that
the same moisture content can be obtained, it can be seen that the cost
savings can be different if the drying condition are different. This
indicates that how to dry it is more important than how much to dry it.
Another observation is that it was an optimal point when the water
content was 9.79\%, but there is no big difference between drying and 15
\textasciitilde{} 20\%. so
\selectlanguage{english}
\FloatBarrier
\end{document}