There is a paradigm shift in the energy field. Promising new technologies are showing success in research level developments. Others are being commercialized for the first time and struggling through startup and shakedown. Still others are showing their frailties and their limits in the marketplace. And some may yet be a long daydream away.
The mix of energy sources we might see 20 years from now is yet unknown. We know some of the elements, and some of the elements we know well.
Such change brings new challenges as well as new opportunities for improvement. Economic and political seismology impose additional layers of complexity.
This session is intended to reflect on real-time development and early commercialization of thermophilic anaerobic digestion (TAD) in the U.S. and how one team is approaching Environmental Health and Safety concerns relative to such development. It follows last year's session, which compared energy and emissions data from a number of alternative fuels on a well-to-wheel basis using the GREET model from Argonne Labs.1One of the fuels examined at the 2008 PDC was compressed natural gas (CNG) derived from the TAD process. The modeling data reflected considerably better energy and emissions data from CNG than for any of the alternatives to conventional diesel included in the study. This result was magnified for greenhouse gas emissions, where CNG reflected a carbon negative performance on a well-to-wheel basis.
Last year's data revealed the following summary data regarding emissions of CNG from both extracted and renewable feedstocks as compared with low sulfur diesel.
(Table in full paper)
The GREET Well-to-Wheel model provides a full life-cycle assessment of energy and environmental performance of transportation fuels. For fossil fuel natural gas, this includes contributions from extraction and processing, as well as how efficiently and cleanly the fuel burns. For TAD technology, the steps include cultivation and/or collection of biomass and processing.
In its most simple terms, a TAD operates as follows: when organic waste decomposes it gives off methane and carbon dioxide. TAD holds organic matter in an oxygen-free environment until it decomposes and then captures the gases that otherwise would be emitted into the environment.
The installation is planned to process 621 tons per day of organic waste in an oxygen-starved in-vessel system at temperatures from 50°C (122°F) and 58°C (136°F). This compares with mesophilic systems which are more common in the United States and run at temperatures ~98°F.
The four products of the digestion process are: methane, carbon dioxide, water and compost. There are no other by-products from this operation.
However, a weakness identified in the study was the use of Argonne's GREET model to calculate the well-to-wheel emissions of the CNG from renewable sources. The model did not have a component for CNG from renewable sources, and surrogates were required to estimate comparable results. While feedstock and fuel use surrogates could be found, no reliable model was presented for production of CNG from renewable feedstocks.
