A diesel generator (DG) set is a type of internal combustion (IC) engine that is mainly used in small industries requiring low captive power, or as a backup option to support operations. In recent years, technological developments in DG sets have been aimed at reducing noise and emission levels, and improving efficiency.
Focus on efficiency
There is a growing scope for technology improvements with the increase in the size of DG sets. This provides manufacturers with several options for improvements in engine design and geometry, and in the fuel control mechanism.
One of the key ways that manufacturers are increasing efficiency is through the development of large-sized DG sets with ratings of up to 5 MW for critical standby power and/or for continuous operations. With the introduction of modern, high efficiency turbochargers, it is possible to use an exhaust gas-driven turbine generator to further increase the engine’s rated output. The net result is lower fuel consumption per kWh and an increase in overall efficiency. A diesel engine is able to burn the poorest quality fuel oils, unlike a gas turbine, which is able to do the same with only expensive fuel treatment equipment. Slow speed dual-fuel engines are now available, using high pressure gas injection, which gives the same thermal efficiency and power output as a regular fuel oil engine.
Hybrids and multifuel technologies are emerging solutions for captive power plants to help improve reliability and efficiency, and achieve cost savings. Hybrid power plants combine at least two different sources of energy. These systems could have or not have a storage facility.
The most common hybrid technologies used are solar photovoltaic (PV) and diesel engines. Meanwhile, multifuel technology-based plants allow the use of biomass feedstock and coal interchangeably. In a solar PV-diesel hybrid system, the control system seeks to maximise the load on the PV system and minimise that on the DGset. In a plant with a battery set-up, besides meeting the excess load, the genset also recharges the battery if needed. This system is quite scalable and can be deployed from a few kW to a MW at least. System sizes can be expanded easily as per the requirements. Additional gensets can also be used. In this case, if adequate power is available from the PV array, some of the gensets may be completely shut down. Small- to medium-scale solar PV-diesel hybrid systems are an emerging option for captive generation purposes. The actual configuration of these systems varies as per the requirements, location, solar radiation levels, etc.
Solar-wind hybrids have also been installed for captive generation purposes, albeit on a very small scale and mostly for non-industrial applications where loads are lower. Telecom tower operators are also opting for hybrid solutions. The key advantages of hybrid power plants are cost efficiency (they reduce dependence on expensive diesel fuel), clean energy (facilitates the fulfilment of environmental obligations) and uninterrupted power supply (helps resolve intermittency issues, and ensure stable and high quality power supply).
Another way of improving engine efficiency is through complementing DG sets with waste heat recovery (WHR) systems, particularly in industries that use DG sets to meet most of their captive needs. A WHR system requires waste recovery equipment to recover heat from steam and transform it into a useful form for utilisation. This is done using energy conversion devices like regenerators, recuperators and economisers. By using a WHR system, energy consumption can potentially be reduced by 5-30 per cent.
Emission and noise control
The Central Pollution Control Board has been regulating emissions from diesel generators at the manufacturing stage (through product certification) since 2005. In March 2016, the Ministry of Environment, Forest and Climate Change (MoEFCC) notified new environment standards for gensets running on liquefied petroleum gas (LPG)/liquuefied natural gas (LNG) or diesel with LPG/ LNG, or petrol with LPG/LNG. These standards have been notified for the first time. In all the three fuel modes of operation, a three-tier classification has been adopted (see table). These standards will control the air and noise emission profile across different categories of gensets and would be revised every four to five years, once emission quality data and technological details pertaining to the gensets are available.
The notification specifies the maximum permissible sound pressure level for gensets with a rated capacity of up to 800 kW as 75 decibels (dB) (A) at 1 metre from the enclosure surface. It is also mandatory for DG sets to be provided with an integral acoustic enclosure at the manufacturing stage itself. The noise norms will be effective from January 1, 2017. The acoustic enclosure will be designed for a minimum insertion loss of 25 dB(A) or for complying with the ambient noise standards, whichever is higher. If the actual ambient noise is higher, it may not be possible to check the performance of the acoustic enclosure or acoustic treatment. Under such circumstances, the performance may be checked for noise reduction up to the actual ambient noise levels, preferably between 10 p.m. and 6 a.m. The manufacturer will have to offer the user a standard acoustic enclosure of 25 dB(A) insertion loss and a suitable exhaust muffler with an insertion loss of 25 dB(A).
These standards have mandated certification for gensets in terms of “Type Approval” and “Conformity of Production” for air emission as well as noise emission. Manufacturers are required to obtain the certification for engine products by the empanelled agencies, which will help in regulating the unorganised sector. This will help in curbing illegal imports of gensets, which are observed to have higher air and noise emission values.
The emission norms in India cover carbon monoxide (CO), nitrogen oxide (NOx), particulate matter (PM) and hydrocarbons, and are specified based on the number of grams of these compounds present in the diesel exhaust when 1 kWh of electricity is generated. The currently available technologies for reducing NOx emission include water injection, fuel water emulsification, delayed fuel injection, inlet air-cooling, intake air humidification, and changes in the compression ratio or turbochargers.
Selective catalytic reduction (SCR) is one of the most cost-effective and fuel-efficient diesel engine emissions control technologies available. Under this, a liquid-reductant agent is injected through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as the diesel exhaust fluid (DEF). The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), which are natural component of the air. These are then expelled through the vehicle tailpipe. SCR technology is designed to permit NOx reduction reactions to take place in an oxidising atmosphere. The technology is called “selective” because it reduces the levels of NOx using ammonia as a reductant within a catalyst system.
Conclusion
In order to make the new technologies available to consumers and promote operations and maintenance practices, regulatory and institutional interventions are required along with awareness programmes. In addition, in view of the government’s increased focus on emission control, genset manufacturers need to develop technologies that offer effective emission control and optimise fuel consumption.