Natural Gas Spark Ignition Engines…what, why, when, and where?

Issue 1 and Volume 116.


By Todd Shaw, Business Development Manager, Mitsubishi Power Systems Americas, Inc.

The reciprocating internal combustion engine is a well proven and familiar technology with decades of use for stationary power generation. Unfortunately, familiarity occasionally results in assumed understanding, and potential misunderstanding as technological advancements are made.

Case in point, a search of “Electricity Generation” on Wikipedia reveals an article that includes a reciprocating engine discussion best summarized by the select phrases “small electricity generators are often powered by reciprocating engines”, “reciprocating engines…are often used for back up generation,” and “large power grids also use (engines)…to feed power during certain circumstances.”

Although the free encyclopedia that anyone can edit is occasionally off-point, in this circumstance it appears to be in agreement with the perception shared by many experienced professionals. In their defense, there are a couple of reasons for the misperception including historical engine applications (stand-by power for example) and the significant increase in electricity consumption over the past 10 to 15 years which led to a focus on large capacity technologies with high power density. In truth, however, there are many market applications that are well suited for the flexibility, reliability and maintainability afforded by today’s modern reciprocating engine.

Before discussing engine opportunities, let us first define “what” from the title of this article. The type of reciprocating engine that forms the basis of this discussion is a 60 Hz medium speed natural gas-fueled lean burn spark ignition (SI) engine (hereafter Engine). This technology operates between 300 rpm to 1,000 rpm, depending on engine size and manufacturer. A typical 60 Hz Engine operates at 720 rpm and delivers full load capacities from approximately 3,650 kW to 8,500 kW; at present some engine manufacturers are introducing even larger newly developed engines. In comparison high speed engines operate between 1,000 and 3,600 rpm and offer continuous duty capacities up to nearly 2,000 kW.

Another significant difference between medium and high speed engines is simple cycle efficiency. Published simple cycle efficiencies for high speed engines range from approximately 35 percent to 40 percent, whereas simple cycle efficiencies for lean burn Engines range from 45 percent to 48 percent +, depending on engine size and manufacturer (efficiencies based on ISO 3046).

“Why” select an Engine? In addition to high thermal efficiency, there are a number of factors including operational flexibility, increased availability, and simplicity of maintenance. Flexibility is achieved by operating an Engine (or a block of engines) at either full or partial load. For example, the Mitsubishi Model 18KU30GSI engine, which has a generator output of 5,500 kWe, is designed for continuous operation at or above 45 percent load. When installed as part of a 10 unit power plant, this provides the Operator with a continuous power output range of 2.47 MW to 55 MW. Operator flexibility is afforded by the timing of when to fire units 2 – 10 based upon load requirements for a specific time of day or season. Considering that typical time from initial engine start to full plant load is <10>

As in most investment decisions, Life Cycle Cost (LCC) is often the main driver for selection. A good rule of thumb for “when” an Engine(s) may be the appropriate solution occurs when planned operation exceeds ~1,400 hours annually (i.e., an availability of 16 percent or greater considering 8,760 hours). Buyers should note fuel costs also follow engine performance at higher altitudes/hot weather. Engine heat rates, unlike combustion turbines, are minimally impacted by high/hot conditions – thus lowering LCC.

Finally, our question of “where” highlights current projects specified for 100 MW or less of generation capacity. Recent market trends demonstrate interest in the use of engines for cogeneration, demand side management, curtailment, and grid stabilization requirements produced by increased renewable system operations. Both regulated and non-regulated utility markets require wind firming and load following capability. In the West and Midwest the desire to produce 100 to 150 MW engine blocks is expected to also continue. In the near future, look for growth in the area of distributed generation; a very cost effective solution for load balancing and transmission line constraints. Buyers should also be aware of regulatory benefits for <25>

Considering the modularity, efficiency, and thermal off-take associated with today’s modern high efficiency Engines, it is clear the medium speed SI engine is well worth consideration when developing today’s generation portfolio.

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