Home Magazine Exploring the possibilities Part 1

For local governments managing the collection and disposal of refuse, a new paradigm exists. Solid waste is now considered a resource. Specifically, some components of the solid waste stream can be converted into energy. The concept of pursuing conversion technologies for solid waste management to promote landfill diversion, generate renewable energy and/or reduce greenhouse gases can be relatively simple. However, actually determining what technology to choose is considerably more complicated.

But don’t despair. Local governments considering such possibilities already are ahead of the game, searching for ways to make the best decision for a community in the short-term and for the foreseeable future. Such options can be identified by choosing technologies appropriate for the community’s location, size, budget and waste stream.

To do so requires a thorough, methodical approach that takes a number of factors into account. A five-step process is recommended for consideration.
 

Step one: explore the universe

The first step is to become familiar with various alternatives available —the universe of technologies that could convert your waste into power.

The following is a summary of eight technologies most frequently considered by waste managers.

• Mass burn combustion. Whether it’s a modular starved-air system (historically used for smaller applications of less than 400 tons per day) or a field-erected excess air system, such technologies combine refractory-lined combustors to reach desired capacity with ash as a residual. Tubes form the incinerator walls, allowing water to circulate as part of the steam-generation process, which in turn is used to create electric power. Some facilities use steam turbines to produce steam for sale to an end user.

• Advanced thermal recycling. This is the term given to mass-burn technology plants that are enhanced —either with a preprocessing system similar to a material recovery facility (MRF) added to remove recyclable materials from the municipal solid waste (MSW) prior to the MSW being introduced into the furnace and/or with an advanced emission-control system designed to capture and recover components in the flue gas, converting them into marketable byproducts.

• Refuse-derived fuel (RDF) combustion. RDF systems, which have been used for decades to improve the quality of MSW leading up to combustion, recover materials and generate energy. RDF can be burned in different types of combustors. Several material processing systems typically are used in an RDF plant, including shredders, magnets, eddy current separators, trommels and picking stations. Different combinations and arrangements optimize results.

• Pyrolysis. Pyrolysis typically occurs at temperatures ranging from 750 F to 1,500 F and degrades feedstock without the addition of air or oxygen. This process produces oils and fuel gases that can be used directly as boiler fuel or refined for higher quality products, such as engine fuels and chemicals. Solid residue from pyrolysis, often called “char,” contains solid carbon and most of the inorganic portion of the feedstock. Burning gases produced during the pyrolytic reaction in a separate reaction chamber releases significant thermal energy, which can be used to produce steam for electricity generation, heat the pyrolytic reaction chamber or dry feedstock entering the reaction chamber.

• Conventional gasification. These technologies cover fluid-bed gasification and fixed-bed gasification, which involve the thermal conversion of organic, carbon-based materials with a limited supply of air/oxygen in the presence of internally produced heat—typically at temperatures of 1,400 F to 2,500 F. This process produces synthetic gases (syngas) composed primarily of hydrogen and carbon monoxide (CO). Inorganic materials are converted either to bottom ash with potential beneficial reuse for making ceramic-like bricks or paving stones, or to a solid, vitreous slag with potential beneficial reuse in roofing tiles or asphalt filler.

• Plasma arc gasification. Developed for the metals industry in the late 19th century, plasma arc technology—a collection of free moving electrons and ions formed by applying a large voltage across a gas volume at reduced or atmospheric pressure—uses extremely high temperatures to break down feedstock. Plasma can reach temperatures of 7,000 F and higher, breaking up the molecular structure of organic material to produce simpler gaseous molecules such as CO, hydrogen and carbon dioxide (CO₂), while inorganic material is vitrified to form a glassy residue.

• Anaerobic digestion. This biological process, known as AD, involves the microbial breakdown of large organic molecules into biogas, which can be treated and combusted in engines, turbines or boilers to produce power, or otherwise processed to create a fuel comparable with natural gas (sometimes referred to as biomethane). The process also produces a residue that contains inorganics, nondegradable organics and other materials—solids that may be cured in standard composting processes.

• Mechanical biological treatment. This approach combines mechanical treatment of the incoming waste stream with biological treatment of the organic fraction of the waste. It is common in Europe, where landfilling of untreated waste is limited. Recyclable materials often are recovered, and remaining materials are used as feedstock for a thermal treatment process.

The preceding list is not an exhaustive one. Other technologies also are marketed for converting MSW into energy. However, evidence of application to solid waste is limited.

Step two: ask threshold questions

Once you are familiar with the universe of possibilities, you should focus your search on what may work in your community. Two key threshold questions should initiate the technology screening process:

1. Has the technology been used to manage the targeted waste streams? Some technologies provide volume reduction of waste, generate heat and operate efficiently. Be sure to weigh the prospects for each technology in terms of how each would address your actual waste stream. If no technical history reflects the proposed technology has been applied to the targeted waste feedstock, then excluding the specific technology from any further review is recommended. For example, plasma arc gasification has been used to manage various hazardous and industrial wastes, but historically has not been commercially applied to convert mixed MSW.

2. Is there a reference facility? Project developers may advocate the successful application of select technologies to MSW without a specific reference facility. A reference facility is a conversion facility where the select technologies have successfully converted the targeted waste streams to energy on a continuous basis. The distinction between a commercially operating and a demonstration facility is that the commercially operating facility has operated on a continuous basis; the demonstration facility usually has not. However, the existence of a facility that has successfully demonstrated the applicability of the technology to the targeted waste stream provides a basis for further consideration. Without either of these types of reference facilities, then excluding the specific technology from any further review is recommended.

Part II of this feature, which will appear in the May/June 2014 issue of Renewable Energy from Waste, will look at three additional steps to aid in the conversion technology decision-making process, including: identify and apply the assessment criteria; develop comparative matrix and rank the technologies; and seek assistance.

The author is solid waste and resource recovery manager in Minneapolis-St. Paul for the Environmental Global Practice at Burns & McDonnell, Kansas City, Missouri.