Wood drying and some issues of efficient energy use in drying
The article was published in: INTERNATIONAL SCIENCE AND
GREEN ENERGY DEVELOPMENT CONFERENCE, Hanoi, 2012. ISBN: 978-604-913-094-6,
Natural Science and Technology Publishing House, PP.374 - 379
Nguyen Khac Minh City
Summary:
Based on a survey to assess some drying systems currently
popular authors came up with some solutions to overcome these weaknesses in
order to improve efficiency effective use of energy in wood drying. Which has proposed
several models of wood drying effect is Using cogeneration on the basis of wood
drying steam capacity is relatively large and uses heat pumps to drying at a
small scale in schools drying temperature of 600C ≤ t.
Abstract:
Based on a survey of current Popular wood drying system, the
authors came up with some solutions to Overcome the Disadvantages in order to
Improve Energy Efficiency in dry wood. The article has proposed a model of wood
dry mà effective use of the thermoelectric generator simultaneously for steam
drying facilities and heat with Relatively large capacity pump used for
small-scale drying at the drying temperature of t ≤ 600C.
I. BACKGROUND
At present wood and wood products has become one of the
major export items of Vietnam. Wood consumption also contributes to promoting
reforestation, green wasteland, barren hills and contributing to hunger
eradication and poverty reduction for people in remote areas as well as
environmental improvement. To improve the physical properties and quality of
the wood kiln drying is necessary and drying stage is energy-consuming and
greatly affect the production schedule in the wood processing enterprises
today.
Through surveys in a number of establishments producing
furniture we found drying systems in these facilities generally still many
problems to be studied and overcome in order to improve efficiency in energy
use also as reduced investment rate for simultaneous drying system can keep the
quality of drying wood.
II. STATUS DRYING WOOD IN VIETNAM
1. The drying system popular in Vietnam
Due to many reasons such as product requirements,
characteristics of the material drying systems existing in our country mainly
wood drying ÷ 15 bar thickness 60 mm, with a thickness of large panels with
long drying time is beyond the scope of this article, the thin sheets with a
thickness of 1 ÷ 3 mm to produce plywood usually natural exposure.
Regarding energy drying system most popular system of drying steam in addition to system use combustion of biomass for heat exchanger air - smoke, some system of modern drying as drying with heat pump, by solar energy or vacuum drying is initially being studied. However the application is limited mainly due to high cost or high energy costs. On drying equipment is still the most popular use of the drying chamber and drying tunnel but have lower energy costs, but due to many reasons such as major investments, the stability of the product is not high so rarely used. Chart drying system uses steam chamber is shown in Figure 1.
Regarding energy drying system most popular system of drying steam in addition to system use combustion of biomass for heat exchanger air - smoke, some system of modern drying as drying with heat pump, by solar energy or vacuum drying is initially being studied. However the application is limited mainly due to high cost or high energy costs. On drying equipment is still the most popular use of the drying chamber and drying tunnel but have lower energy costs, but due to many reasons such as major investments, the stability of the product is not high so rarely used. Chart drying system uses steam chamber is shown in Figure 1.
Figure 1. Diagram of wood drying kilns steam
- Wood stacked slot cross wind;
- Calorifer steam;
- Moist discharge outlet and air supply;
- Axial Fans can reverse;
- Temperature sensor, humidity wood
Structural drying chamber: drying chamber can often drying
20 ÷ 100 m3 each batch is divided into two parts of blocks of wood drying
between the two heads and above the air channel and location for installation
of equipment such as axial fans, staging Calorifier, the temperature sensor
dry, wet, wood moisture, moisture balance of wood ... wooden blocks are stacked
in layers, in between the bars Statistics thickness 20 ÷ 30 mm form slots for
passing drying agent.
Parameters of agent drying (hot air): Due to drying, if
drying conditions are not guaranteed wood easily deformed or hardened surface,
so the evaporation from the surface of the wood must be kept at a moderate pace
. This is done by maintaining the relative humidity of the drying agent is
relatively high in the early stages of the drying process and the humidity of
the wood is in the range 23 ÷ 30%. The temperature of the drying agent is also
controlled (t ≤ 65 ° C),
usually at the end of the drying process the humidity was raised to balance the
moisture of the wooden stick. The speed of drying agent through the wood block
is usually maintained at about 1 ÷ 2 m / s.
The control of the parameters of the agent drying is
performed as follows: At each stage of the drying process parameters
temperature thermometer dry, gradient wood moisture (the ratio between the
average humidity of the wood and humidity should by wood with agents drying)
are predetermined according to the drying process of each type of wood when
there is a difference in temperature opens solenoid valves grade steam for the
calorifier, the moisture balance will valve exhaust moisture .
The drying system commonly used wood fired boiler with wood
shavings or head pieces discharged during processing.
2. Some of the irrational in terms of energy in existing drying system
a. In terms of energy use: In the wood drying process energy
consumption includes consumption of heat and electricity below we consider each
issue.
+ About Heat: First, according to thermodynamics the use of
biomass for heating at a temperature t ≤
65 °
C is a huge waste (losses Exergi in boiler and drying system high) to heat in
this temperature some effective solutions such as using a more refined thermal
energy of solar radiation ... however due to biomass use is a byproduct of the
manufacturing process and is inexpensive to make this problem less get proper
care. A solution to improve the thermal efficiency cogeneration will be
discussed in the following section.
Capacity of Calorifier: through surveys at a number of
establishments choosing heat supply capacity of Calorifier inaccurate and
larger requests (activity coefficient over time about 0.25 ÷ 0.4) this as
parameters of the drying process is constantly changing with the biggest margin
was at first not arrange sensors and thus drying process is built from
empirical premises it can be one of the reasons for prolonging drying time
increases in energy costs.
+ In terms of power consumption: Power supply drying system
including the fan for drying, for the fans and the fan hood, boiler feed water
pump. Wherein the drying fan 90 ÷ 95%. Through the survey, the fact there are
some issues related to the kinetic energy consumed in the fan as follows:
About structural chamber and the loading blocks of wood, due
to the wooden block sized irregular and methods ratings are not really the
logical gap between the blocks and between walls and ceilings woodpile with
relatively large (0.5 ÷ slot with 0.7 m ceiling) forming the slots as drying
agents traverse woodpile and no effect dryer use. The use of the shield line is
not properly concerned.
The selection of fans (flow, head ...) has not been studied
for real fans fit.
The calorifier large capacity also increases the resistance
line and increase its wind power consumption for fans.
b. About control and control of waste heat and moisture: To
automate the operation of the drying chamber using HELIOS controller or controllers
equivalent drying. The measurement signal is temperature thermometer dry,
moisture in the bottle timber and equilibrium moisture content of wood to the
environment in the drying chamber (EMC), the quantity control is the amount of
heat supplied to the drying chamber (opens simultaneously humidity and wind
valve exhaust valve air supply) system currently has the irrationality follows:
+ large heat capacity coefficient operation time of solenoid
valves for heating Calorifier is about 0.25 ÷ 0, 4 such thermal regime of the
drying chamber changes with large amplitude (however, due to the heat capacity
of timber in the chamber large surface heat exchangers more should head mounted
sensor vibration still within the allowable range )
+ capacity moist exhaust large coefficient operation time of
solenoid valve opens the supply of similar winds make older relative humidity
ng similar changes in temperature.
+ For reasons of saving up the sensor is only fitted at one
end of the drying chamber, but to all the drying process to be used for
reversible fans (with reversible fan it is equipped with two touch sensors at
both ends of the drying chamber) so when reverse control signal changed and
drying parameters at the start not fitting sensors big difference with before
reversing. This, together with the thermal capacity of Calorifier which leads
to the dry heat of the head is not mounted sensors and moisture balance here
fluctuates greatly increases the likelihood of damage to the wood drying
(although these parameters at the top of mount sensor remains in the allowable
range) leads to lowering the settings towards prolonged drying process to avoid
damage to the wood, which increases energy costs as well as increase the
investment rate for drying 1m3 wood.
c. On investment: A survey in a number of establishments
have some investment on the following issues:
+ For some drying chamber due to some reasons presented on
prolonged drying time (about 1.5 times with some documents given) this
increases the investment capacity for drying wood 1m3.
+ For biomass fired boiler: regular operating a load (40 ÷
60% of the rated power, fans often do not run, fan runs about 0.3 ÷ 0.4 fumes
survey time). Boilers are devices that cost about 40 ÷ 50% if this drying
system increases productivity significantly invested.
III. ISSUES TO BE CHANGED TO IMPROVE ENERGY EFFICIENCY IN
THE OVEN USING WOOD ENERGY
a. On the use of energy:
+ on power consumption: As above mentioned losses Exergi in
boiler and drying chamber is large to improve the efficiency of energy use we
can use cogeneration. Saturated steam at a pressure of 8 ÷ 10 bar can be put
into the steam engine (engine turbine or screw) to pull the generator. After
the steam engine pressure 1.5 bar can be used to heat the drying chamber.
According to the calculations of power generated can be supplied from 0.6 ÷ 0.8
of the amount of power needed drying system.
To accomplish this towards the drying system should make
some changes:
Calculator capable of Calorifier heating steam parameters
when changing from 6 ÷ 8 bar to 1.5 bar (Δt's influence heat
transfer process)
Change structural system Calorifier from open 100% capacity
to maintain load bottom opening and closing solenoid valve is only done with
the peak load for all steam flow required for drying systems avoid the steam
flow through the steam engine changed much.
The structural changes also do Calorifier system parameters
in the drying chamber is more uniform and help reduce energy costs.
+ In terms of power consumption: To limit power consumption
to perform some of the following:
- Make uniform blocks of wood put into drying to reduce the
gap between them
- Find solutions to cover the gap between the two top of the
drying chamber (above the ceiling and sides) can direct the flow capacity
plates or hovercraft.
These solutions prevent the take off (no drying effect) of
airflow and increase line speed when passing through wood blocks thus
increasing the drying rate.
Calculating aerodynamic and verified by actual measurement
determined air flow and pressure loss optimized to select the fan, avoiding
choosing fan wrong and axial fans generally have lower performance especially
when operating in areas not suitable
b. On heating and waste humidity:
As above stated capacity of heat supply and waste humidity
are greater as President of the drying chamber fluctuated significantly
(particularly for waste moisture so driven by motor rotates slowly if latency
control big do lost control accuracy). To overcome this problem should be
classified into base load and peak (for both heating and exhaust humidity)
control signals only control section peak load which will reduce the volatility
of drying parameters making process drying and better stability. Capacity needs
of the waste heat and humidity is defined by calculated and verified by measurement,
the actual test.
c. About control, control parameters of the air in the
drying chamber:
For drying system reversible fan on an analysis of the
negative impact on energy efficiency of the system by using only one sensor
attached at one end of the drying chamber to solve the following measures:
- Installation of two sensors at both ends of the drying
chamber, while reversing fan also change sensors used
- when using a sensor if take the signal in the middle the
pile of wood drying (or for drying chamber uses two Calorifier at both ends can
arrange sensors in the space between two Calorifier this.
- through the distribution of load peaks and troughs of the
heating and exhaust moisture and selection of waste heat and humidity level
productivity reasonable to reduce the vibrations of the air in the chamber
parameters drying and finishing the drying process.
d. Change the mode of operation of the boiler:
To reduce the cost of energy fuel for the boiler and promote
the capacity of the furnace needs to be tested identified about activities
economic furnace thereby determining the combustion mode, load mode appropriate
to avoid too-load operation reduces investment efficiency furnace.
IV. CONCLUSION
Above analyzed defects of the system drying popular today
and provide some solutions for this defect in order to improve the efficiency
of energy use in drying through which we propose some model effective drying as
follows:
Using cogeneration facility for steam drying with a
relatively large capacity.
With the drying temperature t ≤
65 °
C using a heat pump for drying at a small scale can effectively use high power
and consistent with the basis on a small scale by not investing boiler capital
high investment and operation of complex research using drying system cellar combined with
cogeneration could be an effective solution for the stability of the drying
process to create conditions for cogeneration stable load . Tunnel drying
parameters for a more stable and the heat loss and power consumption can be
significantly reduced. Yet obstacles is the product to have a large output and
stable in shape, size. These are issues that need further study.