Biofuels and Gasification
Let's start with the basics. Biofuels include ethanol, biodiesel, butanol and others. Biofuel production uses both old and new technologies. Conventional “first generation” ethanol is made by fermenting sugars from plants with high starch or sugar content into alcohol, using the same basic methods that brewers have relied on for centuries. The purest form of biodiesel is straight vegetable oil, but a more refined form produces methyl esters (basically, diesel). At the moment, most of these fuels are made from three kinds of agricultural feedstocks, which are also used for food:
• sugar crops, including sugar cane, sugar beets, and sweet sorghum;
• starch crops, including corn, wheat, barley, rye, cassava, sorghum grain, and other
• oilseed crops, including rapeseed/canola, soybeans, sunflower, mustard, and others.
“Second generation” biofuel technologies employ more sophisticated processes to convert biomass into fuel. These include enzymatic and other processes to turn urban waste (municipal solid waste, or MSW), animal waste and fat into biodiesel, and convert cellulose from grasses and waste wood into ethanol and other fuels. So the Edmonton Biofuels Facility is a second-generation biofuel technology.
After reading a May 6, 2010 press release about the facility, I found a nice diagram of the gasification process as it would look for our facility. (Click the image to see a larger version.)
Step 1- Feedstock pre-treatment
The sorted municipal solid waste which is left over from the Materials Recovery Facility or the Composting Facility or other landfill material is shredded. It is stored in a container that is connected to the gasifier via a front-end feeding system.
Step 2- Gasification
The gasification process converts carbon-rich waste into a uniform synthetic gas (syngas). The heat and the pressure break apart the chemical bonds of the waste material into a syngas. The conversion of waste materials into syngas takes less than 10 seconds. Due to the lack of oxygen in the process, there
Step 3- Cleaning and conditioning of the synthetic gas
The syngas is cleaned and conditioned in order to prepare it for catalytic conversion to methanol and ethanol. The cleaning process is accomplished through a sequential conditioning system, which includes cyclonic removal of inert matter, secondary carbon/tar conversion, heat recovery units, and reinjection of tar/fines into the reactor. The syngas that is produced by this process is ready for conversion into liquid fuel.
Step 4- Conversion into liquid fuel
Using a sequential catalytic conversion process and commercially available catalysts, the syngas is converted to market ready fuels and chemicals. The catalysts rearrange the molecules in the gas into methanol and ethanol.
The Emissions from Gasification
Because there's a lot of concern about "heating up" garbage which will release extremely harsh chemicals like dioxin, I've been wondering about the emissions from a gasification facility. Thanks to Wikipedia, here's some insight into the greenhouse gas emissions:
In second generation thermal technologies, nearly all of the carbon content in the waste is emitted as carbon dioxide to the atmosphere (when including final combustion of the products from pyrolysis and gasification). Municipal solid waste contain approximately the same mass fraction of carbon as CO2 itself (27%), so treatment of 1 metric ton of MSW produce approximately 1 metric ton of CO2. In the event that the waste was landfilled, 1 metric ton (1.1 short tons) of MSW would produce approximately 62 cubic metres (2,200 cu ft) methane via
the anaerobic decomposition of the biodegradable part of the waste. This amount of methane has more than twice the global warming potential than the 1 metric ton of CO2, which would have been produced by combustion.
Waste to Energy Research and Technology Council (WTERT) Canada
"Trash-Based Biofuels Could Alleviate Land Use, Emissions Issues" posted May 4, 2010
In a January 2009 publication released by the Sierra Club and Worldwatch Institute, entitled Smart Choices for Biofuels, gasification and other second generation (or thermal) technologies were highlighted as having a potential advantage over first generation technologies (p.7):
Nearly all studies on the role of biofuels in mitigating global warming and boosting energy security have concluded that “second-generation” biofuels, which rely on non-food feedstocks and offer dramatically improved energy and greenhouse gas profiles, are necessary to make wider use of biofuels feasible worldwide.