The Merits of Staying Dry in Waste-to-Energy Processes
by Brian Schellati
Moisture can be an enemy (sometimes unseen) of many recycling and waste-to-energy (WTE) processes. In particular, when that moisture lingers until it is time to consume the materials that have been diverted from the landfill for the specific purpose of either producing energy or manufacturing a new product, the results are less than optimal.
In the waste-to-energy sector, makers of refuse-derived-fuel (RDF), solid recovered fuel (SRF), processed engineered fuel (PEF) and other engineered products all have reasons to avoid the presence of excess moisture. Converting discarded materials into fuel inherently means the consumer of that fuel desires a lower percentage of moisture than is typically found in municipal solid waste (MSW) or most industrial by products.
Thus, in the waste-to-energy sector, investing in methods to decrease the moisture content as one of the processing and preparation steps can make a lot of sense. Consumers of RDF and other prepared fuels that are made from discarded materials require a high enough BTU value to make their decision to use these alternative fuels worthwhile.
Water and other sources of moisture are inherent in all types of waste material streams, including MSW. The organics portion (food scraps, yard trimmings) of MSW contains considerable amounts of water, and some of that moisture gets transferred to the commingled items that organics come in contact with in bins, trucks and at the transfer station. Among commingled recyclables, half-full beverage containers can provide an additional boost to moisture levels.
The weather plays a role in upping the moisture level when waste and recyclables are exposed to rain, snow or high humidity. One sample of scrap paper we tested actually had a moisture reading of 45 percent, thanks to both of these types of exposure to water and other liquids.
Equipment and technology providers offer several ways for RDF producers and recyclers to use drying methods to bring down the moisture levels of their materials to meet consumers’ specifications. Traditional rotary drum driers can perform this task for smaller and more uniformly sized materials while rolling bed driers (RBDs), available from Van Dyk Recycling Solutions, offer an innovative way to dry a wider range of materials.
RBDs were specifically developed for RDF producers, but we have found them to be extremely versatile and we also use them for recovered fiber drying. The concept is based on a combination of a fluidized bed dryer along with material being circulated inside the dryer—so you get both product circulation and retention. You move the material inside and keep it inside the dryer for an extended period at a lower temperature. This keeps the material below its combustion point so there is no fire hazard. The other major advantage is that it can handle material up to 20 inches in size.
The other benefit is that RBDs have an automatic de-dusting system. Dust is typically the first thing that can combust in a dryer (in some applications). The RBDs we sell also have screens on the bottom to remove fines. That is important because when MSW or scrap paper is wet, you can have grit, sand and broken glass attached to the material. This way, when it dries, those contaminants fall off and get screened out through the bottom.
Plant operators in many sectors have the opportunity to allow automated drying to improve their operations. I am eager to work with my colleagues at Van Dyk Recycling Solutions to help you determine whether drying technology can offer you a competitive edge.