Solid-state digester shows promise

If not for the small flare stack nearby, one would never suspect that the nondescript metal building at the Western Beef Development Centre housed cutting-edge energy research.

This is the site of a one-of-a-kind research prototype designed to extract biogas released during anaerobic decomposition of organic waste in solid form. The solid-state anaerobic digester is one of several projects initiated by PAMI’s Applied Bioenergy Centre, headquartered nearby at Humboldt, Sask. It is one of two solid-state digesters known to be in operation in North America and one of a handful in use around the world.

The process differs from current biogas technology whereby solid waste materials first have to be mixed with water into a liquefied state so that the liquid can be pumped and metered during the biogas extraction process, explains project manager Dr. Joy Agnew.

The feedstock during the developmental phase has been feedlot manure layered with straw. As the system is perfected, other agricultural waste will be tried, such as cull potatoes, packing plant waste and deadstock. The Canadian Food Inspection Agency is considering certifying biogas processing as a way to destroy specified risk material on a case-by-case basis.

All of the action in the solid-state system happens inside airtight compartments called reactors. Passive heating systems built into each reactor promote growth of micro-organisms that naturally occur in organic material under anaerobic conditions at temperatures of 35 C to 50 C. The micro-organisms are active for 30 to 80 days depending on the material and other conditions.

They’ve found that 38 C is sufficient and that the process can be jump-started by inoculating a fresh batch with micro-organisms retrieved from a batch that’s just come out of the digester. Peak gas production varies, but generally occurs around day 15 and tapers off though to completion on day 40.

The gas released from anaerobic digestion of organic material is largely methane and carbon dioxide with trace amounts of other gases. Methane, the gas that can readily be converted to energy, comprises half the biogas released from feedlot manure. Biogas from some feedstocks can be as much as 75 per cent methane.

Composting, on the other hand, is an aerobic process that releases mostly carbon dioxide, Agnew explains. Not only is it difficult to recover carbon dioxide from a composting system, but the chemistry to convert carbon dioxide into energy is still far from feasible for commercial applications.

For the time being, flaring converts the methane to carbon dioxide and water vapour. At a later stage of the research, the biogas will be further processed to create electricity and heat, or upgraded to natural gas to be pumped back into the grid.

In a commercial-scale operation, the biogas would be used for heating and powering the plant itself, the farm or commercial buildings, and homes. Any excess electricity could be sold to the provincial grid.

The pilot facility’s two reactors each hold 10 tonnes of the manure-straw mix and the system is now producing 50 cubic metres of biogas (25 cubic metres of methane) per wet tonne of feedstock.

Putting this into perspective, Agnew says energy from biogas released from continuous operation of the pilot facility would power and heat an average Saskatchewan home for a year. On a larger scale, a 40,000-head feedlot, such as the co-operating Pound-Maker lot next door to Western Beef, produces around 162,000 tonnes of manure a year. Processing it through a solid-state digester would yield four million cubic metres of methane, which could be converted into 110,000 gigajoules (GJ) of energy — enough to power nearly 2,000 Saskatchewan homes. Valued at $5 per GJ, the biogas captured would be worth over half a million dollars.

Solid-state digestion may also offer a way for producers to salvage some value from deadstock. Instead of incurring losses of five to nine cents a pound for deadstock disposal on top of the loss of the animal, they could realize a benefit of approximately $22 from the 4.4 GJ of energy retrieved from a 1,200-pound carcass.

Since the project got underway in 2007, the focus has been on the design and functioning of the reactors and tweaking conditions to optimize gas production, which is now about twice as much as a year ago and on par with production from the bench-scale system that serves as the model for the pilot plant and continuing research. Agnew attributes the initial difference to variables such as compaction of the feedstock in the large reactors that either prevented effective microbe action or locked the gas in the bottom layer.

The controls, alarms, and metering systems housed in portable trailers nearby are standard equipment used in the gas industry.

Agnew notes that the prototype isn’t intended for scale-up. It was designed with research in mind to explore how to optimize gas production and provide recommendations for a commercial facility. Ultimately, it is hoped that a manufacturer will help design a full-scale version and build solid-state digesters for commercial use.

A basic feasibility study comparing the cost of building a solid-state digester to that of a liquid digester to handle manure from a 40,000-head feedlot indicated that a solid-state system could be built for $5 million to $10 million versus $40 million to $50 million for a liquid system.

From the Grainews website: Understanding biofuels

Digestate adds value

Just as valuable, and potentially of more value than the energy created from solid-state digestion of manure, is the digestate, or what’s left over after the micro-organisms have done their work. Agnew says solid-state digestion doesn’t reduce feedstock volume by much. It’s about capturing and creating value from natural gases that would otherwise be lost to the atmosphere or earth over time when raw manure is spread on the land or pushed up and left to degrade on its own.

Digestate can be spread as fertilizer, however, composting could add value to it by further destroying pathogens and weed seeds and reducing the volume. If composted digestate could be bagged for the retail market where soil products currently sell for around $1 a pound, the digestate from a 40,000-head feedlot would be worth $6 million!

Preliminary work with digestate shows that it has a more uniform consistency, less odour and is easier to apply than raw feedlot manure.

Some nitrogen is lost as ammonia and nitrogen gas during digestion, but the total loss is less than the nitrogen lost from stockpiled raw manure. The concentrations of other macronutrients (phosphorus, potassium and sulphur) are not affected and actually benefit from the process because they and the nitrogen are converted into more stable, plant-available forms.

Work to date has led to the use of biochar rather than large amounts of straw as the carbon source to obtain the proper carbon-to-nitrogen ratio for efficient composting. The biochar comes from a torrefaction project underway at the Applied Bioenergy Centre, where they heat biomass to very high temperatures in the absence of oxygen.

Biochar can be used instead of coal to generate electricity and, like ash, as a soil amendment to improve porosity and reduce bulk density of soil, Agnew explains. There is nothing but carbon in biochar, so combining it with digestate creates a nutrient-rich product that also improves soil quality.

Intrigued with the initial findings on the potential of digestate as a manure-management alternative, the Saskatchewan Stock Growers Association (SSGA) applied for and recently received funding from the Canadian Agricultural Adaptation Program through the Agriculture Council of Saskatchewan to advance this line of the research.

SSGA general manager Chad MacPherson says the association is pleased to partner on this project because members see the great potential for environmental and economic benefits for beef producers.

Minimizing the potential for nutrient leaching and gaseous losses when manure is stored and spread adds an environmental benefit that goes hand in hand with economic benefits when those nutrients can be efficiently captured and cost effectively recycled for production on the farm, says MacPherson. Other potential advantages such as its low odour, handling characteristics, and potential to minimize pathogens and weed seeds in land-applied manure are also important to beef producers.

For more information go to www.pami.ca or contact Agnew at 1-800-567-7264.

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