Solid solutions

The need for and use of refuse-derived fuel (RDF) has changed over the last 30 to 40 years. Boilers once needed fuel from waste to be resized, but that became less important as mass-burn waste-to-energy (WTE) facilities proved effective without pretreatment of waste.

Today, a resurgence of refuse-derived fuel demand is being necessitated by newer waste conversion technologies, such as gasification. Also, material recovery facilities (MRFs) and mixed waste processing facilities are producing fuel for a variety of industrial processes with their residual material. These newer processes often require more sophisticated techniques than older systems to optimize the calorific value that can be captured from the fuel.

The trends and processes that are giving refuse-derived fuel more attention these days were the focus of a session at the Renewable Energy from Waste Virtual Conference in early October.

During the session, Nathiel Egosi, president of RRT Design & Construction, Melville, New York, explained that the function of a RDF plant is to take nonhomogenous municipal solid waste (MSW) and produce a high-quality fuel while removing noncombustible inorganic material, high moisture content material, and certain hazardous and recyclable items.

“RDF plants were principally developed to prepare solid waste for cocombustion with coal in a utility boiler,” Egosi said during his presentation. “Later, RDF plants with dedicated boilers were developed to achieve better economics and emissions.”

One of the first such plants was built in the city of Ames, Iowa. It came online in 1972 and still operates. The next generation of plants were built to either cocombust RDF with coal or combust RDF alone in spreader-stoker-type boilers.

Some of the first plants, like Southeastern Public Service Authority (SPSA) in Chesapeake, Virginia, and Miami-Dade County Resources Recovery Facility (RRF) in Florida, used trommels before shredding. “Trommeling was used to remove inerts before shredding to produce a low ash, high British thermal units (Btu) RDF,” Egosi said. “Trommels also were used to concentrate recyclables for recovery before production of RDF.”

Other plants used primary flail mills prior to trommeling. The last traditional plant in the U.S. was a 2,000-ton-per-day system built in Robbins, Illinois in 1995.

“RDF is really a U.S.-developed technology that is proven and [has been] in use for over 40 years,” Egosi said.

He said the difference between RDF plants of today and older plants is the consequence of three things. First, the waste stream has changed dramatically. Second, a lot of lessons have been learned and, third, “mass-burn boilers really do work, so the need to alter MSW and manipulate it in a certain way became obsolete and people moved away from RDF.”

Around 2005, however, renewed interest in RDF arose due to the rise of gasification developers. Oil prices also went up, making the economics of such plants intriguing.

The RDF needed to be produced from MSW to make it suitable for a downstream thermal process, to fire in gasifiers or nonMSW furnace/combustors/kilns and to prepare it for downstream chemical processes, Egosi said.

A number of demonstration and pilot plants were built. One in San Antonio closed due to economics. The future of a plant in Philadelphia is uncertain, and a plant in Edmonton, Alberta, is presently providing RDF to Enerkem’s waste-to-methanol plant.

Since 1985, tipping fees have held flat and the density of the waste stream has changed quite a bit, he said. Since that time, textiles have gone up, glass has gone down, food scraps had a dip in 1990-2000 and metals have remained relatively flat. Paper has gone from 31 percent of the waste stream down to 26 percent. Plastics have experienced the most dramatic change from about 3.4 percent of the waste stream in 1985 to 12 percent of the waste stream in 2017.

Engineered fuel is a term that is used when an RDF is further treated or blended with other waste types to achieve a higher and more consistent Btu value, lower moisture content and more predictable emissions, Egosi said.

MEASURING QUALITY

Brian Schellati, director of business development at Van Dyk Recycling Solutions, Stamford, Connecticut, said during his presentation that the company has completed more than 1,700 recycling projects, 250 of which are turnkey MRFs. Of those, only five have been making fuel from mixed waste. But, unlike Europe, Schellati noted, “it has mainly just been a growing market over the last couple of years. More and more processing is being done to create a fuel from mixed waste.”

The incineration or gasification of this material comes with the risk of unacceptable levels of inerts, chlorine and water content. These issues can lead to poor efficiency in gasifiers and overburn situations in kilns because of the reduced energy content from noncombustible inerts. They also can cause increased downtime in light of precipitation and buildup of salts in kilns and increased exhaust emissions. These contaminants also can cause damage from corrosion in thermal systems.

“This results in a poor thermal substitution rate, higher maintenance costs and more downtime,” Schellati said. “Therefore, it is necessary to measure and check these critical values.”

Schellati said contaminant levels currently are measured by doing spot tests. The samples are taken to a laboratory, but often, the results are not satisfactory because the spot tests are not representative of the entire material makeup and there is a high lag time before the results are available.

During his presentation, he showed an analysis of three small fuel samples taken from the same stream of material, which showed major differences in the results.

In another example, Schellati showed a comparison of results from two laboratories that varied by more than 50 percent. The solution, he said, is real-time online analysis of least 10 percent of the production stream of the fuel.

“We use near infrared sensors to identify the different materials in the fuel in real time and on a moving average,” he said.

Some of the key values of interest are chlorine content, water content and heating value of the fuel.

PVC plastic is the main material that can be recognized to measure chlorine content. The moving average over 5 minutes that is calculated is far more precise and useful than the spot results from the laboratory, Schellati said.

“In conclusion, online analysis in real time provides continuous monitoring and measurement of the entire material stream, supplies the producer and the buyer with instant data and, most importantly, ensures that the end user gets consistent product quality for optimal efficiency. It also minimizes costly maintenance and downtime,” he said.

LEARNING FROM EXPERIENCE

Jonathan Menard, executive vice president of sales and strategic positioning for Machinex Technologies, Plessisville, Quebec, said during his presentation that he is observing a change in the market in the U.S., U.K., France, Spain and Australia. Over the last eight years, the company has built more systems for the MSW and commercial and industrial (C&I) materials markets with the aim of generating RDF or SRF (solid recovered fuel) and recyclables recovery, despite usually building single-stream systems.

Menard said the difference between SRF and RDF is quality. RDF is a lower quality fuel than SRF. Its calorific value is between 10 to 20 megajoules per kilogram, and it has no particle size requirements. The moisture restriction also is vague, between 35 to 40 percent maximum. “And the main difference is, in Europe, there is no regulation regarding RDF,” he said.

The SRF calorific value is 20 megajoules per kilogram or higher. The size ranges from around an inch to an inch and a quarter. Another difference, he said, is that it is a regulated fuel.

He said Machinex has built four plants in the U.K. over the last five years that are producing RDF and SRF. The first one the company built was Veolia Warren Farm in Fareham, England. It is a 15-ton-per-hour plant processing C&I and civic and amenity (C&A) waste with the goal of recovering fines, ferrous, heavies, PVC (polyvinyl chloride) and producing a SRF product. It was Machinex’ first project dealing with SRF. Menard said the chlorine content was low at 0.4 percent because of customer requirements, and therefore, Machinex put an optical unit in the system to remove PVC.

Machinex’s second plant, completed in 2013, was Public Power Solution, Swindon, England. It is a 15-ton-per-hour SRF and RDF plant that processes municipal solid waste and C&I. The system processes a variety of materials with a high moisture content and uses a rotating drum dryer to convert RDF into dry SRF that meets European standards.

The third plant Machinex built in the U.K. was Shanks Waste Management in South Kirkby, England. It helps the surrounding area of Wakefield district achieve a 95 percent landfill diversion rate, Menard said. The 30-ton-per-hour system recovers mixed paper, plastic film, mixed plastic, ferrous and nonferrous metals and 60 millimeter fines.

The fourth plant Machinex is building in the U.K. is at Levenseat Renewable Energy Ltd, in Lenark, Scotland. It is a 42-ton-per-hour C&I and MSW plant. At the time of the REW Conference, the project was two weeks into construction but had been planned for about two years.

Levenseat uses thermal drying of fines to improve RDF generation and landfill diversion. The business model is based on a low calorific value to allow for more RDF to be burned in the gasifier. This allows for more material coming in.

Menard advocates these types of systems at the front end of a waste conversion project. He said the added value of mechanical pretreatment is first to convert more waste material into valuable commodities as an RDF and SRF, significantly increase landfill diversion and give a chance to increase recycling capture.

“What we learned through all these projects is that it is one thing to generate an SRF and RDF, and it is much more difficult to generate a homogeneous one,” Menard said. “And this is what all these advanced thermal plants require because they don’t have to keep compensating on their side, so you try to deliver something homogenous. That is the key to success.”

The author is editor of Waste Today and can be reached at ksmith@gie.net.