Project Utilisation of sludge gas for power generation through aero gas turbines
Introduction: Sludge gas from municipal sewage treatment plants is a valuable by-product, containing a high proportion of methane (CH4, 53 to 68 %), which has a calorific value in the range of 19-17 to 24.53 MJ/Nm 3. Since the gas is not easily transportable, it is convenient to utilise it in the vicinity of the sewage treatment plants, generally located at the outskirts of towns and cities.
One way to utilise this renewable energy source is to use it as fuel for internal combustion engines for mechanical or electrical power generation. In the context of the present acute shortage of electrical power in certain areas, such a scheme would certainly be helpful, particularly for standby power generation for peak demand times and for supplying industries located in the vicinity of the treatment plants. While the use of reciprocating gas engines for operation on sewage gas is well known, the present project uses gas turbines, the main advantage of which is that the capital cost would be considerably lower as compared to reciprocating engines, particularly if the turbines are derived from aero-engine applications, after their normal flying duty is completed. Such engines are available at a nominal cost within the country and can be modified to operate on gas.
The other advantages of a gas turbine system are the ease of installation (Ie light weight, low requirement of cooling water), modular maintenance, quick starting and reliable operation. They also have a high temperature exhaust which can be utilised for process heating, thereby increasing the overall thermal efficiency of the system. These engines are in running condition but not certified for aircraft use.
They represent a potential of about 250 MW of generating capacity, which can be harnessed in the manner described and thus save the country over Rs. 100 crores. It may be mentioned that the Dart engine is manufactured indigenously by Hindustan Aeronautics Limited (HAL). Further, there are plans for indigenous development and manufacture of an industrial gas turbine operating on a coal-based fuel. Such an engine could be easily adapted to work on sludge gas also, and because of its proposed advanced design features, the overall efficiency will be pretty high. Thus, a continuous supply of gas turbines will be available in the country for operation on sludge gas.
These engines could also be operated in dual fuel mode (ie, Diesel + sludge gas or kerosene + sludge gas), to meet any operational requirements. A certain amount of R&D work is involved in converting such engines to operate successfully on sludge gas.
Sludge gas production: Sludge gas is a product of anaerobic fermentation of organic matter present in sewage. In a treatment plant the concentrated sewage sludge is placed in large closed chambers called digestors, for periods up to 4 weeks. During this time fermentation of the sludge takes place due to the action of naturally occurring methanogenic bacteria, which results in the formation of a gas rich in CH 4 and also containing carbon dioxide (CO2), nitrogen (N2) and a trace of hydrogen sulphide (H2S). At the end of the digestion process, the slurry is stabilised (ie rendered pathogeni cally harmless) and can be dried to yield a valuable fertiliser. An important practical consideration for gas availability at a sewage plant is the problem of gas leakage through hairline cracks invariably present in digestor domes.
Repair of cracked domes presents many problems and a complete solution is yet to be found. One method of overcoming the problem is to devise an evacuation system which continually sucks off the generated gas, which is then stored in a gas holder, thereby reducing the positive pressure differential in the digestors and hence the leakage. The practicality of such a system was first demonstrated by Pai et al (1978) which gave an encouraging turn to an otherwise hopeless situation. There is of course no reason why similar figures cannot be achieved in India also where the climatic factors are generally favourable for this purpose.
Several treatment plants already exist in the country and more are planned in view of increasing population concentrations at metropolitan cities and consequent pollution problems. It is interesting to estimate the energy availability from this renewable source, given the above variability in the generation rate, in addition to which the variation in CH, content has also to be considered. Based on measurements at a sewage plant in Bangalore, a range from 53-4 to 68.3% can be taken as representative of this. With these figures the rage of energy availability per million of population served by the plant can be estimated (table 1). The thermal power feasible is in the range 2-10 MW per million population
Modifications to aero-engine : The acre gas turbine engine is primarily designed for operation on a liquid fuel such as kerosene (Aircraft Turbine Fuel, ATF). In order to operate the engine on a gaseous fuel such as sludge gas, changes have to be made in the fuel injection and control system. Further, for operation on ground, certain sub-systems such as the oil cooler and the governor have to be modified. The R&D efforts in the present project with respect to a Rolls Royce “Dart” turbo prop engine are briefly described .
(1) Development of gas injector: In the “Dart” engine, the fuel (kerosene) is injected into each of the seven combustion chambers through an atomizer, which sprays the fuel in small droplets in the form of a hollow cone. Figure 1 shows a schematic view of a combustion chamber. The fuel spray is introduced downstream of a swirler which imparts a strong swirl to a portion of the combustion air and creates a region of recirculating flow which helps in stabilising the flame in the primary combustion zone.
The objective of the present development was to devise a suitable injector nozzle for sludge gas which could be retrofitted with minimum changes to the combustion and ignition systems. The limitations were mainly that the injector should be introducible into the existing swirler hub. Since the carlorific value of sludge gas is about half that of kerosene (on a mass basis), it follows that for the same thermal input, the mass flow is twice the value. Further since the density of gas is about 2 orders of magnitude less than that of the liquid, it is necessary to use as large a pipe as possible to keep down the pressure losses.
In order to test various gas injectors, a combustion test rig was set up at the Koramangala & Chellaghala (K & C) valley sewage treatment plant. Here one of the combustion chambers of the Dart engine was set up with an independent supply of air and fuel (sludge gas) at essentially atmospheric pressure conditions.
The basis of such testing is that the gas velocities through the combustor are kept at values corresponding to engine conditions. The performance of any injector which works well at atmospheric pressure conditions would be better under engine conditions where the inlet pressure and temperatures are higher. Several injector configurations were fabricated and tested in this rig. These included injectors with multiple radial holes, a single axial hole and a variable radial slit injector
.
The last-mentioned injector injects the gas in the form of a radial fan jet. The slit gap could be adjusted prior to insertion of the fuel gun into the chamber. The combustion chamber exit was provided with an array of 8 chromelalumel thermocouples so that the temperature distribution at the exit could be determined. The array could be rotated about the combustor axis, to obtain additional data points. The combustor was fitted with the original igniter spark plug.
The injector geometry had an important influence on the combustion characteristics, from the view of obtaining easy ignition, stable combustion and good temperature distribution at the combustor exit. A desirable temperature distribution at the exit is such as to result in the long life of the turbine. The area -averaged temperature traverse quality factor (TTQF), defined as (Tpeak-Taverage)/(delta(Tcombustor)) is a measure usually employed to characterise this feature; a low value being preferable. Amongst the injectors tested the radial-slit was found effective in obtaining good ignition and temperature traverse quality. The slit-width was found to influence the temperature quality factor quite significantly . Subsequently, gas injectors based on this design were fabricated and installed on the engine. A manifolding system was developed to ensure equal supply of gas to each of the seven combustors.
(2)Development of dual fuel injector: The gas injector was further modified to permit operation in a dual fuel mode, ie gas and kerosene. This injector was initially developed in the combustion lab as an air-blast atomizer, and subsequently tested on the engine in single and dual fuel operation.
(3)Accessories and instrumentation: Other modifications to the engine included the development of a digital RPM indicator, a water-cooled oil cooler and high energy ignition units. The digital RPM indicator, apart from indicating engine speed, functione d as an overspeed trip to shut off the sludge gas flow through a pneumatic system in case of turbine speed exceeding a present value. The oil cooler was modified for operation with water as coolant, since for stationary application, the air flow is inadequat e. For starting of the engine, two high energy ignition units operating on 220 V AC were developed in the pilot plant of the laboratory. Other systems developed for operation of the engine were the starting system based on lead acid battery bank, LP fuel feed system for Am, mechanical linkage system for operation of throttle, trimmer, hp cock, speed governor and gas valves, etc.
The instrumentation included panel instruments for indicating major engine variables such as RPM, compressor pressure, jet pipe temperature, torquemeter pressure, oil temperature etc. (4)Demonstration plant: A demonstration plant to investigate the operation of a Rolls Royce Dart turbo-prop engine on sludge gas has been set up at the K & C valley sewage treatment plant of the Bangalore Water Supply and Sewerage Board (BWSSB). This treatment plant has a capacity of 168 million litres per day and provides primary sewage treatment, namely sedimentation followed by digestion of the sludge.
When this project was conceived in 1976, the treatment plant had no facilities for gas storage and the availability of the gas was negligible. Investigations indicated that though gas generation was present, most of the gas was leaking away through numerous cracks in the digestor domes. A pilot gas holder of 28 m 3 was set up and a blower installed to extract the gas from the digestor and fill the gas holder. With this system it was possible to minimise the overpressure in the digestor and hence the leakage. It was possible to collect substantial quantities of gas and the proposal to set up a demonstration plant was taken up.
The layout of the demonstration plant is shown in figure 3 and a general view in figure 4. It consists of a gas collection and holder system, a gas conditioning and compression system, a gas turbine engine and an operating system. The gas collection system consists of a gas blower unit 1-5 kW (2 HP) which extracts the gas from any or all of the four digestors and transfers it to the gas holder. The gas holder is of the conventional moving bell type with a capacity of 2266 m 3 (figure 5). It stores the gas at a pressure of about 90 mm of water column.
The gas conditioning system is a filter filled with material for absorbing hydrogen sulphide. Initially steel turnings are being used for this purpose. The gas compression system comprises a 2-stage sliding vane rotary compressor driven by a 90 kW increased-safety motor, an inter-cooler, oil separators, after- cooler and a cyclone. The gas turbine engine is as discussed above and coupled to a variable- pitch propeller for absorbing the power developed. In the next phase, it is proposed to replace the propeller with a generator for electrical power generation. A gear box to match the propeller speed (1200 rpm) to the generator speed (1500 rpm) would also be required.
, (5)Engine operation and performance: The engine was initially operated on its normal fuel and with the original injectors, to establish the functioning of the various systems. Thereafter, one of the seven combustors was fitted with the gas injector described in §3″1 above, with the remaining ones retaining the liquid fuel injectors. The engine was operated in this mode to establish satisfactory ignition and combustion of the sludge gas within the engine. Subsequently, all the seven combustors were fitted with gas injectors and operation with sludge gas alone was attempted. For controlling the start-up of the engine, a by-pass was provided for the gas after the gas compressor, so that it flows back to the gas holder.
The gas flow was also controlled by means of a gate valve upstream of the compressor inlet. It was found that the ignition, acceleration and running of the engine were smooth and easy even when operating purely on sludge gas. The running time was initially limited to a few minutes owing to the small size of the pilot gas holder. The engine has been operated in the dual mode to estimate its performance. The shaft horsepower is estimated from the torque pressure indicator, which is related to the engine torque through a calibration curve.
To ascertain a suitable operating point for ground running, the engine has been operated at several speeds in its recommended cruise range (13,800 to 14,000 RPM). Figure 6 shows the engine performance when operating with Art alone as well as in the dual fuel mode, at a speed of 14,200 RVM. Due to limitations in the gas flow rate of the installed gas compressor, the engine cannot at present be loaded fully when running on gas alone. Hence liquid fuel upto about 27 ~o (in heat value) is injected to obtain sufficient loading in these tests. (6)Techno-economic aspects: In order to make even a tentative estimate of the techno-economics of gas turbine systems operating on sludge gas, it is important to recognize the large variability in the processes concerned. These relate mainly to the generation of the sludge gas and the engine performance parameters.
Thus,two comparative evaluations are carried out: the first one (evaluation I) considers the lower and upper practical limits for gas generation and gas turbine efficiencies in order to get an idea of the range of power generation potential and costs; the second one (evaluation II), considers various engine and system options for an average gas generation situation. This evah?ation helps in placing the various options in perspective and brings out the influence of sludge gas pricing policy on the overall economics.
Comparison is made with a reciprocating engine system operating on diesel or sludge gas. 6.1 Evaluation 1: The electrical energy that can be generated depends on the efficiency and ratings of the gas turbines available. In the present context of using available aero-derivative engines, it is pertinent to examine the Rolls Royce Dart series 6 engines (which power Viscount and Fokker F-27 aircraft), the Dart 7- series which power the AVRO 748 aircraft and the AI-20 Series-A engines which are surplus with IAF. These engines will be operated at derated conditions in order to obtain a longer time between overhauls.
The extent of derating will have to be obtained by conducting trials on demonstration plants. Section 3 of table 1 indicates the range of efficiencies which may be expected and the corresponding electrical outputs. The worst case corresponds to the Dart 6 operating with lean gas which gives an overall efficiency of just under 14%. The AI-20 engine operating with rich gas will give nearly 7 times this output at a system efficiency of around 20% .
6.2 Evaluation 2: It is well known that the efficiency of simple cycle gas turbines is generally lower than that of reciprocating engines which have higher compression ratios. One technique to improve the efficiency of the gas turbine is to incorporate a heat exchanger between the turbine exhaust and the compressor outlet. Significant improvement in efficiency of the order of 50% can be achieved with an additional heat exchanger and engine modifications. In the present evaluation the effects of attaching a heat exchanger, with an effective value of 0.8, to the engine system are considered.
Another important factor considered is the pricing of the sludge gas which will naturally influence the final economics. Three possibilities are considered, viz: —-sludge gas is supplied free of charge; — gas is charged equivalent to coal on a heating value basis, say Rs. 0-23/Nm^3; –gas is charged equivalent to Diesel oil on a heating value basis, say Rs. 2.21/Nm^3 7. Concluding remarks: The tests with the demonstration plant set-up have indicated the basic practicability of the proposed system, in respect of engine modifications, operation and performance.
Of course, further testing to demonstrate the endurance of the engine is still required. The indications of the combustor tests as well as the fortuitously low sulphur content of the gas (as a result of absorption in the gas holder), suggest that long life between overhauls should be possible. Thus ,there is every reason to pursue the proposal further by setting up actual generating plants wherever possible. Also , every effort must be made to set up and optimise sewage treatment plants wherever necessary, in order to reduce pollution and to utilise the by-products such as sludge gas and fertilisers to the maximum extent possible. References : Trevelyan W E 1975 Tropical Science 17:193-209. Pal B R, Abbey D K 1978 Bio-gas production from sewage:
A preliminarystudy, NAL-TM-PR-000/I-78 Barber N R 1977 Industrial Gas: 20-21