As the demand for finite resources increases, there will be a greater need for food companies to demonstrate sustainability across all areas of their business. Onsite AD, where full use is made of both the heat and power generated by waste materials, offers an obvious solution.
By Matt Hale

Sustainability has never been higher on the agenda for food and drink companies than it is now. Not only does operating in an ethical and environmentally responsible way make sense for both planet and pocket, but it is clear that this is what consumers want, too. Research from Globescan revealed that 92 percent of consumers think that food companies should focus their efforts on securing the future sustainability of food, with many also believing that farmers should be paid more for their produce. So, what more could America’s food producers be doing to increase their sustainability credentials?

Making the Most of Waste
In recent years, some of the country’s most forward-thinking food companies have developed a truly circular approach to resource use. Alongside minimizing the volume of waste they generate, they are also turning the unavoidable fraction that does occur into renewable energy, for use in their onsite operations. Anaerobic digestion (or AD)—in which organic matter is naturally broken down to produce energy and biofertilizer—has taken big strides in the U.S. over recent years, with more than 2,000 AD plants now in operation.

While the vast majority of these AD facilities treat wastewater, and more than 240 are situated on farms, there is also a huge potential for onsite industrial plants processing feedstock such as food waste and residues, ranging from vegetable peelings and sugar beet pulp, to liquid malt waste and distilling residues. The benefits for the companies operating these plants are multiple—reduced waste disposal costs; reduced energy costs; security of energy supply, with reduced reliance on fossil-fuel derived power; carbon mitigation; superior green credentials; and creation of a nutrient-rich biofertilizer. Furthermore, the fact that these plants have an onsite use for the power they produce means they are less affected by changes to state and federal legislation on renewable energy.

Take the Heat
However, any AD facility that wants to maximize its returns also needs to be making use of its full heat output, not just its power output. The AD process generates plenty of surplus heat—most commonly, heat produced by biogas combustion in a combined heat and power (CHP) unit, but also via digestate pre-heating, pasteurization, biogas upgrading to biomethane and digestate concentration. Ensuring that this heat is used either within the AD process itself or within other onsite operations can make a big difference to a plant’s efficiency and therefore profitability.

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The majority of shoppers think food companies should focus their efforts on securing the future sustainability of food.

By using heat exchangers within an AD plant, surplus heat can be taken from one process or place and transferred to another. Common everyday examples include domestic radiators (which transfer heat from a boiler to a room) and car radiators (which take heat away from the engine). Two of the most common types supplied to AD plants are Plate Heat Exchangers and Tubular Heat Exchangers. As their name suggests, Plate Heat Exchangers consist of a series of metal plates to transfer the heat between two liquids. However, while they provide a large surface area for thermal transfer, they are generally only suitable for simple fluids which do not present a risk of fouling, such as water, milk and oils. In contrast Tubular Heat Exchangers (also known as shell and tube heat exchangers) consist of one or more tubes within a larger tube or shell. Because the tubes can be designed in a number of ways (for example smooth, corrugated or scraped) the basic design principle can be designed for a wide range of materials including viscous fluids and materials containing particles. As you can see, there are many different models and refinements and it is advisable to consult a specialist who can explain the benefits of different types and perhaps offer different solutions. When specifying a new heat exchanger, either for a new process, or to replace an existing unit, a direct replacement may not be the best solution. The science of thermodynamics and heat transfer is developing all the time and new designed of heat exchanger are likely to be much more efficient than older ones. Retro-fitting a new heat exchanger can often be a cost-effective alternative to replacing an entire production line.

Potential Uses for Heat in the AD Process
When it comes to making full use of the heat, there are a number of options with the AD process itself, including: preheating feedstock; for pasteurizing; to reduce the volume of digestate; or to upgrade biogas to biomethane. For onsite plants within the food industry, it can also be used for space heating, cooking, heating liquids, or pasteurizing and sterilizing foodstuffs. In addition, large sites may have significant office and staff facilities, where there may be the scope to install district heating systems.

As the demand for finite resources increases, there will be a greater need for food companies to demonstrate sustainability across all areas of their business. Onsite AD, where full use is made of both the heat and power generated by waste materials, offers an obvious solution.

Matt Hale is International Sales and Marketing Director at HRS Heat Exchangers. Headquartered in the UK, HRS Heat Exchangers Ltd operates at the forefront of thermal technology, offering innovative and effective heat transfer solutions worldwide, across a diverse range of industries. With more than 35 years of experience, they specialize in the design and manufacture of an extensive range of tubular, corrugated and scraped surface heat exchangers. All products are designed in accordance with the ASME standard. For more information, call (770) 726-3540 or e-mail info@us.hrs-he.com.

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