CHAPTER 3 DEVELOPMENT OF GASIFICATION-ENGINE- GENERATOR SYSTEM

Transcription

1 22 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,

2 23 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.

3 24 Co-current Gasifier Cyclone Separator Dust Filter Producer Gas Cooler Tar adsorber Air Producer Gas-Air mixer Cleaned & cooled PG Air Blower Vertical 4-Stroke, Compression Ignition, Constant Speed, Water Cooled and Direct Injection Type Reciprocating Engine Diesel Electrical generator Figure 3.1 Schematic diagram of gasification-engine-generator system Table 3.1 Major specifications of gasification-engine-generator system Gasifier Type Packed bed, co-current, fully downdraft, throat formed by a convergent-divergent part Diameter 300 mm Diameter of throat 100 mm Height 1050 mm Max. wood feed rate 6 kg/h Engine Generator Type Vertical, 4 stroke, direct injection (DI), water cooled, diesel Type Direct coupled to engine, single phase, A.C., 50 Hz Compression Rated output 3 kva 18.5 :1 ratio Efficiency 90 % Rated output rpm Voltage 230 V Bore dia mm Current 13 A Stroke length 112 mm Speed 1500 rpm

4 Gasification System 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. Biomass Air PG Ø286 P1/T1 P2/T2 P3/T3 P4/T4 P5/T5 P6/T6 P7/T7 P8/T8 All dimensions in mm 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

5 26 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 biomasscharcoal 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.

6 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/nm 3 and particulate content must be reduced to atleast 50 mg/nm 3 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 kw e 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-enginegenerator systems. Table 3.2 Waste water analysis of 100 kw e gasifier power plant Sl. No. Parameter Measured value Permissible Limit 1 ph Electrical conductivity (mho) Suspended solids (mg/l) Total solids (mg/l) Total volatile solids (mg/l) Total dissolved solids (mg/l) COD (mg/l) BOD (mg/l)

7 28 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 hightemperature 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. Ø116 Ø75 40 All dimensions in mm Figure 3.3 Cyclone separator

8 29 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 Al 2 O 3 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). Ø330 Ø All dimensions in mm Figure 3.4 Dust filter

9 30 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. PG PG Water Water Ø300 All dimensions in mm 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

10 31 heterogenous tar conversion at high temperatures (Morf 2001). The full sectional front view of the tar adsorber is depicted in Figure 3.6. Ø230 Ø244 All dimensions in mm 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

11 32 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 Engine System given in Table 3.1. The major specifications of the engine system have already been 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.

12 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. Mixture 45 PG Air All dimensions in mm Figure 3.8 Producer gas-air mixer

13 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 multihole diesel injector. This design can hold the amount of liquid diesel which impinges on the piston cup walls to a minimum. Figure 3.9. The photographic view of the entire engine system is shown in Figure 3.9 Engine system 3.4 INSTRUMENTATION 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

14 35 Table 3.3 Parameters measured and instruments used in gasification system Sl. No. Parameter Instrument 1 Biomass quantity Weighing balance (1g) 2 Gasification air flow rate 3 Gasifier pressures P1, P2, P3, P4, P5, P6, P7, P8 Orifice meter with U-tube manometer U- tube manometers containing 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 7 Time Stop watch Depth rod and measuring tape 8 PG temperature at gasifier exit K- type thermocouple 9 PG pressure at gasifier exit U-tube manometer 10 PG temperature at dust filter exit K- type thermocouple 11 PG pressure at dust filter exit U-tube manometer 12 Water temperatures at inlet and exit of PG cooler J-type thermocouple 13 PG flow rate Orifice meter with U-tube manometer 14 PG pressure at tar adsorber inlet U-tube manometer 15 PG pressure at tar adsorber exit U-tube manometer 16 PG sampling before and after PG scrubbing and cooling section for tar and particulates measurement 17 Weight of tar residue 18 Weight of particulates 19 CO,CO 2,CH 4 contents in PG T & P apparatus as per European standard Electronic analytical balance (0.0001g) Electronic analytical balance (0.0001g) NDIR sensor based gas analyser 20 H 2 content in PG Thermal conductivity H 2 analyser

15 36 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 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 m 3 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.

16 37 Table 3.4 Parameters measured and instruments used in engine system Sl. No. 1 Engine air flow rate Parameter 2 Diesel consumption rate Instrument Orifice meter with U- tube manometer Graduated burette and stop watch 3 Engine speed Digital tachometer 4 Diesel injection quantity/cycle Measuring cylinder 5 Diesel injection pressure Bourdon pressure gauge 6 Diesel injection pump control rack position Steel scale 7 Engine exhaust gas temperature K-type thermocouple 8 Cooling water temperature at inlet and exit of engine J-type thermocouple 9 Cooling water flow rate 10 O 2 and CO 2 contents in engine exhaust gases Measuring cylinder and stop watch Electro-chemical sensor based O 2 analyser 11 Generator voltage AC Voltmeter 12 Generator current AC Ammeter 13 Generator power Energy meter and stop watch 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.