Biogas - Synthesizing Substrates
At Bio-En Power’s Elmira, Ontario, biogas plant, it isn’t unusual for odd items including trailer hitches, steel bars, and garbage cans to show up in the waste streams the facility takes in, but they usually don’t cause any setbacks. “Our equipment is built to take this odd type of stuff,” says Earl Brubacher, manager of operations. “At worst, we have to open up the machine to remove the item, but there’s no real hardship.”
The facility, which started up in March 2014, is just about at capacity, which is 70,000 metric tons of mixed waste annually. “We’re getting there, we’re probably at 85 or 90 percent right now,” Brubacher says. The plant is part of a larger group, Cornerstone Renewables, a sourcing organization that he says being a part of has a number of advantages. “If one digester goes down for some reason or can’t take something, the waste company doesn’t have to worry about where it goes—they just get told to take it to another digester,” he says.
Trucks that haul fuel, which is done five days per week, include liquid tankers, roll-offs, walking-floor or hoist trucks, depending on the type of waste. “We see a lot of food waste, waste from [waste]water treatment plants, rejected batches of offspec food material, packaged products…anything in the food chain, from the farm to the fork.”
Liquid waste goes through a fairly simply process when brought to the plant—it arrives via tanker truck and is pumped into storage tanks, which are capable of holding about 750 tons of waste. “It doesn’t sound like a lot, but when you’re doing 70,000 tons per year, that’s about two days’ worth—it gets you over a weekend,” Brubacher says, adding that batches are made every day, so material is tanked for around 34 hours or less before it is sent to the digester.
For solid material, it is brought to the plant receiving area where it is dumped and mechanically separated into two streams—organic slurry and contraries, Brubacher explains. Some items may require depackaging and are sent through a depackaging line, a proprietary process that he couldn’t discuss in detail. Plastics, metals and other undesirables are sent into a compactor bin, and eventually the landfill, and remaining slurry is pumped into the storage tanks.
On challenges, Brubacher says the facility isn’t set up to receive dirty material, meaning it has to be cleaned up prior to being processed, and occasional full truckloads of tin cans can cause some processing delays. “We can handle it, but a high volume of tin cans in one slug is a bit of a challenge,” he says.
The digester Bio-En Power deploys is what Brubacher describes as second generation. “The vast majority of digesters out there are first-generation, or single-stage digesters, and we’re two-stage,” he says. “Part of the anaerobic process happens in the first stage; the environmental conditions are best for those types of bacteria, and then it enters into a second stage, a different environment for a different bacteria and the rest of the process.”
A single digester has four stages, Brubacher adds—hydrolysis, acid, acetic acid and methane phases. “We split them—we have hydrolysis and acid in one part, and the acetic acid and methane in another stage. We separate them to provide the most ideal conditions for each group of bacteria.”
Annually, about 1,300 miles directly south from Bio-En Power’s Elmira facility is Harvest Power’s 5.4-MW, combined-heat-and-power biogas plant in Orlando, Florida, which, although is close to double Elmira’s capacity, processes a very similar mix of waste. About 120,000 tons of organic materials is taken in from sources in the region, including Walt Disney Resort, where it is located.
Every day at any time, the Central Florida Energy Garden receives waste into a reception pit, where material is slurried if required, and pumped into storage tanks where it is mixed and fed into a digester. While liquids are generally mixed and pumped directly into storage tanks, notes Jeremy Goodfellow, Harvest Power vice president of energy operations, solids must be preprocessed, broken down and decontaminated. “For the health of the digesters, we need to ensure there are no contaminants that can kill bacteria—for example, antibacterial soaps, which are amongst the facility’s unaccepted materials—or materials that can damage equipment, such as large pieces of metal, or cause high levels of undesired materials in our effluent or fertilizer, including heavy metals or chemicals.”
Material is sorted out through the mechanical preprocessing system and post digestion, while tanks must to be cleaned as part of scheduled maintenance activities, Goodfellow says. The residual material is removed by a waste hauler for recycling if possible, or for final disposal.
Another way to keep substrate in check is regular lab testing, according to Goodfellow, as well as education and outreach to the waste generators and their employees.
A vast array of materials has proved to be a challenge, but one not without solution, Goodfellow adds. “There is a wide range of materials to deal with, and the composition of the organic materials can change over time. Preprocessing to remove physical contamination—plastic, metals and glass—is one of the most challenging aspects, but is required to ensure contamination does not flow into the digestion system.”
Article cited from: http://goo.gl/6jq8cZ