David, I should clarify that I used the term burning while meaning co-firing. I assume that biomass co-fired in a thermomechanical plant would produce energy that would displace fossil fuel carbon emissions equivalent to about 30% of the original biomass carbon. This number is based on thermodynamic efficiency limits. Am I overlooking something or is 30% in the right ball park? I will repeat the table with that modification in order to be explicit:
1. Burning (e.g., co-firing) with carbon capture (BECS), somewhere higher than 100%, maybe 115% (30% fossil emission avoided, 85% CO2 sequestration); 2. Burial, including CROPS, about 90% 3. cellulosic ethanol with carbon capture, about 80% 4. biochar about 70% (50% in the soil, 20% energy capture) 5. burning (e.g., by co-firing) or cellulosic ethanol without carbon capture, about 30% 6. Leaving it on no till soil, less than 10% As for the long run or the short run, I think that depends on the urgency of the problem. Some of us feel that the atmospheric CO2 problem is urgent now, but we are in the minority. Judging by the priorities of our established institutions, fuel production is the 1st, 2nd and 3rd priority, but a comprehensive policy toward technical means to lower GHGs is missing. = Stuart = Stuart E. Strand 490 Ben Hall IDR Bldg. Box 355014, Univ. Washington Seattle, WA 98195 voice 206-543-5350, fax 206-685-9996 skype: stuartestrand http://faculty.washington.edu/sstrand/ From: David Keith [mailto:[email protected]] Sent: Thursday, September 16, 2010 8:38 AM To: Stuart Strand; [email protected] Cc: [email protected]; [email protected]; James Rhodes Subject: RE: [geo] Carbon sequestration workshop Sep 9-10, Heinz Center, Washington DC Stuart, Thank you for this. We are getting closer to a sensible evaluation. While we still have an economy with huge carbon emissions, a ton of negative emissions is the same as a ton not emitted. So while cofiring biomass in coal-fired power plants without capture does not rank on your list at all, because there is no capture, it still avoids emissions of carbon. And because the capital cost of retrofitting plants large coal plants for biomass co-feed is very low and their combustion efficiency (for the biomass) is high this can be a very cost-effective way to reduce carbon emissions with biomass. Of course, it's not sexy and it's limited. I think there are two analysis to be done, over the next X decades when carbon emissions are high, the relevant metric is not sequestration efficiency but carbon mitigation efficiency. We need to find ways to reduce emissions which are cost effective and of low environmental impact. Where biomass can help, because it is a limited resource, we need to find ways to do this they provide a lot of avoided emissions per unit biomass. However, there are several dimensions of trade-off here and one does not want to maximize along only one dimension and forget all the others. For example, just maximizing the efficiency per unit biomass and ignoring cost or other environmental impacts makes no sense. In the long-run, when the emissions intensity of the rest of the economy is lower, then negative emissions per unit biomass, the metric that you are focusing on, becomes more important. I certainly agree with your last point, that we need to broaden the range of options under consideration for managing carbon emissions, and think about some of these interesting hybrids. -David For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
