Capturing energy from waste goes global

By Bryan Sims | May 04, 2011

Like the U.S., a myriad of companies across the globe are employing tried and tested methods to convert the millions of tons of waste generated each year into renewable energy. Brazil is no exception, according to Francisco De Souza, CEO of ECOS Environmental Consulting in Brazil. De Souza was one of three speakers at the International Biomass Conference & Expo in St. Louis on the Exploring Multiple Approaches to Energy Capture from MSW panel, which addressed the need for transforming a liability into an ever-increasing asset within the bioenergy sector.

De Souza presented startling statistics pertinent to waste generation in Brazil. For instance, from 2009 to 2010, Brazil generated six times more waste than the population growth. “This increased accumulation of MSW in Brazil is demanding more waste management services that focus on waste valorization processes,” De Souza said. In total, Brazil generates approximately 68.5 million tons of waste annually, according to him, with the southeast region generating the most at approximately 47.8 million tons annually.

According to De Souza, there’s potential for heat generation from refuse-derived fuels (RDF) like MSW materials in Brazil aimed at serving a range of clients involved in various industries, such as pulp and paper plants, cogeneration units and power utility facilities. One client in particular that has expressed immense interest in RDF of late, he said, is cement manufacturing companies in Brazil. But why the specific interest from cement producers?

“It’s all about sustainability,” De Souza said. “Fuel is a major cost for the cement industry. They want balanced fuel supply sources due to supply and logistical constraints. They want to be recognized as a sustainable industry.”

“The idea is to produce the same quantity of ordinary Portland cement using less natural minerals, less fossil fuels and using less carbon dioxide,” he continued. “If they use alternative fuels, like RDF, they can reach those goals.”

Votorantim Cimentos, a leading cement manufacturer in Brazil, is one of the interested cement manufacturers that will soon be utilizing MSW as a fuel to offset its energy and production costs at its largest cement kiln operations in Curitiba, the capital of the State o Paraná located in the south region of Brazil.  De Souza said there is already a winner for the Curitiba Municipal Solid Waste Management and they expect to sign the contract in a couple months. He added that the Votoratim cement plant located near Curitiba has plenty of capacity to valorize and use all the RDF produced for project.

Chris Lyons, manager, marketing and product management for San Diego-based industrial gas turbine manufacturer Solar Turbines Inc., addressed the importance of recognizing the different composition of renewable gases, such as landfill gas or biogases, and how they interact with the different designs of combustion engines. Solar Turbines has installed over 13,700 combustion units worldwide.

Lyons referred to the Wobbe Index as a useful tool when introducing renewable gaseous fuels into a combustion unit, such as landfill or digester gases, which are methane rich, or gasified biomass or waste, which are typically hydrogen rich.  The Wobbe Index, according to Lyons, is calculated by taking the lower heating value of the fuel and dividing it by the square root of specific gravity normalized to air. In practical terms, Lyons said the Wobbe Index is a way of assessing how large the fuel system and injector flow areas must be for a given fuel and pressure drop requirement. 

“Basically, the lower the Wobbe Index, the higher the flow area or fuel supply pressure must be for that fuel,” Lyons said. “Fuels with the same Wobbe Index have the same supply pressure requirements for a given fuel system.”

Lyons continued, “We’re not really worried about the heating value. If you look at natural gas, it’s got a Wobbe of about 1,000 Btu per cubic foot. For combustion turbines, why Wobbe is so important, hydrogen has a BTU content of 200 Btu per cubic foot, but it’s a low specific gravity so the Wobbe value is about 700 Btu per cubic foot. So, it has to do with shoving that much fuel into a fuel injector and shoving that into a turbine.”

Not only should biomass developers be mindful of gas properties prior to introducing it to the combustion turbine, corrosion can be an issue as well. There are two types of hot corrosion: high temp hot corrosion and low temp hot corrosion. Lyons advised to be aware of these corrosion types in design consideration for combustion turbines. He said Solar Turbines is developing new technologies, like high chromium blades, that can eliminate corrosions.

Another concern is the start-up of a combustion unit, Lyons said. “What I mean by start-up is you have to get the fuel to idle, which takes a couple minutes, and then introduce the gas,” Lyons said. “But, you can’t start on a high-hydrogen fuel for safety reasons, particularly because if you have flame-out.” For those that use landfill gas, he recommends to start on natural gas first, but “you can use a variety of different fuels to start-up, such as diesel, kerosene or propane too,” he added.

Another element to consider for developers considering the use of landfill gas and/or digester gas, Lyons said, is to make sure the combustion engines are free of siloxanes, which are silica derived from chemical fractions commonly found in MSW and other waste fractions, such as shampoos, detergents and so forth. Failure to remove siloxanes in the gas can cause harmful deposits on piston rods in the combustion engine. Lyons said Solar Turbines has developed siloxane removal equipment to circumvent this issue.