Hydraulics aids trash-to-energy conversion

Hydraulics is known for generating large forces from compact packages. But it's also extremely rugged, often performing without fail under operating conditions that would quickly do in other motion-control technologies.

Take, for instance, the case of Montenay Energy Resources of Montgomery County Inc., (montenayenergy.com),a waste-to-energy facility in Conshohocken, Pa.

Each day, Montenay Energy processes tons of household garbage and municipal solid waste at its 20-acre site. The facility incinerates the waste and uses the thermal energy to fire boilers and drive turbines which ultimately generate electricity for 30,000 homes.

For every 100 tons of trash, Montenay Energy produces more than 50 MW of electricity and reduces the volume of solid waste sent to landfills by 90%. The Environmental Protection Agency, among others, approves of this process as an alternative to discarding tons of garbage in landfills, especially since the garbage never stops coming.

Recently, Montenay Energy upgraded the system that controls the rate at which trash is fed into the incinerator with the help of Bosch Rexroth Hydraulics, Bethlehem, Pa. (boschrexroth-us.com). Typically, a ram feeds the trash into a large incinerator and then extracts the ash. Brutally hot and dirty conditions demand an extremely durable system to keep the boilers fired 24 hr a day. Hydraulics was an obvious choice.

Montenay previously used a main hydraulic power unit with 24 submersed gear pumps rated at 3,675 psi. The gear pumps set the flow rate for multiple sets of identical cylinders that moved grates and fed trash into the boiler. The grates move plates inside the boiler that shuttle trash along as it burns. The problem with the design, says John Polidore, Montenay's plant operations and maintenance manager, was that the use of fixed-displacement pumps caused the grates to intermittently move and stop for a various periods, depending on process conditions. When stopped, the grates frequently seized, halting their operations. This, in turn, required removing the unit from service for repairs. What's more, the gear pumps were stacked together and submersed in a tank, making them difficult to access and repair.

Bosch Rexroth Service Engineer Keith Metz suggested an approach that replaced the 24 gear pumps with four A10VSO-DR Size 18 piston pumps. The pressure-compensated units produce about 5 gpm flow at 1,800 rpm and have a rated peak pressure of 5,000 psi. A pressure controller maintains constant pressure within the pump's operating range so it supplies only the amount of hydraulic fluid required.

Flow then passes through Model 2 FRE 6 proportional flow-control valves. The two-way valves let an electrical signal control fluid flow independent of pressure and temperature variations. Each valve assembly comprises a proportional solenoid with an inductive positional transducer, a metering orifice, and pressure compensator. The company's VT 5010 electrical amplifier cards with electrical position feedback control the valves, and Montenay's existing PLC commands the cards.

Bosch Rexroth and its distributor Airline Hydraulics, Bensalem, Pa. (airlinehyd.com), assembled a power unit for the feed system on one boiler to test the design. And after proving the unit could successfully control and synchronize the cylinders, the duo revamped the power units for each boiler using four top-plate, pumpmotor groups — each with A10VSO pumps and Baldor 25-hp electric motors. Previously, one electric motor powered six gear pumps. The one-pump-per-motor design enhances reliability. The hydraulic companies were also responsible for manifolds, field piping, and other components necessary to complete the installation. Each boiler required eight valves and driver cards, and associated electronic hardware.

Polidore also wrote the frontend software program that monitors internal boiler conditions and commands the hydraulics to react to operating conditions. "Using conditions such as temperature and pressure, the hydraulic system controls the boiler in-feed motion," says Polidore. "The system is capable of varying the in-feed speed based on boiler operating parameters." The pressure-compensated pumps and proportional valves control the hydraulic flow throughout the cylinder cycle to optimize feed rates and combustion efficiency.