Anax-Star Turboexpander Guide

The Anax-Star Turboexpander® “A clean energy solution from Anax Power” Broadway Plaza, 350 Broadway, Suite 350, Santa Monica California 90401

Anax-Star Turboexpander Guide

Clean Energy from Pressure Reduction There are streams of energy all around us, embedded in everyday functions and utilities that we take for granted. For all the good that solar panels and wind turbines provide, these technologies have two significant drawbacks: they are intermittent and expensive. Energy from waste also comes in many forms, such as biomass burning, or high temperature heat reclaim for thermal generators. These technologies, too, have downsides. Biomass requires huge quantities of combustible material, generating high carbon emissions. Thermal generators, such as the Organic Rankine Cycle, require the waste heat at relatively high temperatures, and have a low heat to electricity conversion ratio. Now there is a new approach to renewable energy that neatly steps past all of these downsides, drawbacks and disadvantages. It provides a way to recover energy with a high conversion ratio, improving the efficiency of our natural gas supply pipelines.

The Anax-Star Turboexpander®(ASTE) is a proprietary technology that is based on well-

established scientific principles, integrating world-leading expertise in several fields to deliver clean energy from waste.

The ASTE combines natural gas pressure reduction with low-grade waste heat to convert the enthalpy reduction in the pressure let-down to grid-ready electricity. The gas is delivered to the downstream users at the right pressure and temperature, while the waste heat is absorbed back into the gas

stream. The electricity output is available for use locally or for export to the distribution grid at renewable energy rates.

Clean Each ASTE package configured for waste heat recovery will generate up to 6 MWh per day with zero carbon emissions. This level of electrical generation could be worth over $250,000 per year if sold to the grid or could be used to offset site electrical consumption to avoid the need for a larger electrical service equipment when processes are being expanded. Putting waste heat back into the gas supply also reduces heat emissions from the site and can reduce site water consumption by up to a ton (2,000 pounds) per hour.

Cool The ASTE system does not need the waste heat to be any warmer than 150°F – a much lower temperature than most ORC systems which, which need the heat to be available at a minimum of 300°F, if the system is to be cost effective. Using low-grade waste heat allows the ASTE to be used in more systems than if it were to require warmer heat. The ASTE’s heat transfer system for cooling the inverter’s power electronics also allows the efficiency losses from the electronics to be recovered and fed back into the gas network.

Compact At the core of the ASTE is a two-stage turboexpander running at speeds up to 25,000 revs per minute. This high speed enables the turbine wheels to be smaller than normal, with a diameter of less than 6” each. The turbines use patented active magnetic bearings which are oil-free and are smaller than any comparable load-bearing technology. To capitalize on wheel’s size the generator uses world-leading permanent magnet technology, packing a 350hp generator into a module no bigger than a squeezebox. The system uses back-to-back plate-shell heat exchangers to feed the waste heat into the gas stream in two stages. Finally, the entire 250 kW skid is only seven feet wide, seven feet high, and thirteen feet long, capable of fitting into a standard intermodal shipping container.

Variable The combination of a permanent magnet generator and ultra-high frequency inverter enables the ASTE to operate across a wide range of inlet pressures without losing stability. This stability solves a major obstacle to turbines achieving widespread acceptance in gas pressure reduction applications; the need for stable inlet and outlet conditions in order to deliver at the correct frequency. The gas distribution system uses a wide pressure variation to buffer stocks of gas within the system, so the inlet pressure to the gas regulating system can be highly variable. The ASTE rides these variations with ease.

Versatile The ASTE is sealed within a hermetic pressure casing and can stand inlet pressures up to 975psig, enabling it to be used in the main backbone of the gas distribution network. The high pressure variant is optimised for an outlet pressure of 450psig with a nominal inlet pressure of 750psig (but capable of varying between 900psig and 600psig). There are also two medium pressure variants of the ASTE. The high ratio version is optimised for an inlet pressure of 350psig and outlet pressure of 80psig and the low ratio version is optimised for 300psig inlet and 180psig outlet.

Adaptable One of the unique benefits of the ASTE package is that it has been designed to suit a wide variety of applications by adding one of the auxiliary heat packages alongside the basic unit. If waste heat at 150°F is not available to power the energy conversion process, then a gas engine can be deployed to provide a suitable source of heating together with some auxiliary power generation. The gas engine can be a recip engine or a microturbine CHP unit. Alternatively, if the ASTE is used on a gas regulating station, for example in a citygate application, then a gas-fired reheater can be used to provide the necessary reheat in the expansion process. Heat is applied to the ASTE before the first stage of expansion, heating the incoming gas to 130°F and between the two stages, reheating the gas to the same level to ensure that the outlet from the package is never lower than 50°F.

Strong The ASTE package is assembled on a single steel skid, making it easy to transport. The frame is epoxy painted for long-life corrosion resistance. All of the unit gas and water pipes are welded steel, also epoxy painted and the wetted parts of the heat exchangers are corrosion-resistant 316L stainless steel. The ASTE casing and the heat exchangers are designed in accordance with ASME VIII, with all welding to ASME IX. The package unit is designed to be installed outdoors and can even be placed in hazardous locations (Class 1 Division 2 Group D Temperature Class T3). All skid mounted equipment meets this requirement as a minimum, including the control panel. Electricity generated is metered via a revenue grade meter. The control panel is manufactured in type 304 stainless steel and conforms to NFPA 496: Control Panels for use in Hazardous Locations, Class 1 Division 2 for Purged and Pressurised Enclosures. The frame and pipework are designed to be suitable for installation in seismic zones 1 – 4. Safety valves and control valves meet the requirements of the American Petroleum Institute’s standard API617, including the safety valves in the water cooling circuit. The complete package is certified to UL1741 and is suitable for grid connection.

Secure The ASTE incorporates a sequence of safety devices to protect the unit from off-design conditions. The expander has a bypass valve to handle excess gas flow and to equalise pressures in the event of an emergency stop. Each heat exchanger is fitted with a pressure relief valve, piped to atmosphere. The electrical circuits are protected against power isolation with an anti-islanding relay and the inverter includes an integral speed regulator to monitor the turbine speed and ensure that it does not exceed safe limits. In the event of an inverter fault, the system will shut down using dynamic braking resistors and a bypass valve to bring the turbine to rest quickly and safely. The bearings are designed to bring the shaft to rest safely in the event of a sudden loss of power to the magnetic bearings.

Smart The unit control system incorporates a full diagnostic suite which monitors key parameters such as temperature, pressure, shaft speed, electrical output and bearing function and posts warning, alarm and shutdown alerts. The unit records electrical output and can provide a metered record of performance by remote connection. The control panel is equipped with a touch screen interface panel which provides access to real-time operating parameters, performance trends, control setpoints and instrument calibration.

Capacity

Power Output

Inlet Pressure

Outlet Pressure

Inlet Temp

Outlet Temp

Preheat Input

Reheat Input

Gas Flow

Gas Flow

Heat Conv

PR

kW psig psig °F °F MMB/h* MMB/h* scfm lb/min % - High Pressure Range

250 750 200 60 59 0.71 0.76 6711 294.5 58.0% 3.56 250 750 325 60 82 1.09 0.81 10274 450.7 44.9% 2.25 250 750 450 60 108 1.73 1.11 16330 716.4 30.0% 1.65

Medium Pressure Range 250 350 80 60 58 0.60 0.67 6162 270.3 67.2% 3.86 250 325 130 60 83 0.92 0.71 9463 415.1 52.4% 2.35 250 300 180 60 100 1.59 0.74 16489 716.4 36.5% 1.62

Low Pressure Range 250 180 40 60 62 0.61 0.63 6433 282.2 68.7% 3.57 250 150 60 60 85 0.94 0.66 10047 440.8 53.4% 2.21 250 120 80 60 110 2.05 0.82 21917 961.5 29.8% 1.42

*MMB/h is one million BTU/h. 1MMB/h = 293kW of heat The table shows typical performance criteria for three pressure ratings of the ASTE, with three pressure ratio bands for each range. All models within the three pressure ranges are based upon a common generator and shaft platform but the turbine impellor is custom-designed for the operating range to ensure that the expander operates at optimal efficiency in that application. This is achieved with waste heat at 150°F. The heat conversion percentage shows the amount of waste heat that is converted to useful electricity. An Organic Rankine Cycle system with heat available at 250°F would only achieve a heat conversion percentage of about 12.5% and would require a cooling tower or other cooling system to dispose of the residual heat once it had been processed. In contrast the ASTE converts far more of the heat – between 30% and 70% depending on the operating conditions – to electricity. The remaining heat is used to warm the gas flow passing through the regulating station, preventing condensation and protecting pipes from low temperature embrittlement. None of the heat recovered is rejected to atmosphere through cooling towers. In fact the ASTE provides a profitable way to dispose of excess heat from combustion processes, gas engines or kilns without the high capital and operating costs of running cooling towers.

Principle of operation Gas from the upstream supply network passes through a pre-heat exchanger which raises the gas temperature to 130°F. The gas passes through the first stage of expansion, dropping the pressure to an intermediate level and converting some of the thermal energy to electricity and cooling the flow. Next, the gas passes through a secondary re-heat exchanger, raising the gas temperature back to 130°F.

A second stage of expansion brings the gas back down in temperature, extracting electrical energy and lowering the pressure to the necessary outlet value. The low pressure, warm gas is fed into the downstream distribution network. Variations in inlet gas pressure are handled automatically by the control logic which varies the turbine speed to match the gas flow requirements and maximises the recovered energy. If the demand for gas is higher than the flowrate provided by the ASTE then a bypass valve opens to direct additional gas to the downstream network. If the pressure differential across the pressure let down station is too high, leading to an electrical overload condition, then the inlet regulator valve will modulate to maintain the optimal operating pressure differential across the machine. This combination of inlet regulation, bypass and active control of the heat input enables the unit to operate over a wide range of inlet and outlet conditions.

Worked Example For a gas-driven piston engine the supply is at 350psig and the gas is delivered to the engine set at 80psig. If we take the supply temperature as 60°F then we can add about 1.30 MMB/h of heat to the gas supply in order to recover 250kW of useful electrical output power. In a gas engine-driven power plant this heat would come from the cooling jackets of the engines. The plant would otherwise require a cooling system to get rid of the heat and stop the engines from overheating. A gas flow of 6100 scfm (which is about 2kg/s, or 265lb/min) would be sufficient to deliver the 250kW output, which means that four ASTE units could be deployed on a site with ten generator sets each delivering 16MW of power, generating a further 1MW by capturing the waste heat from the engine jackets and feeding it back into the gas stream. This would raise the plant efficiency by 0.625%. If the additional power is sold to the grid at a renewable rate of $0.14/kWh then the four ASTEs would provide income of over $1m per year, assuming a utilization of 8,000 hours per year.

ASTE Hybrid If the ASTE installation was applied to a site which was a major user of gas but did not have any available waste heat, heat could be provided by running an auxiliary generator. One 250kW ASTE would require input of 1.3 MMB/h of heat. Assuming that the unit delivered an electrical output of 35% and heat output of 45% (of the total gas input), a 295kWe CHP unit would then be required to augment a 250kW ASTE. The total gas consumption of the combination system would be 45.7 scfm, which is 0.75% of the gas flow through the expander. This ASTE Hybrid combination would provide 545kW of electrical power to the grid, of which 45.9% would be delivered from the pressure let down turbine (renewable) and 54.1% from the CHP plant (non-renewable). If there is no waste heat available and it is not feasible to add an auxiliary generator then the required heat could be provided by burning a small proportion of the process gas. For the above example, with an inlet pressure of 350psig, outlet of 80psig, generating 250kW, the heat input of 1.30 MMB/h would require about 20.5scfm of gas, which is 0.34% of the gas throughput.