Carbon Capture and Storage
Ramgen’s Low-Cost, High-Efficiency CO2 Compressor Technology
CO2 compressors represent a large fraction of the enormous capital and operating cost penalties of any CCS system. The CO2 compressor power required for a pulverized coal power plant with an amine-based capture is approximately 8-12% of the plant rating, depending on conditions. A 1,000-MW PC plant would require 120 MW, or 160,000 hp, at an estimated $180 million for today’s 3 x 50% configuration. The ammonia-based processes consume about 1/2 of the power at less than ½ the cost, but still represent a considerable expense.
The CO2 compressor power required for an IGCC power plant is approximately 5% of the plant rating. A 600-MW PC plant would require 30 MW, or 40,000 hp, at an estimated $45 million for the same 3 x 50% configuration.
Both of these values are based on current estimates of the “state-of-the-art” integrally geared turbo compressor at nominal discharge pressure of 2200 psia, and do not include installation costs at 35 to 50 percent increment.
Ramgen’s shock compression technology represents a significant advancement in the state of the art for many compressor applications, and specifically for CO2 compression. The principal advantage of Ramgen’s shock compression is that it can achieve exceptionally high compression efficiency at very high single-stage compression ratios, resulting in a product simplicity and size that will lower both manufacturing and operating costs.
The novel Ramgen technology concept addresses the two greatest objectives identified by the Department of Energy for the Capture and Storage of CO2 – lower costs and better efficiency.
Ramgen’s Super-Sonic Shock Wave CO2 Compressor
Ramgen Power Systems, Inc. received a 4-year $11.0 million DOE grant to begin the development of a CO2 compressor capable of developing the required 100:1 pressure ratio in two stages of compression.
Ramgen based its original design concept on a two-stage, integrally geared compressor, with a common driver, following the pattern of the conventional designs, but with two stages instead of eight. Figure 1 shows a drawing of one such candidate configuration, along with a dimensional scale to indicate its approximate size for a 10,000 hp unit. The final specifications of the system will be defined as a deliverable of the program.
The reason that existing CO2 compressor designs are so expensive is, in part, because the overall pressure ratio is 100:1, and, in part, because CO2 requires stainless steel construction in the presence of water vapor. But by far, the most significant impact on cost is an aerodynamic design practice that limits the design pressure ratio per stage on heavier gases such as CO2.
Standard turbomachinery design practice is to limit the inlet flow Mach number to less than 0.90 at the inducer blade tip to avoid generating shock waves in the blade passages and their accompanying losses. This is typically done by adjusting the stage speed. The Mach number itself is a function of molecular weight and therefore the effect is more pronounced on the heavier-than-air CO2. This inducer blade tip speed limit results in a pressure ratio per stage limits of approximately 1.7 to 2.0:1 on CO2. At these stage pressure ratios, eight stages of compression are typically required to reach an overall pressure ratio of 100:1
MAN Turbo 10-stage 200:1 CO2 Compressor
This issue is further complicated by the need to intercool the CO2 between each compression stage. The heat of compression discharge temperature associated with these very low stage pressure ratios is approximately 200°F, which, as an inlet to the next stage, is too hot to achieve good efficiency, but lacks the thermal driving force for cost-effective heat exchanger selection. This heat is also of insufficient quality to be of practical use elsewhere in the process. The only option is to reject virtually all the compressor electrical input power through intercooler heat exchangers and supporting cooling towers, themselves a significant capital and installation expense.
Intercooler selection is made even more difficult by the need for low-pressure drop designs and the requirement to use low heat transfer effectiveness 304 corrosion-resistant, stainless steel construction. Air cooled heat exchangers, often required in arid climates, exacerbate the problem with their generally lower approach temperatures and require substantial fan horsepower, often overlooked in the compressor power evaluation.
Ramgen, on the other hand, designs its rotors to create and manage shock structures and can realize the full effect of shock waves to generate substantial pressure ratios, efficiently. The proposed Ramgen CO2 compressor concept would achieve the required 100:1 pressure ratio in two stages of compression, each rated at 10:1 (10 x 10 = 100). This configuration would feature a conventional intercooler between the first and second stages as well as an aftercooler, if required by the application. In the CCS application, it is likely and preferred that the capture process itself would become the intercooler and aftercooler.
Ramgen intends to offer its compressors at approximately 40-60% of the price of the current integrally geared, state-of-the-art designs.
In addition to the obvious cost advantages and as a direct result of the Rampressor being able to achieve single-stage compression ratios of 10-12:1, stage discharge temperatures are estimated to range between 450-500°F, depending on inlet gas and cooling water temperatures. This offers the potential for significant heat integration, without compromising compressor performance. The combined compressor and heat recovery creates an even more impressive energy efficiency advantage by recovering 70-80% of the electrical input energy in the form of useful heat. Potential uses for the available heat are to regenerate amine solutions or pre-heat boiler feedwater.
Ramgen’s Discrete-Drive Single-Stage Configuration
Ramgen Discrete-Drive HP Stage
At a minimum, this discrete-drive approach allows better matching of each stage to its specific process flow, including side-streams. In addition, the inherent variable speed capability of these optional drive systems provides operational flexibility that the developers see as very desirable. It should be noted that the inlet conditions to the first stage of compression are fixed by the process and relatively constant. The Inlet conditions on the subsequent stages are a function of the cooling medium, itself driven by the changing ambient conditions.
In simplistic terms, the first stage wants to run at constant speed, and subsequent stages before the next capture system flash level will want to run in a variable speed mode. The compressor, however, must be selected to meet the hot condition requirements and forcing multi-stage designs to operate at the same speed will drive one or the other off its optimum efficiency.
The Ramgen baseline drive utilizes a conventional low-speed motor and simple, single-step external speed-increasing gearbox, with other variations available to suit customer and contract requirements. High-speed permanent magnet motor drives or steam turbines are both feasible direct drive alternatives.
Ramgen intends to leverage high-speed motor development to the greatest extent possible by matching our rotor speeds and power requirements to existing high-speed motor designs and developments. Direct-drive steam turbine configurations would also offer the same degree of operational flexibility, and in addition to providing another opportunity for heat recovery. This product simplification with the use of existing driver systems substantially reduces development cost and risk.
This single-stage discrete drive approach also substantially reduces
Ramgen’s development cost and time. This schedule acceleration and
design simplification will also allow Ramgen to submit proposals to participate
in a number of the emerging opportunities to supply field demo units.
The first is that production times will not be dependent on the very limited and long lead time supply of precision, high-speed AGMA class 12/13 gearing, a chronic supply chain constraint.
The second is that discrete drive and integrally geared designs are rated differently. Geared units are nominally power rated, based upon predefined gear center distances and fixed design speeds.
Single stage units are rated on rotor diameters. The operating speed of a single-stage unit is adjusted within mechanical limits to meet the head or pressure required and the frame size is determined by the capacity required at that operating speed.
The important understanding is that as the stage is driven faster, it produces significantly more flow, and the rotor size can be reduced for a given capacity required. In practical terms if the HP stage is rated to the full 2200 psia nominal discharge pressure, the rotor will run faster and is likely to be two sizes smaller than it would be if it were rated at 1200 psia. The associated first cost savings is significantly more than the cost of the cryo-pump that would be required to boost the pressure from 1200 to 2200 psia.
Successful Ram 2 Test
The DOE has reviewed these test results and has authorized Phase II work to begin on a CO2 specific design, suitable for pilot-scale field demonstration.
Of considerable importance is that the design tip speed of the 10:1 pressure ratio CO2 compressor rotor is sufficiently low so as to allow for a shrouded rotor design concept, which eliminates the tip clearance and associated tip leakage effects and greatly simplifies the mechanical design.
Dresser-Rand Invests in Ramgen
Dresser-Rand is recognized by many scientists and engineers as the leading compression technology company in the world and they are the ideal partner for Ramgen in completing the development of its supersonic shock wave compression. Dresser-Rand has the credibility and capability required for the scale of roll out that will be necessary to make a difference with CO2 emissions and climate change.
Dresser-Rand will bring its considerable resources to bear on “productizing” the Ramgen technology. Of particular interest is that Dresser-Rand does have a large scale test capability that could support testing of a commercial scale unit. D-R is also a world leader in the supply of steam turbine drives which are of considerable interest as prime movers for the CCS compressor application.
Ramgen Development Plan
The plan also includes specific efforts to characterize the CO2 and its behavior in the supercritical regime in a series of static test rigs that are the equivalent of wind tunnel testing. These tests will also validate the impact of impurities contained within the gas, depending on application. Several of the key industry users have offered to assist us in identifying the range and variability of these components in their applications
At the same time, Ramgen will be working with CC&S System developers to maximize heat integration with their respective processes. To date the CC&S developers have been designing their processes without much insight on compressor cost and performance. Most are now beginning to realize that a cost effective CO2 compressor is key to their success and Ramgen is supporting their efforts to lower cost and improve efficiency.
The high-pressure ratio per stage capability of the Ramgen technology is the key enabling capability necessary to achieving this goal.
That HP-16 would scale to an HP-24 to cover 500-600MW depending on assumptions. The matched LP stages, should they be required, would range from 24 to 40 inches.
We have investigated single train units for 800MW plants and consider them both feasible and within our capabilities to scale and deliver on the same commercial terms and schedule.