By Arvind A. Patel, P.E., Nelson Rosado, P.E, and Adam Redd, P.E
Today’s U.S. power market continues to shift away from large baseload centralized coal-fired generation for multiple reasons, including low natural gas prices. Renewable generation continues to rise and is becoming a significant portion of many power producers’ portfolios. In areas where energy from decommissioned centralized units is being replaced with renewables, the demand for small, fast responding, simple cycle, natural gas or dual fuel, peaking capacity is increasing. Reciprocating Internal Combustion Engine (RICE) technology has been gaining popularity as a primary power generation resource since it offers the flexibility to meet these needs. This article will provide answers to frequently asked questions related to this technology and identify a few consideration items.
|RICE Technology offers several advantages over combustion turbines, making them a very attractive option. Photo courtesy: MAN Diesel & Turbo|
Isn’t RICE a New Technology?
RICE technology dates back to the 1800s. Outside the U.S., RICE units operating on liquid fuel are commonly used for power generation with sizes ranging from several kilowatts to over 40 MW. In the U.S. power market, internal combustion engines have historically been limited to small emergency services such as backup generators or pump drives. Recently, medium speed natural gas or dual-fuel capable RICE units in the 9- to- 18-MW range have been gaining popularity for various applications.
What are the Common Applications for RICE Technology?
Power generators (utilities, municipalities, cooperatives, IPPs) as well as industrial users and institutions are considering RICE technology for various services. These services generally include:
Peaking Capacity: Provide peaking power at a high efficiency to reduce fuel costs.
Backup for Renewable Generation: Power output from renewable resources can vary significantly over a short period of time. A fast responding highly efficient resource that is also capable of starting/stopping frequently is required for this load following application.
Black Start Capability: Small RICE units have historically been utilized for emergency backup generation. Larger units may be utilized to black start a facility since they rely on few auxiliary systems to support startup.
System Regulation / VAR Support: With large rotating (coal) generators being removed from service and more renewable (PV) based power flowing through the transmission system, RICE units can be a cost effective option for regulation / VAR support.
Distributed Generation / Microgrid: This application relies on the generation being located close to the load. The loads are generally small, ranging from hundreds of kilowatts to about 100 MW. The small size and modularity of RICE units makes this an attractive technology for this service.
Cogeneration/Combined Heat & Power (CHP): The waste heat from RICE generation units can be recovered to produce either hot water and/or steam. While RICE CHP applications are common in the international market, the US is expressing increased interest in this area.
RICE or Combustion Turbine, Which Technology?
RICE technology offers several advantages compared to combustion turbines making them an attractive option. This is especially the case for simple cycle facilities under 225 MW that will be relied upon for peaking and/or ancillary services. However, each application is unique and needs to be evaluated holistically on a case by case basis. While Table 1 provides a high level comparison of RICE and combustion turbine technologies, additional key factors such as permitting, expected hours of operation, life cycle costs, market conditions, etc. also need to be considered during the selection process. Additional information is provided in the following sections.
|RICE vs. CT & Cost Trend|
Heat Rate: Figure 1 shows nominal output vs heat rate for commonly used combustion turbines in today’s market. The purple squares represent nominal 25 – 100 MW aeroderivative CTs used for peaking applications while the blue diamonds represent frame turbines. It can be seen that while RICE units range from 9 to 18 MW, the full load heat rate is in the range of 8,100 – 8,700 BTU/kWH which is better than most CTs.
|Heat Rate Plot|
Performance – Flexibility, Response Time, Turndown: As renewables become a larger portion of baseload generation, there is an increasing demand for reliable, dispatchable power that not only can be placed online quickly, but can also be started and stopped frequently due to changing load conditions.
RICE units are capable of ramping up to full load in less than 5 minutes and are able to operate at about 33 percent of their nominal rating without compromising heat rate. This technology also does not incur a penalty for frequent starts/stops since maintenance cycles are based on hours of operation. Combustion turbines generally ramp at a slightly slower rate (10 – 15 minutes) and can turn down to about 40 percent of their rated output, however, heat rate is compromised.
Due to their smaller size, multiple RICE units would be required to achieve the output of a single large CT; for example 5 x 18 MW RICE vs 2 x 45 MW CT. In this example, the minimum output from this RICE facility would be about 6 MW whereas the CT facility would be 18 MW. While the use of multiple RICE units may be advantageous for turndown, reliability, redundancy, system response time, heat rate, etc. capital and O&M costs need to be evaluated to determine which technology is most cost effective for the application.
|The Fairmont Energy Station is a 25-MW project equipped with four Cat G16CM34 generator sets. The plant was commissioned in 2014. Photo courtesy: Caterpillar Inc.|
Sensitivity to Ambient Temperature: The power output of a combustion turbine varies significantly with compressor inlet temperature. As the ambient temperature increases above ISO conditions, power output decreases. To counter this effect, common industry practice it to utilize an inlet air cooling technology (ie evap cooler, chiller, wet compression, etc.) to reduce the compressor inlet temperature and maximize performance. As a point of reference, in a dry desert climate (about 105F), combustion turbines can experience a 15 percent to 20 percent reduction in output relative to ISO conditions.
Reciprocating internal combustion engines are less susceptible to changes in ambient temperatures. The output vs temperature correction curve for RICE is essentially flat and reduction in power output generally starts to occur when the ambient temperature climbs above 105F. For this reason, inlet air cooling is not utilized on RICE units.
Water Consumption: As previously stated, combustion turbines typically utilize an inlet air cooling technology to increase output. These technologies not only consume water, but also have stringent requirements for water chemistry. Water treatment systems are required to produce, store, and pump the process water. Depending on the supply water quality, treatment system, and type of inlet air cooling technology utilized, a process waste water stream may also be generated requiring disposal. The addition of this infrastructure can significantly increase project costs.
RICE applications do not require inlet air cooling, therefore water usage is significantly reduced. Additionally, air cooled heat exchangers (ie radiators) are typically utilized to reject heat from a closed loop cooling water system. Since this system only requires periodic make-up (similar to topping off the radiator in an automobile) permanent water treatment systems are not required.
Minimum Fuel Gas Pressure: Whether it is CTs or RICE, each OEM specifies the minimum required fuel gas pressure for their product. The required natural gas supply pressure for combustion turbines can range from appx. 400 to over 900 psig. A soft correlation also exists where the units with the lower heat rates (better efficiency) require higher gas pressures.
RICE technology requires gas pressure in the range of appx. 75 – 150 psig at the power island. The lower fuel gas supply pressure requirement becomes very attractive in areas where the guaranteed supply pressure is low. Adding on-site compression or contracting a higher guaranteed pressure from the gas supplier can significantly increase overall project costs.
Noise Considerations: Combustion turbines are typically equipped with enclosures which provide sound attenuation. These enclosures may also provide equipment protection allowing CT units to be installed outdoors if required.
Compared to combustion turbines units, RICE units are quite loud with typical sound levels exceeding 110 db. For this reason, it is common practice to locate RICE units inside of a building (engine hall) to provide sound attenuation. Although much larger in size, the engine hall also serves to protect the equipment similar to the CT enclosures. If RICE units are to be installed near sensitive noise receptors, serious consideration should be given to performing a noise survey to determine what measures need to be taken to reduce noise emissions. While noise from the engine systems may be mitigated by the building design (siding, insulation, etc.), exhaust and/or inlet silencers, low noise radiators, and other reduction features may also be required to reduce overall noise emissions. The engine hall, supplemental systems (ie. HVAC, lighting, maintenance crane, etc.), and noise reduction features can significantly impact the total installed cost of a RICE facility.
What Does the RICE OEM Scope of Supply Include?
The scope of supply can vary based on a project’s needs and overall contracting approach. In general, the RICE OEM typically provides the following: RICE gen-sets & auxiliary skids, radiators, emissions control systems, ductwork, stack, silencers, starting system, switchgear, control system, and integral platforms/supplemental steel. The owner (or EPC) is responsible for providing the buildings (engine hall, administrative offices, warehouse, maintenance shops, etc.), HVAC systems, main power transformer, site development, foundations, utilities, commodity materials, and installation.
How Much Does a RICE Facility Cost ($/kW)?
For the same size area developed on a project site, RICE units provide less power per square foot (power density) compared to other technologies. For this reason, the cost of a RICE facility is very sensitive to the MW (number of units) installed.
Figure 2 shows a trend for MW Installed vs Installed Costs ($/kW) for RICE facilities utilizing 9 MW units. The installed cost on a $/kW basis decreases significantly as additional units are installed. After appx. 50 MW (ie. 5 – 6 9MW units), the slope of the curve starts to level off as the economy of scale benefits are realized. For small facilities (appx. 27 MW and less), costs associated with site development, balance of plant, construction, etc. can range between 65 and 75 percent of the total cost. In comparison, for larger installations (about 80 MW and larger), these costs are between about 40 and 50 percent.
|How Much Will it Cost?|
Larger 18-MW RICE units are gaining popularity in today’s market. Although not shown, for these units, a similar trend exists, though there is significantly less sensitivity (flatter curve) in the 18- to- 36-MW range. After 54 MW, the curve starts to flatten in a similar manner as that for the 9 MW units.
The estimated capital cost of a facility must take several factors into consideration which include but are not limited to; equipment costs, estimated material quantities, site location, site type, noise restrictions, market conditions, labor rates, constructability, indirects, contingency, etc. These factors can significantly impact the cost of a RICE facility and therefore project specific estimates should be developed in lieu of relying on industry benchmark data.
To provide some points of reference, the installed cost of a 2 x 9 MW RICE facility can range from $1800 – $2700/kW while a 12 x 18 MW RICE facility can range from $990 – $1400/kW. American Public Power Association indicated in a 2016 report the cost for new CT and RICE based capacity as $854/kW and $1,496/kW respectively. The US Energy Information Administration (EIA) released a report in April 2013 titled “Updated Capital Cost Estimates for Utility Scale Electricity Generating Plants” showing $973/kW for an 85MW Conventional CT while RICE technology was not identified. In the same report, EIA also stated “It should be noted that all estimates provided in this report are broad in scope. A more in-depth cost assessment would require a more detailed level of engineering and design work, tailored to a specific site.”
|South Texas Electric Cooperative’s Pearsall Power Station, equipped with 24 Wartsila 34SG engines. Photo courtesy: Wartsila.|
What Else Do I Need to Know About RICE Technology: As previously stated, the determination of RICE vs CT technology needs to be evaluated on a case by case basis since each application is different. While RICE may provide advantages to CTs in specific applications, there are several non-obvious items that a potential owners/operators should be aware of. A few of these items are discussed below.
European Content: While US suppliers offer RICE technology, the engine-generator sets in the 9- to- 18-MW range for power generation facilities are manufactured in Europe. For this reason, the use of European codes, standards, design philosophies/practices, and material supply must be allowed to some degree.
Auxiliary Power: RICE units are capable of ramping up to full load in less than 5 minutes. To achieve this, the engines need to be kept warm during standby (not operating). This is typically accomplished by utilizing electric heaters in the cooling water system to maintain temperature. The electrical load can account for about 1 to 3 percent of the total auxiliary power consumption. Depending on the location of the facility, the expected dispatch, the amount of time the units may be in standby, etc. this cost should be taken into consideration when evaluating technologies.
Air Permitting: While the reciprocating engines can ramp up quickly to provide power (about <5 minutes), the emissions control equipment on the back end is much slower to respond. Depending on the hours of operation, installation location, time between starts, etc., the SCR systems may require significantly more time to reach operating parameters. During this period, air emissions are not controlled to optimum levels. The facility’s permitting process needs to take this into consideration so emissions limits do not restrict the availability of the flexible resource.
|Sky Global One, a 51-MW gas-fired plant west of Houston, began commercial operation in April 2016. The plant features six 8.6 MW Jenbacher J920 FleXtra gas engines from GE. Photo courtesy: GE.|
Dual Fuel Units: Most dual fuel RICE units in the US are required to operate on natural gas with ultra-low sulfur diesel as the backup fuel. When operating on natural gas, a small amount of the liquid fuel is also consumed. The technology selection process and facility design need to take this into consideration.
Summary: While RICE technology is not new, the application of utilizing multiple 9- to- 18-MW units as the prime mover for a generation facility is fairly new to the US power industry and gaining popularity. The benefits of a fast responding, high efficiency, flexible, dispatchable facility that requires low fuel gas pressure and minimal water make RICE units a very attractive resource. However, due to the low power density, the installed cost ($/kW) of a RICE facility is very sensitive to site specific factors. Each application is unique and must be carefully evaluated holistically to ensure the benefits of RICE technology can be realized cost effectively.
All three authors work for Sargent & Lundy LLC. Arvind Patel is a vice president and project director for RICE Projects & Business Development. Nelson Rosado and Adam Redd are managers of RICE Projects.