Waste Conversion, Re-processedWaste Conversion, Re-processed
October 17, 2011
In a previous column I noted the need for waste conversion technologies to “Scale up or Shut Up.” As part of a grant from the North Carolina Biofuels Center, the Environmental Research and Education Foundation and its collaborators on the project (North Carolina State University, Maverick Biofuels, Waste Industries) have been identifying further challenges that lay ahead for converting municipal solid waste (MSW) to biofuels (and other value-added products), beyond simply scaling up.
Although there are other conversion technologies out there, some of the most promising are pyrolysis, steam gasification and plasma gasification. Despite differences in the process, these gasification technologies create syngas (primarily composed of carbon monoxide and hydrogen) followed by a Fischer-Tropsch (F-T) process, which uses a catalyst to convert the syngas to a biofuel. While seemingly magical on the surface, the F-T process uses the basic science of catalytic reaction (much like a catalytic converter on a car) to reorganize carbon (C) and hydrogen (H) molecules into carbon-hydrogen chains such as C12H23, which is the average molecular formula for diesel (for those of you who, like me, may need to reach back a few years to high school chemistry class).
So, aside from demonstrating scalability, what challenges lay ahead for those hoping to convert MSW to biofuels? One preliminary finding is that gasification technologies have specific feedstock requirements, both in terms of composition and particle size. This means that feeding raw waste material into a gasifier using a wheel loader or cart tipper is likely not going to be an option. Most gasification processes require non-combustibles such as glass and metal to be removed to ensure the process is either functional or efficient. Note that while this may be achieved with a single-stream materials recovery facility (MRF) or a source-separated recyclables program, the efficiency in removing non-combustibles would need to be monitored more closely than it is currently.
Additionally, the moisture content of incoming waste must be 10 percent or less, which may work for mixed residential MSW with separated collection of food and yard wastes. However, without the organics removed, mixed residential MSW typically has a moisture content of between 20 and 25 percent. For non-residential waste, moisture content is even higher.
Commercial waste (including restaurant waste) can have a moisture content as high as 70 percent. Additionally, at certain times of the year such as holidays, residential MSW can experience moisture content spikes well in excess of 25 percent due to increased food waste disposal (and you thought the only consequences of those holiday parties were weight gain and hangovers).
Overly moist waste likely will need to be separated or dried before it can be converted to biofuels. This may add substantially to operating costs on the front end.
To minimize complications and maximize efficiency, particle size entering conversion technologies must be relatively uniform and sufficiently small to sustain a complete or nearly complete gasification reaction. Couple this with a 10 percent or less moisture content and the required material entering a conversion process begins to resemble refuse-derived fuel (RDF). Although a fully processed RDF may not be absolutely necessary, the waste conversion process would require a substantial amount of pre-processing.
This also begs the question of how much waste composition can shift or change before the quality or composition of the generated biofuel is affected. Similarly, how will changes in waste volume affect the process? While some of the pre-processing issues may be addressed by scaling up, issues like moisture content and the effect on quality of the end product will likely remain. These challenges are certainly not insurmountable, but they need to be addressed during development of the technology because compared to other feedstocks like plant biomass or wastewaster sludge, MSW clearly brings its own set of issues to the table.