chapter 3 development of gasification-engine- generator...
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CHAPTER 3
DEVELOPMENT OF GASIFICATION-ENGINE-
GENERATOR SYSTEM
3.1 INTRODUCTION
To achieve the objectives of the research work which called for
extensive experimentation, a complete gasification-engine-electrical generator
system was required. There was also a need to have flexibility in the system
so that any modifications or alterations can be implemented at a later stage
during the course of research work. The modifications may be needed due to
variations in bioresidues, gasification air supply pattern, etc. Moreover, to
measure the various parameters, many instruments were required to be
incorporated in the system. As no such system was available commercially to
suit the requirements, it had to be designed and developed exclusively for the
research work. The design and development were based upon these
requirements.
3.2 CONCEPT
The experimental set-ups used by earlier researchers were reviewed
and relevant information obtained from them was used as one of the
ingredients of the present design. The following specifications formed the
bases for the present design:
Diesel engine of 3.7 kW capacity to be driven,
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Wood pieces of dimensions 45mm x 45mm x35 mm to be used,
Bioresidues like loose GroundNut Shells (GNS) also to be tested,
Fully co-current flow scheme to be evaluated in gasifier,
Producer gas (PG) to be dry cleaned and cooled.
Diesel engines of 3.7 kW (5 hp) are largely used for electrical
power generation and irrigation of farm lands in India. To drive a 3.7 kW
engine connected to an electrical generator, a maximum biomass feed rate of
about 6 kg/h is required. The design procedure of a simple gasifier is given in
Appendix 1. Wood reapers of cross section 45 mm x 35 mm are largely
available in timber mills. They are cut to 45 mm length to get wood pieces
suitable for feeding into the gasifier. GNS in loose form is also abundantly
available in the vicinity. Since the generated PG has to drive an engine, its tar
content should be minimum. To get low tar content in PG, fully co-current
flow scheme has to be evaluated in downdraft biomass gasifier. As water is
scarce as well as the tar and particulates contaminated water should not be
directly discharged out, PG has to be dry cleaned and cooled. A design was
evolved considering these aspects and the entire system with necessary
instruments was developed.
3.3 DESCRIPTION OF THE SYSTEM
The overall system used for conducting the experiments consists of
gasification system and engine system. The gasification system consists of
co-current flow gasifier, air blower, flaring pipe, cyclone separator, dust filter,
PG cooler, tar adsorber, and associated instruments. The engine system
consists of a PG-air mixer, diesel engine, electrical generator, and associated
instruments. The schematic diagram of the overall system is depicted in
Figure 3.1. The major specifications of the system are given in Table 3.1.
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Figure 3.1 Schematic diagram of gasification-engine-generator system
Table 3.1 Major specifications of gasification-engine-generator system
Gasifier
TypePacked bed, co-current, fully downdraft, throat formed bya convergent-divergent part
Diameter 300 mm Diameter of throat 100 mm
Height 1050 mm Max. wood feed rate 6 kg/h
Engine Generator
TypeVertical, 4 stroke,direct injection (DI),water cooled, diesel
TypeDirect coupled toengine, singlephase, A.C., 50 Hz
Compressionratio
18.5 :1Rated output 3 kVA
Efficiency 90 %
Rated output 3.7 kW @ 1500 rpm Voltage 230 V
Bore dia. 84.5 mm Current 13 A
Stroke length 112 mm Speed 1500 rpm
Co-currentGasifier
CycloneSeparator
Producer GasCooler
Vertical 4-Stroke, Compression Ignition,Constant Speed, Water Cooled and Direct
Injection Type Reciprocating Engine
Electrical generator
DustFilter
Taradsorber
Air Blower
Producer Gas-AirmixerAir
Diesel
Cleaned & cooled PG
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3.3.1 Gasification System
3.3.1.1 Co-current Flow Gasifier
The co-current flow gasifier is basically a packed bed reactor with
biomass and air entering at the top and flowing downwards along the gasifier.
The full sectional front view of the gasifier is shown in Figure 3.2.
Co-current flow gasifiers have been experimented in the past by few
researchers. A transparent open core downdraft gasifier was developed by
Milligan et al., (1994) wherein both air and biomass entered at the top of the
reactor and travelled downwards along the gasifier.
Figure 3.2 Co-current flow biomass gasifier
The gasifier has been designed to have variable configuration i.e., it
can be used as downdraft, updraft, throat type or throat-less type gasifier.
Depending upon the experimental requirements, any particular configuration
P1/T1
P3/T3
P4/T4
P2/T2
P5/T5P6/T6P7/T7P8/T8
PG
AirBiomass
Ø286
All dimensions in mm
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can be chosen and used for any type of biomass. In the present study,
downdraft, throat type configuration was selected for conducting experiments.
The air supply from a blower is regulated by a valve and its flow rate is
measured by an orifice meter No. 1 made of stainless steel. Air enters the
gasifier through a pipe provided at the top. The biomass is fed through a
feeding port, which is also provided at the top of gasifier. The feeding port is
kept closed during operation of gasifier except during feeding. The gasifier
has been fabricated out of 3 mm thick mild steel sheet in the form of a
cylindrical shell with tappings at regular intervals of 10 cm for pressure and
temperature measurements. Sampling-cum-viewing ports are also provided
along the gasifier height. The gasifier is lined inside with refractory cement
to withstand high temperature. It has a stirring arrangement to spread and to
agitate the biomass bed during gasification. The residual char/ash present on
the grate is cleared by rocking the perforated grate by means of a handle. The
bottom ash collection chamber has a pipe on its side through which PG exits
the gasifier. The char/ash which gets accumulated in the ash chamber during
continuous operation of the gasifier is removed through an ash port.
Due to the presence of many components in the PG flow path, the
resistance to gas flow is high. If engine is allowed to draw gas through these
components by itself, then required amount of gas generation and its supply
cannot be achieved. To overcome the pressure drop and to enhance the
quantity of PG admission to the engine, the blower is provided in the
upstream side of the gasifier. So, the entire gasification system is under
positive pressure. Bhattacharya et al., (2001) also operated a hybrid biomass-
charcoal gasification system by blowing air to the gasifier at three levels
along its height. If air is sucked into the gasifier by means of centrifugal
blower or engine, then open-top gasifier may be used.
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3.3.1.2 Producer Gas Scrubbing and Cooling Section
The hot PG generated in the gasifier contains tar and particulate
matter. So, it should not be supplied to the engine directly. The tar content
should be reduced to atleast 100 mg/Nm3 and particulate content must be
reduced to atleast 50 mg/Nm3 before supplying PG to the engine (Hasler and
Nussbaumer 2000). Generally, the generated PG is scrubbed and cooled by
means of water before supplying to the engine. The contaminated water is
then discharged out after certain period of operation.
The measured parameters of effluent water resulting from a typical
100 kWe biomass gasifier based power plant after 30 hours of its operation are
given in Table 3.2 and are compared against the permissible limits for safe
disposal. The COD to BOD ratio is 0.715; but it should be less than 0.65 to
dispose the water without pre-treatment. Therefore it becomes necessary to
pre-treat the effluent water before disposal. It may be economical for large
capacity gasifier power plants, but not for small scale gasification-engine-
generator systems.
Table 3.2 Waste water analysis of 100 kWe gasifier power plant
Sl.No.
ParameterMeasured
valuePermissible
Limit1 pH 6.5 7 – 82 Electrical conductivity (mho) 1.11 ---3 Suspended solids (mg/l) 670
10004 Total solids (mg/l) 3505 Total volatile solids (mg/l) 13506 Total dissolved solids (mg/l) 10007 COD (mg/l) 608 1008 BOD (mg/l) 850 150
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The new philosophy used in configuring the cleaning systems is to
eliminate the particulates in dry form, without significantly contaminating the
cooling water. This can reduce water quantity and water treatment load
(Dasappa et al., 2003). Cummer and Brown (2002) also reported that high-
temperature removal of particles, tar, and alkali from PG without the use of
high energy or water inputs is most sought-after. Considering these aspects,
in the present research, the PG was dry cleaned and cooled without making
direct contact with water. A cyclone separator, a dust filter, a PG cooler of
shell and tube type, and a tar adsorber were designed and fabricated for dry
cleaning and cooling of PG.
Cyclone Separator: A high efficiency dry cyclone separator is
used to remove the particulates from PG. The full sectional front view of the
cyclone is shown in Figure 3.3. The hot dust laden PG enters through a
rectangular duct while the cleaned PG leaves through a circular pipe at the
top.
Figure 3.3 Cyclone separator
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Ø75
Ø116
All dimensions in mm
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Dust Filter: It is a cylindrical shell containing four filter elements
also called as candle filters. These filters are top-held inside the shell and are
fabricated of SS mesh (No. 100). Each candle filter has mesh open area of
approximately 35 %. The dust particles bigger than 150 micron are retained
on the mesh and the cleaned PG comes out at the top of dust filter. Even
though a higher mesh No. can be used for filter fabrication, the associated
higher pressure drop prohibits its usage. Figure 3.4 shows the sectional front
view of dust filter. The inlet gas temperature must be kept above 300°C to
avoid moisture and tar accumulation on the candle surface (Engstrom 1998).
For hot gas cleanup, candle filters made up of ceramic material may also be
used. The ceramic candle filters are generally made up of Al2O3 and SiC
(Babu 1995). The pressure drop across the candle filter increases with more
and more dust deposition on its surface. The combination of cyclone
separator and candle filter constitutes an efficient system for hot gas cleaning
(De Jong et al., 2003).
Figure 3.4 Dust filter
Ø330
33Ø250
All dimensions in mm
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Producer Gas Cooler: The PG from dust filter enters gas cooler
which is a shell and tube type heat exchanger. It has a configuration of one
shell pass and two tube passes with cooling water flowing in shell side and
PG flowing in tubes. The sectional front view of PG cooler is shown in
Figure 3.5. The flow rate of PG is measured by an orifice meter No. 2
provided after PG cooler.
Figure 3.5 Producer gas cooler
Tar Adsorber: The cooled PG at about 50°C enters tar adsorber at
its top for final tar removal. The adsorber contains a bed of evenly sized and
stabilized charcoal particles which function as adsorbents. The tar molecules
which are the adsorbates diffuse from the bulk of PG to the surface of the
charcoal, forming a distinct adsorbed phase. The attractiveness of charcoal
for solving the tar problem is related to its low cost and natural production
inside the biomass gasifier (El-Rub 2008). Biomass char can also be used for
Ø300
PGPG
Water
Water
All dimensions in mm
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heterogenous tar conversion at high temperatures (Morf 2001). The full
sectional front view of the tar adsorber is depicted in Figure 3.6.
Figure 3.6 Tar adsorber
All the components of gasification system were fabricated and were
arranged as per the layout shown in the Figure 3.1. The photographic view of
the gasification system is shown in Figure in 3.7. In the system, dust particles
and (or) condensate separated from PG were collected at the bottom of every
component by means of gas tight collectors. From the description, it may be
known that the PG does not come into direct contact with water anywhere in
the system but it is dry cleaned in the various components of PG scrubbing
and cooling section. Because of that, there is no generation of contaminated
effluent water from the gasification system and the question of its safe
disposal does not arise. If wet scrubbing method (PG and water contact
directly) is adopted, due to contamination and accumulation of tar and
Ø244
Ø230
All dimensions in mm
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particles, the recirculation water has to be drained out after certain time. The
effluent water is acidic and poses environmental problems if not safely
disposed.
Figure 3.7 Gasification system
3.3.2 Engine System
The major specifications of the engine system have already been
given in Table 3.1.
3.3.2.1 Air Filter with Air Flow Measuring Tank
In a diesel engine, the air filter is connected to the inlet manifold of
the engine directly. In the present research, as the engine has to be run in dual
fuel mode also, the air filter is connected to the engine through a PG-air
mixer. For the purpose of engine air flow rate measurement, an air tank fitted
with orifice meter No. 3 is connected to the air filter.
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3.3.2.2 Producer Gas-Air Mixer
The PG-air mixer is at the junction of gasification system and
engine system. Its function is to mix the cleaned and cooled PG from
gasification system with the engine air which has been sucked through air
filter and to supply the mixture to the engine. It has three ports, one each for
air flow, PG flow, and mixture flow. The sectional view of the mixer is
shown in Figure 3.8. A valve is provided in air supply pipe and another one is
provided in the PG supply pipe to regulate respectively the quantities of air
and PG entering the mixer. Both air and PG should be mixed homogeneously
before it is supplied to the engine in order to achieve complete combustion
inside the engine cylinder. Since good turbulence is required for thorough
mixing, the PG-air mixer volume has been kept small.
Figure 3.8 Producer gas-air mixer
45°
Air
PG
Mixture
All dimensions in mm
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3.3.2.3 Engine-Generator Set
In consists of a direct injection, compression ignition, diesel engine
directly coupled to an electrical generator. The combustion chamber of the
engine is formed by a bowl-in-piston with swirl and a centrally located multi-
hole diesel injector. This design can hold the amount of liquid diesel which
impinges on the piston cup walls to a minimum.
The photographic view of the entire engine system is shown in
Figure 3.9.
Figure 3.9 Engine system
3.4 INSTRUMENTATION
3.4.1 Parameters Measured in Gasification System
A number of parameters were measured in various experiments
conducted using the system. Table 3.3 lists the measured parameters in the
gasification system and the instruments used for their measurement. All
thermocouples were calibrated by the manufacturer. The thermocouple
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Table 3.3 Parameters measured and instruments used in gasificationsystem
Sl.No. Parameter Instrument
1 Biomass quantity Weighing balance (1g)
2 Gasification air flow rate Orifice meter with U-tubemanometer
3 Gasifier pressures P1, P2, P3, P4, P5, P6,P7, P8
U- tube manometerscontaining water
4 Biomass bed temperatures T1, T2, T3, T4,T5, T6, T7, T8 K- type thermocouples
5 Gasifier surface temperatures t1, t2, t3, t4 J-type thermocouples
6 Biomass bed height Depth rod and measuringtape
7 Time Stop watch8 PG temperature at gasifier exit K- type thermocouple9 PG pressure at gasifier exit U-tube manometer10 PG temperature at dust filter exit K- type thermocouple11 PG pressure at dust filter exit U-tube manometer
12 Water temperatures at inlet and exit of PGcooler J-type thermocouple
13 PG flow rate Orifice meter with U-tubemanometer
14 PG pressure at tar adsorber inlet U-tube manometer15 PG pressure at tar adsorber exit U-tube manometer
16PG sampling before and after PG scrubbingand cooling section for tar and particulatesmeasurement
T & P apparatus as perEuropean standard
17 Weight of tar residue Electronic analyticalbalance (0.0001g)
18 Weight of particulates Electronic analyticalbalance (0.0001g)
19 CO,CO2,CH4 contents in PG NDIR sensor based gasanalyser
20 H2 content in PG Thermal conductivity H2
analyser
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outputs were connected to digital temperature indicators which gave
temperature readings directly. For determining the volatile matter content of
biomass bed particles sampled along gasifier height, an electric muffle furnace
was used. An electric heating mantle was used to evaporate the iso-propanol
solvent used in tar and particulates sampling apparatus. Two numbers of tar
and particulates sampling apparatus were constructed to enable sampling
before and after PG cleaning and cooling section simultaneously. They were
designed and fabricated following the European standard. The detailed
specifications of various instruments and apparatus which were used for
measurements are given in Appendix 2.
3.4.2 Parameters Measured in Engine System
Table 3.4 lists the measured parameters in the engine system and
the instruments used for their measurement. An air tank of 0.076 m3 capacity
fitted with an orifice meter to measure engine air flow rate was also
fabricated. For the measurement of electrical power produced by the
generator, a panel board consisting of voltmeter, ammeter, energy meter was
prepared. A resistance load bank of 3 kW capacity was also created for
dissipating the electrical energy produced by the generator. Suitable
arrangements for the measurement of DIP parameters like diesel injection
quantity, injection timing, and injection pressure were also readied with the
diesel injection system. The detailed specifications of the instruments which
were used for measurements are given in Appendix 2.
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Table 3.4 Parameters measured and instruments used in engine system
Sl.No. Parameter Instrument
1 Engine air flow rate Orifice meter with U-tube manometer
2 Diesel consumption rate Graduated burette andstop watch
3 Engine speed Digital tachometer4 Diesel injection quantity/cycle Measuring cylinder5 Diesel injection pressure Bourdon pressure gauge
6 Diesel injection pump control rackposition Steel scale
7 Engine exhaust gas temperature K-type thermocouple
8 Cooling water temperature at inlet andexit of engine J-type thermocouple
9 Cooling water flow rate Measuring cylinder andstop watch
10 O2 and CO2 contents in engine exhaustgases
Electro-chemical sensorbased O2 analyser
11 Generator voltage AC Voltmeter12 Generator current AC Ammeter
13 Generator power Energy meter and stopwatch
3.5 SUMMARY
The experimental set-up with extensive instrumentation was
designed and developed exclusively for the research work. Pressure tappings
and sampling-cum-viewing ports along gasifier height, dry cyclone, SS candle
filters, a shell and tube type PG cooler, use of charcoal as adsorbent, no
generation of contaminated effluent water are certain novelties of this small
capacity gasification system. It can be used for conducting many types of
experiments in biomass gasification.