Engr. S. Sibte Ahmed Jafri, FIEEE(Pak), FIE(Pak), PE
Introduction:
The term Self Generation Means Generation of Power by an industrial unit for its own consumption. Other terms commonly used for self-generation are in-house Generation or captive power. A closely related term is co-generation which implies that the plant is capable of delivering the electric (or mechanical) power and process heat (usually steam)
It is also possible to extend the generated power to adjacent industrial units. The emphasis being on generation, the distribution arrangement being secondary.
Why Self Generation?
It is generally the duty of the power utility companies (WAPDA and KESC in Pakistan) to provide power to its customers so that all their requirements are met. In Pakistan these companies are under severe pressure due to the growth of load and deficiency in, primarily, the generation capacity. This could be appreciated by considering that demand for power increase by about 900 MW, every year. This clearly shows that availability of power would always be a difficult proposition. One possible way of coming out of the difficulty is to make local or in house provision for the supply of power for the industrial unit. This is self-generation.
Available options:
Possible options available for self-generation (or co-generation) schemes depend on the fuel and prime-movers available. From the experience gained in Pakistan it seems that the choice would revolve around three possibilities, namely:
- Steam turbine
- Gas turbine
- Furnace oil fired diesel engine
Of these it may be noted that the first two can very conveniently operate on furnace oil (cheapest fuel available in the country). Gas turbine in principle requires distillates and operating them on furnace oil, especially for small plants, has yet to be proven. However in case of co-generation it could be possible to consider gas turbine even when operating on distillate fuel.
Comparison of options:
Steam Turbine Plant:
A basic steam turbine plant is based on the traditional Rankine cycle. Steam turbines have been extensively used in industry and power generation. The main advantage which the steam plant offers is its ability to burn difficult fuels. The other advantage which the steam plant offers is its ability to burn difficult fuels. The other advantages associated with steam plants are;
- High reliability
- Low noise level
- Ruggedness
- Low maintenance, and
- Long life
The disadvantages being, apart from the low efficiency.
- High first costs,
- Considerable space requirement,
- Long start time, and
- Long installation time
In small sized turbine the overall efficiency may be as low as 25-30%. Heat requirements could be coupled with power generation and the efficiency may increase to 60% or more. The type of turbine used for straight power generation are the condensing type requiring huge quantities of water for condensation. For the power and process steam needs an extraction (or pass-out) type and the back pressure type of turbine could be used.
Gas Turbine:
- Simplicity of layout,
- Low first costs,
- No cooling water requirements,
- Low auxiliary consumption, and
- Small space requirements.
The problems associated with gas turbines are:
- Low efficiency
- Sensitivity to change in ambient conditions, and
- Maintence problems.
Gas turbine are not generally suitable for use with heavy fuels. The more common fuels used by gas turbines are distillates and gas.
It is interesting to note that the low efficiency of the gas turbines, about 20% in small units, can be improved to levels as high as 80% by heat recovery and most of the recoverable heat is in the high temperature rigion.
Waste heat recovery boilers are essential components in co-generation systems employing gas turbines. These boilers are constructionally boilers and are available in both water-tube and fire-tube design. These boilers are typically of low pressure – drop type and thereby have large volumes.
Diesel Engine:
Diesel Engines are the most familiar type of prime-movers. The reciprocating are very versatile and can use a variety of fuels. The main advantages of these engines are:
- High efficiency.
- Easy maintenance (because of familiarity), and
- Good part-load behavior
The disadvantages being:
- High noise and vibration levels,
- High consumption of lube oil, and
- Frequent maintenance.
The type of reciprocating engines which could be consideration for the self-generation schemes because of the existing fuel cost structure, are gas engines and diesel engines based on furnace oil. The engine speed should be low, i.e. around 500 to 750 RPM to minimize wear and tear.
Heat recovery from the reciprocating engines is possible from the lubricating system, jacket cooling system and the exhaust. The actual amount would vary on the engine design and operating conditions. Most of the heat available is in the low temperature region. However, by the energy available for producing high temperature steam is low is further restricted by the limits on the exhaust temperature. These temperatures should be kept above 200 C to avoid the cold end corrosion. The pressure drop in the exchanger is limited to about 150 mm water column.
The major components of a self-generating power plant could generally be divided into the following heads:
- Prime-mover (for a steam turbine plants this would also include the boiler, condenser, etc.)
- Generator
- Fuel handling, treatment, storage and distribution system
- Auxiliaries (fuel system, lubricating system, intake and system)
- Control system
- Electrical substation and distribution network
- Building and civil works.
Facilities should also be provided for repair maintenance.
Conceptual Planning:
It must be realized that each self-generation scheme is unique in character and no general solution could be offered. The important parameters while considering a scheme are the unit size, installation size and operating strategy to yield the optimum results. In general it could be said that large units offer a better solution. The loads which are fairly constant, have small fluctuations with low variation between the maximum and the minimum, yield good results.
In general, planning for a self-generation project is quite involved and interactive. To give a basic idea the key steps are listed below:
- Preparation of an inception report; general survey
- Preparation of feasibility study
- Pre-qualification of suppliers
- Arrangements for financing
- Preparation of tender documents
- Scrutiny of bids
- Negotiation and award of work
- Installation contracts on the advice of main contractor
- Inspection and fitness testing of the main equipment
- Coordination with the supplier
- Construction with the supplier
- Construction supervision; punch listing
- Performance testing
- Pre-commercial operation
- Handing over to the owner
Application:
On account of uncertainty of supply and prolonged and regular load shedding, industries in the country started considering alternatives to overcome the problem and avoid heavy losses of production. A large number of industrial units acquired high speed diesel engines to provide emergency power. The alternative required low initial investment and was taken as an immediate solution to a rather major problem. However, long duration’s of load shedding, high cost of diesel and frequent maintenance of HSD engine proved that this was not the best of the solutions.
Some of the more enterprising industries went for complete self-generation which required more investment, initiative and dedication on their part to generate power. The idea again special importance in the eighties. Industrial unit in the general, having constant load were the prime candidates. Textile industry was ideally suited on account of consistent load and a very good load factor. The other industrial sector which benefited from this idea was chemical process industry.
On account of low price of furnace oil, diesel engines operating on furnace oil appeared as the most feasible prime-mover for the application. The steam turbine could also qualify but due to its generally high cost and lower efficiency of small sized turbines the option is only viable if there is a need of large quantities of process steam.
Problems in Self Generation:
Some of the typical problems identified in self-generation schemes are as follow:
- Maintenance of inventory for a long period, resulting in blockage of capital.
- Supply and quality of furnace oil.
- Availability of fresh water and its treatment.
- Government’s Policy.
These problems and some of the possible solutions are discussed in some detail below:
- Spares and Maintenance:
In countries where Manufactures have their own engine manufacturing plants or their Network offices with spares warehouse facility, the maximum time required to get a spare part is 24 hours, and therefore there is no need to keep large inventory of spares, especially the insurance spares.
In many cases the operators simply sign up a Services Contract assigning the maintenance of the engine to the representative or the local Network office of the Manufacture.
- Furnace oil Supply:
The supply, especially in the north, is rather erratic. This forces operators to go for very large storage tanks – usually recommended by oil marketing company to enough for 6 weeks is a large quantity fuel, thus posing a serious hazard.
Considerable capital is also tied up in these storage tanks. As an example, a 10MW plant shall have an estimated storage size (based on 6 weeks storage) of 2200 k1 with Rs. 13.86 million tied up (the current HFO cost is Rs. 6,300 per MT). The tankyard cost is also quit high.
In our opinion the storage should at the most be for two weeks, preferably smaller. The duty of storing fuel and supplying it to the user would, and quite correctly, shift to the oil supply companies.
This would result in several benefits in general.
These being:
- Smaller sums tied up (Rs. 4.66 million, instead of Rs. 13.86 million)
- Lower tankyard cost.
- Fire-fighting would becomes simpler.
- Reducing the risk of spillage, fire hazard, etc.
- Sabotage and war risk can be covered better.
It is found that the quality of furnace oil is quite inconsistent. This is because some of the oil is available from the local refineries (mainly NRL) and the rest is imported, mostly from Middle East. Middle East oil is often the product of cracking plants and contains considerable amount of metallic particles (catalyst) of very small sizes, which such oils may not cause harm to the boiler, they could be detrimental to diesel engines. Problems are mainly caused by metallic contaminants and the asphaltenes.
It is recommended that self-generation facilities be supplied with the products of NRL or some equivalent sources.
- Government’s Policies:
It is our belief that GOP while not discouraging small plants, encourages the private sector in setting up their own power plants, preferably in the size around 10-30MW.
The policies related to the import and furnace oil engines should be firmed up so that investors may plan without fear. These policies should cover:
- Import of equipment and spares.
- Cost of oil and lubricants.
- Tariff of WAPDA in buy-sell agreements.
To achieve this objective the following could be done:
Improve and continuously review the buy-sell tariff: The only tariff which is presently available for the buy-and-sell is J-2. This tariff does not meet the requirements of Schemes of the size suggested. Many industrial groups have their units located rather far apart. In such cases the idea is only feasible if possibility of “wheeling is adopted”. The generation unit should be able to sell power to the utility at one point and buy it at another point. Realistic wheeling charges can be applied to such clients.
Regulate furnace oil supply: quality and quantity.
Local manufacturing or local spares warehouse facility should be encouraged.
Local manufacturing of diesel generating sets and auxiliaries would improve the availability of spares and would support the diesel engine industry in general; possible beneficiaries of such a scheme could be the railways, the army, and the fishing industry.
Establish consultancy and guidance cell:
This cell should essentially work in liaison with experienced consultants who would ensure that the operators interested in self-generation follow the general direction which has been recommended by WAPDA; ensure proper selection of size and utilization of the equipment procured.
The cell should be able to provided technical advice on various aspects of operating in parallel with utilities. It should also maintain director and data of self-generators.
Arrange with financial institutions to back up projects, especially in those areas where WAPDA or KESC is under server pressure.
Seminars on awareness and operating experience should also be arranged.
About the author
The author heads one of the oldest. Consultancy firms in the field of electrical engineering in Pakistan. His firm – Jafri & Associates – one of the foremost names in the world of private generation and co-generation.
