Choosing the Power
The alternatives considered included adding particulate traps and other modifications to the diesel exhaust systems. While that might have helped, the DPW would still have faced restrictions on how many hours those units could be run. Barnes says they pressed on.
“We also conducted a number of studies to look at the feasibility of running the grinders on natural gas. Those studies showed two huge problems: first, more than a mile of natural gas utility pipeline would have to be laid to get the fuel here. Second, a natural gas engine of sufficient size to power the grinders would be about as large as the grinder itself and, based on quotes we got, cost roughly a half million dollars each. Electric seemed the obvious choice.”
Up to that point, Barnes and the DPW had been using a pair of Morbark Model 1300 tub grinders for wood and green waste processing at the site. Very satisfied with both the performance of those units and Morbark’s support behind them, they chose to stay with the company but took the opportunity to reevaluate their overall grinding needs.
“The tubs had been real workhorses for us since they were installed,” says Barnes. “But with the growth we were experiencing, we needed to find a way to increase production without adding more grinders. With that goal in mind, after seeing what they could do in demonstrations, we replaced the tub grinders with a pair of Morbark Model 7600 horizontal grinders.”
A Pleasant Surprise
Engineers charged with determining the appropriate size and horsepower of the new grinders, as well as the electrical demands for them, initially ran their calculations and came up with a number that served as something of a benchmark during the installation and startup process. According to Barnes, once the units were up and running, that number was quickly proven inaccurate.
“When we did the original plan, engineers estimated costs based on the belief that, production-wise, it would take a 1000-HP electric unit to do what a 750-HP diesel could do. Based on that, they came up with a 2.5:1 cost ratio, that is, for every dollar we were spending to process with diesel, we should expect to spend $2.50 for the electric units. To get the production we wanted, however, we were factoring in costs based on running the grinders at max power every time they were grinding. We did not anticipate the surprising efficiency of the larger units, a benefit made possible by the control system offered standard on the Model 7600.”
The onboard control to which Barnes refers, the Morbark Integrated Control System (MICS), is designed to monitor hydraulic pressures, temperatures, clutch systems, and engine efficiency, then make automatic adjustments to maximize performance. The MICS also allows utilization of a remote diagnostic system which, by nature of being able to quickly and remotely identify potential problems, will keep downtime — and associated costs — to a minimum.
“We cannot quote hard numbers at this time,” adds Barnes. “But suffice it to say that we were pleasantly shocked with how far off we were in our calculations for the difference in cost between the diesel and the electric. As chance would have it, shortly after we went to electric, the price of diesel soared to nearly $4.00; that helped make the switch look that much better. It has since come down to about $1.60, but we are no longer at the mercy of those wide price fluctuations, and we are getting the level of performance we need to keep pace with the growth.”
To power the pair of Morbark Wood Hogs, as well as a trio of McCloskey trommels that were also converted to electric power, Bakersfield DPW installed a 1.2 MW electric service adjacent to its grinding operation. The 93-acre Mt. Vernon site, which currently processes better than 210,000 tons of wood and green waste annually, has undergone some other changes in recent years to accommodate the steady increase in volume.
“For better than 15 years, our main outlet for the material we grind has been area co-gen plants,” says Barnes. “In the last few years, however, we’ve started working with a biosolids composter who needs material for a bulking agent. So we’ve branched out a bit and now have an alternative outlet in the event demand in one area goes down. We’ve also started taking in industrial food waste and roll-off containers from area commissaries and composting it onsite.”
It should be noted that two-thirds of Bakersfield’s Mt. Vernon site is, in fact, dedicated compost area with two different grades of finished product slated for food and non-food agricultural applications. Since startup, delivering downsized material from the grinders to the windrow area meant scooping it with a loader, riding up a ramp and dropping it into a trailer for the short trip. A new, innovative approach, one that Barnes calls their “poor boy’s transfer station” has improved that process as well.
“It is essentially a roadway with two large holes cut into the top, opening it to two truck lanes, which pass underneath. It allows us to load out wood chips to trucks in the right tunnel and — at the same time, if necessary — load curbside green waste into a truck on the left. It was a nice inexpensive way to both address the issue of efficiency and reduce wear on the loaders caused by repeated lifting and dumping into trailer beds. We have also installed a string of overland conveyors, which takes finished compost from one of our radial stackers to an area where farmers can load it out for use in their fields. Doing that has dramatically reduced the volume of vehicle traffic on site, the distance each vehicle has to travel and, most importantly, emissions from that activity.”
Loftier Goals at Mt. Vernon
Despite processing the massive volumes they do, providing fuel to co-gen plants, creating compost for agricultural use, and more, the Mt. Vernon facility is not a money-maker for Bakersfield DPW, nor had they ever really planned for it to be one.
“Our operating costs are about $7 million/year, and that’s funded largely through our refuse collection,” says Barnes. “On average it costs us about $32 a ton to process material and create a product here, and we can only sell the compost for about $15 a ton, so it’s obvious that’s not the motivation. Our primary goals here are landfill diversion and environmental compliance. We’ve worked hard to get the first part of that in place and, with the switchover to electric power for the processing gear, we have a nice jump on the second part. That we were able to do so in such a seamless manner, and position ourselves for future growth, is a nice feather in our cap. That we had to reevaluate our operating costs downward because of the grinder efficiency is a nice testimony to Morbark and its focus on what’s important for its customers.”