Codigesting Crop Residues
From sewage sludge to food waste to grass clippings, it seems every organic waste stream under the sun is or has been a potential candidate for anaerobic digestion (AD). The whole process isn’t as simple as dumping any amount of manure or food waste into a pit, however. Adequate ratios and the right mix of each feedstock must first be determined in order for a digester to achieve maximum performance.
Researchers today are working to determine what that recipe is, and how AD systems can be improved and optimized to generate the most energy while allowing for flexibility in feedstock input capabilities, including the use of cellulosic biomass such as crop residue.
According to Jun Zhu, professor of renewable energy and environmental engineering at the University of Minnesota , there are issues associated with using manure alone for AD, due to its low carbon to nitrogen (C/N) ratio. Introducing crop residues into the mix can not only enhance methane production, but at the same time reduce the volume of residue materials for disposal, he says.
Zhu’s conclusions were determined through experiments with three crop residues—corn stalks, wheat straw and oat straw. When added to swine manure, all increased the total daily volume of methane gas production, though corn stalks performed the best, followed by oat straw. The only preparation done to any of the feedstocks was the chopping of the straw to allow it to be passed through a 40 mesh sieve, which has 40 wires per inch.
Zhu says the optimum amounts of crop materials added to swine manure were determined by the desired C/N ratios of the mixture, but the ratios change with the amount of manure—the nitrogen source—and each type of ag residue—the carbon source. The importance of getting the C/N ratio right is that the microbes that eat the substrates, or feedstock, need a sufficient concentration of each to achieve optimum growth for the digestion process, hence producing the most methane in the shortest period of time.
Typically, somewhere between a 16-1 to a 30-1 C/N ratio is the AD microbial sweet spot. “We tested C/N ratios of 16-1, 20-1, and 25-1, and found that 20-1 was the best,” Zhu says.
There are at least two characteristics of these feedstocks that make them perform better than wheat straw, Zhu says. “Since wheat straw has significantly higher carbon content than corn stalks and oat straw—46 percent versus 39 percent—the quantity of wheat straw added is less than those of corn stalks and oat straw,” he says. The quantity reduction for wheat straw means a reduction in the amount of easily degraded materials—or food—for digestion, hence resulting in reduced biogas productivity.
The second reason is because crop residues are primarily composed of cellulose, hemicelluloses and lignin, among which lignin is the least degradable material in AD. This is due to the shielding effect of lignin resulting from the intense cross-linking with cellulose and hemicelluloses. “As such, crop residues with higher lignin content will limit the degradation of such fibers,” Zhu explains. “According to [information published by academics] the lignin content in wheat straw, corn stalks and oat straw is 18, 8.4, and 13 percent, respectively. The high lignin content in wheat straw makes its sugars more difficult to use by microbes than that of the other two residues, leading to lower productivity of biogas and methane. Interestingly, the lignin content of these crop residues appears to well explain why corn stalks performed the best in biogas and methane production, followed by oat and wheat straws.”
While adding crop residue to a digester may seem like a simple concept, it isn’t that easy, as most digesters on the market today are traditional liquid-state digesters and are not designed to handle more than 15 percent solids. Günther Bochmann, a project manager at the Biogas Research & Consulting Group in Austria, says he thinks the biggest factor in utilizing ag residues or energy crops in livestock AD systems is process engineering. “Here, a lot of mistakes can be done,” he says.
“Some plant manufacturers think they know everything, and that all plants match to every substrate.”
A researcher at Ohio State University is taking AD system engineering to a new level, by developing a (patent-pending) AD technology that integrates a traditional liquid anaerobic digester with a solid-state anaerobic digester to allow for the expanded use of cellulosic biomass feedstocks such as crop waste and yard clippings, as well as a substantial increase in digester performance.
A Step Further
Assistant professor Yebo Li, who works in OSU’s Department of Food, Agricultural and Biological Engineering, has more than 15 years of experience with process and system development for the production of biofuels and biobased products. He recently scored a $2 million grant from the state of Ohio’s Third Frontier Advanced Energy Program to further develop his system, dubbed integrated anaerobic digestion system or iADs.
Explaining how the technology works, Li says the effluent from the liquid anaerobic digester (less than 15 percent solids) is mixed with lignocellulosic biomass such as corn stover and yard waste and fed into the dry digester. Before being put into the system, the cellulosic biomass generally needs to be shredded or ground to around 1-inch particle size. “The effluent from the liquid anaerobic digester is performed as inoculum and a nutrient amendment for the dry digestion,” he says. In other words, the effluent left over from the liquid AD process is used to treat the solid waste in the solid-state digester. “Most of the lignocellulosic biomass has a high carbon content, and needs a nutrient amendment—supplementation of nitrogen.”
AD systems on the U.S. market today can only process up to 14 percent solids compared to Li’s, which has demonstrated capabilities of processing from 20 to 40 percent solids, resulting in substantially increased biogas production.
“The finished material of the dry digester is stackable and like compost, and the biogas produced in the solid-state digester can be combined with that from the liquid phase digester to be converted in one combined-heat and-power unit,” Li says.
Importantly, the system allows for the use of various sources of cellulosic biomass such as yard trimmings and crop residue. “If we can include lignocellulosic biomass into the digester, it will increase the available amount of feedstocks within a specific transportation range,” Li says. Among feedstocks tested in the iADs, crop residue such as corn stover and wheat straw performed better than yard waste, he says. Like Zhu, Li adds that the composition of the cellulosic biomass influences the performance of the digester. “High cellulose and hemicellulose content will be beneficial, while high lignin content will have a negative impact,” he says.
Other benefits of the system include the elimination of effluent management, and the solids that are left over in the process can be sold as natural fertilizer.
In order to demonstrate the technology on a large scale, Li and OSU have partnered with Ohio-based biogas company quasar energy group, formerly known as Schmack Biogas, to install it at one of the company’s facilities.
“What we came to realize last year, when Dr. Li approached us about the technology, is that it will increase the feedstock types that we can accept in our systems, and it will pretty much double the energy output of a typical system,” says Caroline Henry of quasar. “We’ll be able to accept cellulosic biomass, which in a regular liquid digester we can’t.”
Henry says the initial plan was to install the iADs in its facility in Wooster, Ohio, where the company has an operating digester on the OSU campus, but it will likely be moved to its plant in Zanesville, Ohio, instead. Quasar also has an operating facility in Akron, Ohio, a facility under construction in Columbus, Ohio, and has designed a project for an ongoing five-farm digester project in Rutland, Mass.
Henry was unsure of the distance from the facility the crop waste could be hauled and remain economical, but says feedstocks typically accepted at quasar’s facilities come from a maximum of 60 miles, usually between 40 and 50 miles, to be cost-effective. She says the state’s Third Frontier Advanced Energy Program, which is providing the $2 million grant to fund Li’s research and quasar’s project, will help make that and similar determinations as the technology is commercialized.
Current plans are to initiate installation of the digester this year, according to Henry, and some of the initial planning and engineering has been done already. When complete, the integrated system will be able to process more than 30,000 wet tons of biomass annually and produce more than 750 kilowatts of electricity.
Henry adds that crop waste is abundant in Ohio, where quasar has sited all of its facilities. “The Ohio agriculture industry produces nearly 5.3 million dry tons of sustainable corn and wheat crop residues annually,” she says. “That’s waste that we can start accepting and using in the iADs.”
Author: Anna Austin
Associate Editor, Biomass Power & Thermal