Hayes, Denis and Gail. 2015. Cowed: The Hidden Impact of 93 Million Cows on America’s Health, Economy, Politics, Culture, and Environment. W.W. Norton & Company.
Digesters are more about controlling pollution than generating electricity. If every ounce of manure from 93 million cows were converted to biogas and used to generate electricity, it would produce less than 3% of the electricity Americans currently use (Cuellar, A.D., et al. 2008. Cow Power: the energy and emissions benefits of converting manure to biogas. Environmental Research Letters 3).
Weißbach, D., et al. April 2013. Energy intensities, EROIs, and energy payback times of electricity generating power plants. Energy. Vol 52: 1, 210–221
Producing natural gas from maize growing, so-called biogas, is energetically expensive due to the large electricity needs for the fermentation plants, followed by the agriculture’s energy demand because of fertilizers and machines.
Biogas-fired plants, even though they need no buffering, have the problem of enormous fuel provisioning effort which brings them clearly below the economic limit with no potential of improvements in reach.
“The Maas brothers decided to set up their Farm Power plant right between the dairies, so the manure wouldn’t need to be trucked long distances to the digester, and the finished product could be piped at reasonable cost to nearby fields. With the farmers lined up, all Farm Power had to do was find $3 million to build a million-gallon tank in which to digest manure, a generator, and tanks to hold the stuff coming in and going out of the digester, which included up to 30% pre-consumer food waste—things like cow blood, dead chickens, and fish waste. Food that has not already been digested by animals contains more energy, allowing the anaerobic bacteria in the digester to pump out more methane. The facility can process forty to fifty thousand gallons of manure daily.
This generator and another, which Farm Power operates at Lynden, Washington, generate enough electricity to power a thousand homes. The liquid material coming out of the digester is a better fertilizer than raw manure because it contains far fewer pathogens and weed seeds and doesn’t stink as much. It first flows into a pit; from there, as a more stable manure slurry, it’s piped to nearby fields where it can be pumped through an irrigation nozzle or injected into the soil. The dry residue is turned into sanitary, comfy cow bedding. After the dry matter is squeezed through a screen, it’s loaded into trucks and hauled back to the farms. In the future, Farm Power plans to pasteurize the bedding product. Kevin scooped up some finished product stored at one of the nearby dairies. He held it out, inviting Denis to examine it. The bedding was still hot, and smelled like soil and hay.
Digesters don’t solve every environmental problem. Certain antibiotics in cow manure can kill off the fermenting and methanogenic bacteria that make the process possible. The heat in digesters probably doesn’t destroy most antibiotics. New research suggests some pathogenic and antibiotic-resistant bacteria survive anaerobic digestion. Installing a scrubber to remove sulfur dioxide from the digester gas wasn’t economically feasible for the Maas brothers, so they got a permit to emit some pollution. More nitrogen, phosphorus, and potassium remain in the final product than is ideal. Carbon dioxide is also put in the air, and the trucks hauling waste and bedding burn fuel.”
Smil, Vaclav. 2010. Energy Myths and Realities: Bringing Science to the Energy Policy Debate. AEI Press.
Smil has this to say about China before modernization:
“…biogas digesters were unable to produce enough fuel to cook rice three times a day, still less every day for four seasons.
The reasons were obvious to anyone familiar with the complexities of bacterial processes. Biogas generation, simple in principle, is a fairly demanding process to manage in practice. Here are some of the pitfalls:
- The slightest leakage will destroy the anaerobic condition required by methanogenic bacteria
- Low temperatures (below 20°C),
- Improper feedstock addition,
- Poor mixing practices
- Shortages of appropriate substrates will result in low (or no) fermentation rates,
- Undesirable carbon-to-nitrogen ratios and pH
- Formation of heavy scum.
Unless it is assiduously managed, a biogas digester can rapidly turn into an expensive waste pit, which—unless emptied and properly restarted—will have to be abandoned, as millions were in China. Even widespread fermentation would have provided no more than 10 percent of rural household energy use during the early 1980s, and once the privatization of farming got underway, most of the small family digesters were abandoned.
More than half of humanity is now living in cities, and an increasing share inhabits megacities from São Paulo to Bangkok, from Cairo to Chongqing, and megalopolises, or conglomerates of megacities. How can these combinations of high population, transportation, and industrial density be powered by small-scale, decentralized, soft-energy conversions? How can the fuel for vehicles moving along eight- or twelve-lane highways be derived from crops grown locally?
How can the massive factories producing microchips or electronic gadgets for the entire planet be energized by attached biogas digesters or by tree-derived methanol? And while some small-scale renewable conversions can be truly helpful to a poor rural household or to a small village, they cannot support such basic, modern, energy-efficient industries as iron and steel making, nitrogen fertilizer synthesis by the Haber-Bosch process, and cement production.”