August 20, 2008 | General

Process Control

BioCycle August 2008, Vol. 49, No. 8, p. 41
Equipment and software to monitor key process parameters are becoming increasingly sophisticated, but fundamental understanding of the biology of composting and anaerobic digestion is critical for project success.
Nora Goldstein

MCGILL Environmental Systems operates six composting facilities in North Carolina, Virginia and Ireland. The plants primarily compost industrial and municipal biosolids and food processing residuals, as well as yard trimmings, wood waste and other wood-based bulking agents. When it comes to optimizing performance of its composting process, says Noel Lyons, McGill’s president, “the overall picture is the only way to look at it. All the most wonderful software packages and monitoring systems in the world don’t by themselves guarantee that things will go well.”
Lyons explains that a prevention mentality is at the core of the company’s process control strategy. “With any system – from operations that turn with bucket loaders to fully automated facilities – the key is to keep aerobic conditions everywhere, all the time. Control systems and management practices revolve around that fundamental parameter.” The company puts a lot of emphasis on proper blending of feedstocks as all of its operations compost indoors in positively aerated static piles – making it less feasible to adjust the mix once the piles are constructed.
Plant operators monitor all the basics, i.e., temperatures, oxygen, air flow, etc., but Lyons notes that moisture content is a great indicator of possible problems. “When I walk through the plants, the first thing I look for is any sign of leachate – it tells me if something is wrong,” he says. “Even if it is only one spoonful, it means that some part of the pile is too wet.”
McGill worked with a software company to develop a process control package. It is designed to continuously monitor temperatures; the air delivery rate matches the temperature set points. “We eliminated the process controllers that have the fans go on for a certain number of minutes and then off for a set number of minutes,” says Lyons. “We have done formal experiments on continuous aeration based on temperature set points versus the on-off approach and the speed that piles went anaerobic with the latter was surprising. A good measure of any composting process is not so much the temperature on its own, but heat removal and oxygen uptake.”
The company puts a lot of emphasis on training, but there is always room for more, he adds. “For us, this is particularly important in the area of blending. When we recently opened our plant in Virginia, we picked who we felt was our most experienced operator in blending, and moved him to the new plant for three months. Having someone with tons of experience, who can train new operators, was of enormous value.”
McGill has a technical trainer on staff who is based in Ireland, where the company is about to open a new composting facility. The new plant has the highest level of process control of any of the company’s facilities, with totally enclosed, fully aerated composting bays (each 600 cy capacity) inside of a building. “We see a good opportunity for growth of composting in Ireland, so we wanted to design a facility that would allow us to expand,” says Lyons. “And the first obstacle any composting company encounters when it wants to expand is the history of its operating plant. So having two layers of odor containment – the enclosed bays inside a building – between the process and the outside was very important.”
Tried and true methods of measuring moisture in compost piles have been the hand-squeeze test (if it drips it’s too wet, if nothing is left on the hand it is too dry and if there is a stain it is just about right) or drying a sample (calculating the difference between the wet and dry weights to determine the moisture content). Green Mountain Technologies (GMT), which markets the Windrow Manager software, recently developed a hand-held moisture sensor. The moisture data is fed into GMT’s Windrow Manager software – along with temperature and oxygen readings, C:N, etc. – which uses the data to help oversee the operation and optimize the process.
“We recently sent the first Windrow Manager with the integrated moisture sensor to the University of California, Davis for a carcass composting project,” says Michael Bryan-Brown of GMT. “Researchers used it to measure the rate of drying/moisture release of the carcasses. The initial feedback is that the sensor provided a good reading, but when the probe hit a bone, it damaged the sensor. The final design issue is the robustness of the sensor, providing it with enough contact area to get an accurate reading while protecting the end of the probe.”
He adds that the benefit of having this hand-held sensor is that moisture is the ultimate parameter that controls the rate of degradation in the pile and the potential for odor generation – much more so than oxygen. “Both parameters are important, but oxygen levels are secondary to moisture,” says Bryan-Brown. “If the pile is too wet, the interstitial space is filled with water, preventing air circulation. If it is too dry, it affects the quality of the end product.”
Green Mountain worked with Reotemp to manufacture the sensor for the moisture probe. The company expects to overcome the robustness hurdle, as it was able to do with its oxygen monitor. Its oxygen probe has a membrane sensor that is subject to wandering depending on the amount of moisture and certain gases present. “We have gotten around that by automating the recalibration of the probe so that the operator only has to take a few pumps of ambient air – which we know is 21 percent oxygen – and the software on our Pocket PC resets the probe,” notes Bryan-Brown.
Like composters who have customers relying on them for a continually high quality end product, anaerobic digestion facilities that are selling power to the grid or gas to the pipeline have an added level of urgency to stay up and running. There are a number of variables to monitor or control, from the content of substrates being fed into the front end to the amount of methane versus CO2 and hydrogen sulfide (H2S) in the gas on the back end.
“The whole thing with successful anaerobic digestion is to control all the variables, from the skid loader going down the runway of the dairy barn and pushing manure into the pit that will feed the digester to the electrical current being fed into the grid,” says Marcus Martin, whose companies include GEN-TEC, a vendor of process control technologies, and Martin Machinery, which sells custom generator sets (gensets) and heat recovery units. “You need to bring all those variables down to a few parameters that need to be measured in order to keep the operation running smoothly.”
Digester system designers typically develop the loading rate based on characteristics of the substrates and capacity of the system. New substrates introduced are typically evaluated to ensure they will not upset the biological balance in the digester and to measure their biogas potential. “Feed rates, flows and temperature are set at start-up of the digester, but a pretty big window is allowed to provide the operator some flexibility,” he says. “However, if an operator goes outside those parameters it can set off an alarm so that adjustments can be made before the digester would have to be shut down. Uptime, of course, is in everyone’s interest. In fact, uptime is a big word with the power buyers. Some digester maintenance contracts are based on uptime.”
Control equipment marketed by GEN-SET on the back-end of the digester – between the digester and the generator set or pipeline, i.e., the gas producer and the gas consumer – revolves around knowing what is in the gas and then determining the feed-in rate. “The focal point is to use all the gas produced; the second highest priority is to burn it efficiently,” explains Martin. “To use all the gas, you need to know how much is produced, which relates to pressure in the digester and flow volume. What we specifically track is the BTU content of the gas – the percent of methane, which relates back to the substrates being added. Periodic H2S sampling is done, an important parameter to track in terms of scheduling maintenance intervals.”
The fuel consumption rate of the genset is based on the value of the fuel; a higher biogas content increases the output of the engine. Conversely, a lower fuel value “derates” the engine. “If you are burning biogas with 30 percent methane content in an engine designed for 50 percent, our process controller derates the engine,” notes Martin. “Otherwise, the engine would shut off. We just developed a software product that will enable us to poll all the projects using our equipment on a daily basis, monitoring digester performance on the output side. If there is a significant change from the last time we polled, we will alert the operator.”
Anaerobic digesters are designed with a specified gas production rate based on their capacity, but in many instances, says Kristofer Cook, Managing Director of Bioprocess Control Sweden AB, that capacity isn’t fully used. His company developed an on-line monitoring system called Biogas Optimizer that enables digesters to operate close to their maximum capacity. “We conduct a prestudy to analyze the production system, substrate(s) and process dynamics over a period of time, as well as collect real time data,” explains Cook. “We measure pH, gas flow and biogas composition, including methane content. All of this data is used by Biogas Optimizer to control the loading rate to the digester.”
The monitoring system is set up to make its determination based on pH readings from a sensor located on a recirculation loop connected to the digester. “The holy grail of digester monitoring is volatile fatty acids (VFAs), which provide an early warning of system distress,” he adds. “VFAs will kill bacteria and crash an entire system. The problem is that equipment to measure VFAs on-line is very expensive, sensitive, requires extensive calibration and is not fully reliable and tested today. Bioprocess Control solved that challenge by using pH, which is an overall indicator of a healthy production system, and soft sensor technology. Our mathematical model extrapolates on the pH data, and then controls the loading rate.”
The control tool works in manual, semiautomated or automated modes. Facility operators typically start out in the manual mode, taking suggestions from Biogas Optimizer to manage the organic loading rate. The semiautomated mode works within the upper and lower boundaries of the gas production rate; the fully automated mode totally controls the loading rate, reducing the need for process monitoring.
The Händelö biogas plant in Norrköping, Sweden, owned and operated by Svensky Biogas AB, is designed to treat 24,000 metric tons/year of substrate, 90 percent of which is stillage from a nearby ethanol plant. Retention time in the mesophilic digester is 40 days. The average biogas production rate was 2.61 m3/m3/day; the organic loading rate was 4.0 Kg VS/m3/day. Estimated annual biogas production was 1.89 million m3/year. Biogas Optimizer was installed at the Händelö facility, and configured for manual mode. After 43 days, the average biogas production rate increased to 3.23 m3/m3/day, a 24 percent increase, with an 18 percent increase in the organic loading rate. “The operator changed the organic loading rate five times during that period, which resulted in the 24 percent increase in the average biogas production rate,” says Cook.

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