Figure 1. Relative dust/particle sizes. Source: Donaldson Company Inc.
Erosion is a problem caused by large particles (usually 10 microns and larger) hitting and damaging the turbine blades. In extreme examples, unfiltered air can cause very severe blade erosion. In one case from the late 1970s, of a static air filter system with bypass doors that opened to unfiltered air in the desert during a sandstorm, turbine blades were almost completely destroyed. This is what can happen when no air filters are used.
Today, however, erosion can be avoided with even the most basic filter technology, and modern air filter options tend to focus on capturing the finer particles (5 microns and smaller) that lead to corrosion, compressor fouling and the plugging of cooling holes on the turbine blades.
Corrosion is caused where particles, chemicals, or vapours in the air react with a turbine blade’s metal properties and cause damage to the blades.
Comparison of filters taken from adjacent turbines which had run for the same number of hours, but with a different filter. The cleaner blades were behind the filter with the finer filtration capability – the one that could (and did) remove more of the particles in the 5 micron range that cause blade fouling
Fouling is the build-up of particles on the downstream side of the compressor blades, which reduces the efficiency of the turbine system as well as increasing the turbine’s heat rate (and thus fuel consumption).
Plugged cooling passages on the turbine blades are another problem that filtration can solve. Turbine blades can get extremely hot, which isn’t surprising since power generation is achieved using a combination of fuel and high-pressure air. To help cool the turbine, blades are designed with small air passages. Without proper air filtration to the turbine, fine particulates can collect in these cooling passages, contributing to higher temperatures and potential damage to hot section components. Although most turbines are equipped with sensors so the system will shut down if temperatures get dangerously high, the life of these components can be shortened if this is an ongoing problem.
The role for reciprocating engines
The final in our series of articles on industrial-scale CHP discusses the place in distributed generation for industrial sites for CHP systems based on reciprocating engines. Engines are well suited to meet intermittent and variable electrical loads, and may be the best choice when a limited amount of heat recovery is required. Continuing improvements to technology can only expand the role for engines, writes Michael A. Devine.
Global impetus for CHP began in the 1990s with the convergence of two forces. First, the electric power industry began to move toward competition and market-based pricing. That change helped drive investment in generating capacity away from capital-intensive, high-risk centralized power plants and toward smaller, less costly generating systems close to end power users. Many of these distributed power systems offer potential for CHP.
Three Caterpillar G3516 gensets burn natural gas to produce
2.9 MW of electricity, plus heating and cooling for the
Grancasa Commercial Centre in Zaragoza, Spain
Secondly, growing concern over global climate change, embodied by the 1997 Kyoto Protocol, forced national governments to look for ways to reduce emissions of carbon dioxide and other greenhouse gases. CHP inherently reduces those emissions by extracting more useful energy from fuel.
Seeing those developments, major engine manufacturers began developing high-efficiency, low-emission reciprocating engine-driven generator sets that drive down the costs to install and operate on-site power systems, thus making distributed generation and CHP easier to justify on cost grounds. In the immediate future, engine-driven generating technology is poised to contribute significantly to growth in CHP worldwide.
PROVEN GENERATING TECHNOLOGY
Reciprocating engines have been generating electricity for decades, and their use is widespread. Engines perform well in power generation because they are inexpensive to install and own, serve small-scale loads efficiently, perform reliably over a wide temperature range without de-rating and are easy to service and maintain. In CHP service, heat recovery from engine cooling and exhaust systems is relatively simple and economical.
Diesel and natural gas engine-driven generators serve a wide range of industrial requirements. Broadly defined, they include: