INTRODUCTION
Global concern on environmental issues and the fast depletion of fossil fuels
have imposed a great strain on developed countries to find an alternative source
of fuel which is environmental friendly as well as non-depleting. Biomass is
one of the renewable energy resources which is capable of displacing large amounts
of solid, liquid and gaseous fossil fuels. The solid biomass materials can be
conveniently used as an alternative fuel through a process known as gasification.
Gasification is the process of converting solid fuel into a gaseous fuel through
a thermo-chemical conversion process. The process involves the utilization and
conversion of biomass in an atmosphere by using air or steam as a gasifying
agent to produce a low or medium heating value gas. The generated gas, known
as producer gas is a mixture of carbon monoxide (CO), hydrogen (H2),
methane (CH4), carbon dioxide (CO2), nitrogen (N2)
and water vapor (H2O). However, the gasification process also produces
fine dust, ash and condensable compounds of tar (Sridhar
et al., 2001) which must be cleaned and removed to a permitted value
if the producer gas is intended to be used as fuel in an IC engines.
| Table 1: | Composition
of producer gas |
 |
The typical composition of producer gas obtained from the downdraft gasification
process on percentage volume basis over the last three decades is shown in Table
1.
Today, due to the increase of fuel prices and environmental concern, there
is renewed interest on the gasification technology. Biomass gasification is
one of the biomass conversion technologies that can be effectively used and
utilized for decentralized power generation plants and thermal applications.
The technology can be realized in two ways: directly used in the boiler for
steam production followed by running a turbine and treated the producer gas
to run a gas turbine and IC engines.
Typically, the temperature of gasification is in the range of 800-900°C
and the producer gas has a heating value of 4-12 MJ Nm-3 (McKendry,
2002). The main reason of low heating value is due to the dilution of the
product gases with nitrogen from air during the gasification process. Therefore,
the producer gas is difficult to liquefy or compress particularly in small-scale
gasifiers (Russel, 2008). In the current practice the
producer gas is used immediately once it is produced and quantity and the flow
rates of the producer gas are slightly above ambient condition.
Compressing and storage of producer gas are viewed as attempts to store the
gas and to be used at the later stage. Generally, the only equipment required
to compress and store any type of gas are a compressor and a pressure vessel.
However, the main problem with compressed gas storage is the low storage density,
which depends on the storage pressure. Higher storage pressures will increase
capital and operating costs (Wade, 1998). The producer
gas can be compressed to a higher pressure than atmospheric, due to the fact
that the producer gas has poor ignition or a delay in ignition characteristics
(Singh et al., 2007). Therefore, the producer
gas cannot be self-ignited unless there is a source of ignition such as spark
plugs in Spark Ignition (SI) engine or diesel fuel in Compression Ignition (CI)
engine (Sridhar et al., 2005).
Based on earlier research, compressing of producer gas has been tried by using
switchgrass as a biomass fuel in gasification process (Asma
et al., 2006). The producer gas obtained was compressed using an
air compressor and stored at 869 kPa of gauge pressure. The compressed producer
gas was then converted into ethanol using microbial catalysts. The composition
of producer gas recorded was 16.5% of CO, 15.5% of CO2, 5% of H2,
4.5% of CH4 and 56% of N2.
A feasibility study conducted by the US Department of Energy’s
National Technology Laboratory (2008), has revealed that the Integrated
Gasifier Combined Cycle (IGCC) which operates the gasifier continuously can
produce and stores the syngas instead of using it immediately. The compressed
and stored syngas from gasification process may be used to produce electricity
in gas turbines during periods of peak demand or when the tariff of electricity
is at the highest price.
It has been seen so far that not many literatures have been done on the compressing and storing of producer gas from biomass gasification. Therefore, there is still room for researchers to study the concept and develop compressed producer gas for future use, particularly in the small scale power generation. Hence, the present study is aimed to develop and evaluate the feasibility of compressing the producer gas from biomass downdraft gasification process.
THEORETICAL ANALYSIS
Compressing producer gas in a cylinder by using air compressor: An air
compressor can generally be defined as a device that is used to increase the
pressure of gas or vapor, typically air or a mixture of gaseous and vapors.
The basic principle of the device is taking a volume of gas or vapor and increasing
it to a pressure higher than atmospheric pressure in a closed tank by reducing
its volume. The gas in the pressured condition is stored in the tank at a specified
pressure depending on its type and specifications. The gas can be stored in
high pressure cylinders ranging up to 6000 psi (410 bar), normal pressure cylinders
ranging between 2000 and 2500 psi (140 and 175 bar) and low pressure cylinders
ranging up to 480 psi (34 bar). The common type of air compressor is the reciprocating
air compressor which usually uses a piston within a cylinder as the compressing
and displacement element. Single stage air compressors are generally used for
pressures in the range of 70 to 120 psi while two stage air compressors are
for pressures between 125 to 250 psi (Frankel, 2004).
Figure 1 shows a gas being compressed inside a cylinder by
the movement of a piston. The gas is compressed from volume V1 to
volume V2 and as the piston moves, the pressure is raised from pressure
P1 to pressure P2 and the temperature also rises from
temperature T1 to temperature T2. The final pressure depends
on the final temperature, which depends on the degree of cooling that takes
place during the process (Iynkaran and David, 2004).
|
| Fig. 1: | Compression
of gas in the cylinder |
When air flows into the pipe, its mass depends on the pressure, temperature
and composition prevailing at the compressor inlet. When pressure and temperature
are considered, the actual volume of air flow is usually stated as Free Air
Delivery (FAD). In an air compressor, the FAD is basically a volume of air drawn
into a compressor from the atmosphere. After compressing and the cooling process,
the air is returned to its original temperature but at a higher pressure. However,
the term FAD does not mean that air is at standard conditions due to altitude,
barometer and temperature that may vary at different locations and at different
times.
By considering the initial air conditions P1, V1 and T1 and the final compressed air conditions P2, V2 and T2, using the ideal gas law:
Usually it is assumed that the compression is perfectly cooled, the temperature remains constant and the final pressure is the lowest possible. Hence, the process is called isothermal and obeys Boyle’s Law, PV = constant. Therefore, the temperatures are cancelled and the volume of FAD (V1) from Eq. 1 becomes:
The actual rate of FAD is determined by taking into account the atmospheric pressure (P0) and the time (t) taken for building up pressure in the compressor tank from atmospheric pressure to a specified gauge pressure. Therefore, Eq. 2 can be substituted as:
As mentioned earlier, Eq. 3 is relevant where the compressed air temperature is the same as the ambient air temperature by assuming perfect isothermal compression. In the event the actual compressed air temperature at a discharge pressure of T2 is higher than the initial air temperature of T1, the FAD has to be corrected by a factor as in the following equation:
Gasifier-air compressor flow diagram: The selection of a suitable air
compressor to be coupled with a downdraft gasifier is the most important criteria
in this research project as illustrated in Fig. 2.
|
| Fig. 2: | Gasifier-air
compressor systems |
The amount of biomass and producer gas flow rate has to be properly calculated
and balanced in order to avoid too much waste or an excess of producer gas generated
from the gasification process. At any condition, the amount of producer gas
supplied to the air compressor should be higher than the amount of gas delivered
by air compressor. Otherwise, air will be entering the system and mixed with
the producer gas in the compressor tank. For theoretical calculations, the efficiencies
of the downdraft gasifier and the cooling system were taken as 70 and 95%, respectively
(Sivakumar et al., 2008; Bhave
et al., 2008).
Other parameters involved in the calculations are including biomass specific fuel consumption, low heating value of producer gas and the biomass fuel feeding rate.
EXPERIMENTAL METHODS AND PROCEDURES
Experiments have been conducted on a single stage reciprocating air compressor in which the output flow rate of the compressed producer gas was regulated at 130, 150 and 170 L min-1 constantly, using a calibrated flow meter as the specifications is shown in the Table 2. The discharged output pressure was set at 2.0, 2.5 and 3.0 bar pressures, respectively.
Experimental setup: The experimental setups consisted of a downdraft gasifier, a producer gas cooling and cleaning system and air compressor. The downdraft gasifier was specifically designed and developed by Universiti Sains Malaysia (USM) to generate an acceptable quality of producer gas, consistently. The specifications of the gasifier and the air compressor used in the experiment are given in Table 3.
The 50 kg biomass fuel is fed into the gasifier through the top opening at
a feeding rate of 20 kg h-1 as shown in Fig. 3.
Air was supplied to the gasifier using a rotary blower, in which the capacity
was higher than the required airflow rate. To prevent pressure build-up and
the overloading of the motor of the blower, a by-pass valve was used to discharge
some air to the atmosphere. Airflow to the gasifer was measured by using a calibrated
rotameter in the range of 6-120 m3 h-1 and controlled
by using a ball valve.
| Table 2: | Specifications
of air flow meter |
 |
| Table 3: | Specifications
of the gasifier |
 |
|
| Fig. 3: | Reactor
of downdraft gasifier |
Air entered the combustion zone and the producer gas generated went out near
the bottom of the gasifier at a temperature of about 550°C.
When the steady operation of the gasifier was achieved, the hot producer gas was allowed to pass through the attached cyclone to remove all particles and dust. The producer gas was then passed through a heat exchanger for the condensation of water as well as for the cooling process. The cooled producer gas then passed through a holding tank, followed by a filtering system to remove tars and other fine particles for cleaning process. The final temperature of the producer gas measured was around 35°C before entering the air compressor as shown in Fig. 4.
In this study, the gasifier was started for about 30 min earlier to stabilize
the gas and at the same time the air compressor was also started to compress
the normal air from surrounding.
| Table 4: | Specifications
of air compressor |
 |
|
| Fig. 4: | Downdraft
gasifier-air compressor systems |
After checking the final gas outlet temperature of gasifier, the ball valve
was fully opened to allow the producer gas to be compressed by air compressor.
A single stage reciprocating air compressor was used to compress the producer
gas as the specification is given in Table 4. Standard engine
air filter and a pressure reducing valve were installed at the intake manifold
and the discharged pipe of the air compressor respectively. The intake FAD of
producer gas from gasifier was fixed at 670 L min-1 as per air compressor’s
specification. The producer gas was compressed from an initial pressure of 0
psi gauge pressure and temperature of 30°C, to an operating pressure of
7.6 bar gauge pressure. The compressor motor will re-start when the pressure
in the tank drops to a set pressure of 4 bar.
Air inside the air compressor was first purged out to discharge any remaining air inside the compressor tank to the surrounding. The process was then repeated until the compressor tank was fully displaced by producer gas. The producer gas was then flared continuously at the producer gas ports throughout the experiment as an indicator for the presence of gas from the gasifier.
Hawlett Packard Module 4890 gas chromatography was used to measure the volume percentage of CO, H2, CH4, CO2 and N2 to determine the heating value of producer gas. To measure the composition with the gas chromatography, producer gas samples were taken from the sampling unit with gas sample containers through a sampling point in the gasifier.
RESULTS
Flare of producer gas: The capability of the producer gas to fuel engine or process heat application can be determined by the flaring of gas through a provided port in the biomass gasification process. As shown in Fig. 4, the gas was able to be flared just after the gasifier reactor; the other port of gas flare was located after the air compressor. It was found that the gas which flared had a blue flame without any smoke as compared orange/yellow flame in the gasifier. This phenomenon implies that the compressed producer having a good quality, almost free tars and particles which is needed for engine applications. However, it is still containing water vapor which is not desirable and should be removes before gas enters the engine.
Producer gas flow rate: Figure 5-7
show the performance of the air compressor in terms of producer gas output flow
rate and pressure within a specific time period. An analysis of the composition
of the producer gas showed that it consisted of approximately 17.5% CO, 2.3%
CH4, 10.25% H2, 16.2% CO2 and 54% N2.
It was observed that the producer gas flow rate was fairly constant over the
entire range set of pressures. The gas flow rate slightly fluctuated at the
beginning due to the rapid rising of the pressure in the tank and the time taken
for setting-up the required flow rate. After 5 min, the flow rates were seen
to be almost stable towards the end of the experiment. The percentage of the
flow rate drops was in the range of ±5.2% in 130 L min-1,
±4.7% in 150 L min-1 and ±4.5% in 170 L min-1.
Discharge pressure: The discharge pressures were difficult to maintain as the air compressor motor ran on and off between the operating pressure to the minimum set pressure throughout the experiment. Therefore, it can be seen that the pressure fluctuated greatly at a higher discharge pressure as compared to a lower set pressure. The pressure drops calculated were ±6.5%, ±8% and ±10% for pressure of 2.0, 2.5 and 3.0 bar, respectively. The higher pressure drops implies that the higher capacity of air compressor should be used when higher producer gas flow rate is required.
DISCUSSION
As mentioned, there are not many literatures been done on the compressing and
storing of producer gas from biomass gasification. Based on earlier study, the
common flame color obtained from normal operating temperature of biomass gasification
is an orange/yellow flame as discussed by Wander et al.
(2004). However, a research conducted by Energy Research
Centre of Netherlands (2008) has revealed that the clear blue flame producer
gas flare was an indication of cleaned and free of tars of producer gas generated.
For compressed producer gas, Roy et al. (2009)
has been successfully carried out an experimental on simulated producer gas
in which the air and producer gas were mixed and kept constant at two bar gauge
pressure to fuel the IC engine. The study has also recovered the supercharged
producer gas from updraft gasifier in determining the effect on engine performance
and emission in the dual fuel IC engine. Therefore, this was a preliminary study
to see whether the compressed producer gas from downdraft gasifier could maintain
its flow rate at pressure higher than atmospheric and the quality of the gas
generated.
|
| Fig. 5: | Discharged
flow rate and pressure at 2.0 bar |
|
| Fig. 6: | Discharged
flow rate and pressure at 2.5 bar |
|
| Fig. 7: | Discharged
flow rate and pressure at 3.0 bar |
CONCLUSIONS
Compressing and storage of producer gas from downdraft gasification process
has a number of advantages. This study has proved that the producer gas from
the downdraft biomass gasification process can be compressed and stored for
future use, particularly in a small-scale stationary power plant. However, the
higher capacity of air compressor must be selected if the producer gas needs
to be compressed and stored at higher pressure. The following conclusions were
drawn from the present investigation undertaken. The important findings can
be listed below:
| • |
Good quality producer gas with a blue flare was obtained when the producer
gas was compressed to a pressure higher than atmospheric |
| • |
The flow rate and the pressure of the producer gas were almost constant
throughout the experiment; therefore, they could be varied to suit the required
applications |
| • |
High pressure output will result in a higher mass flow rate of the producer
gas |
ACKNOWLEDGMENT
The authors would like to express sincere appreciation for the assistance of Mr. Norman for his co-operation and assistance, thanks for the kind help. Financial support from the Federal Land Development Authority Grant No: 304/PMEKANIK/6050134, an agency under the Ministry of Agriculture and Agro-Based of Malaysia, is gratefully acknowledged.
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