Originally posted by: BrownTown
I know what a fuel cell is, and the reaction is exactly the same as combustion. In the case of hydrogen that is 2 H2 + O2 => 2 H2O, the only difference is the way in which is goes about doing this, the electricity is produced in a different manor, the energy is still all coming from exactly the same source by exactly the same reaction. I assume you know this based on your statements, but your apparent assumption that I do not know it is incorrect. The undisputed fact that bacteria exist which produce electrons does not change the fact that they are neither efficient or cost effective at doing so in relation to other methods of extracting the energy from biomass.
No, the reaction in a fuel cell is not the same as combustion. The fuel cell catalyzes the process, increasing both efficiency and selectivity by altering the activation energy profiles of the potential reactions. The reaction is also fundamentally different, since it involves a catalyst. It is true that the net reaction is the same, but there are intermediate steps involving the catalyst. Involving the catalyst also removes undesirable side reactions, increasing efficiency. Your statements regarding bacterial efficiency are demonstrative of this. Your inability to grasp the fundamental utility of "efficiency" is more glaring still. Bacteria can be very efficient. Indeed, most living things are very efficient. Obviously many other engineers have found this approach to be cost-effective, since this technology has already been implemented at several full scale plants. You say that you know, but you have said nothing to support this in any of your posts so far.
Well certainly MOST bacteria produce CO2, some may not, the statement "bacteria do not release CO2" is wrong since it is not true for the majority of cases, but again, I am assuming based on your research you have a certain bacteria in mind which may or may not produce CO2. However it is important to note that depending on exactly what this sedimented sludge is, it may simply break down rapidly into CO2. For example trees obviously lock CO2 into biomass, but when leaves fall off they are rapidly converted back into CO2 by decomposers. If the bacterial waste will also decompose rapidly then the CO2 is still getting out.
Sludge is
the solidous product of wastewater plants and is extremely stable. At the plant I worked at, it was stored in a giant warehouse until people could come by and pick it up to spread on their crops (as long as the crops weren't for human consumption). I could walk into this building without any problems, which would obviously not be true if it were decomposing at all into CO2, since CO2 is an asphyxiant at higher concentrations.
This is likely the root of the problem here in that we are approaching this from different angles since I am approaching it from an electrical utility type background and you are from a wastewater treatment type background. However I still maintain what I have said previously which is that a bacteria based approach is more expensive and less efficient than simply cofiring the biomass. Your superior knowledge concerning the use of bacteria in breaking down biomass will likely allow you to win any sort of argument concerning bioreactors or bacteria and I will concede that point to you, however I find it unlikely that you will be able to disprove my original premise here, if you have some evidence concerning the cost and efficiencies of this process that show it to be superior then I would be interested, and in turn I can provide you with papers showing the efficiencies of modern coal boilers to be 40%, showing the use of biomass in these boilers, and showing the costs associated with this method if you want to dispute any of those points.
It may be more expensive and less efficient in the short run, but there are several reasons that these plants don't burn biomass (EPA regulations ranking high among them, but also the long-term utility of these fuel cells). A simple cost-benefit analysis shows that for many wastewater flows, this type of fuel cell is extremely cost-effective and can actually power the entire plant. The plants that I am aware of that have currently implemented this technology actually kick power back to the grid as they are able because they can produce a surplus depending on the time of day. As for the supposed "poor" efficiency and cost, you never mentioned those in a meaningful way in the post I quoted above. You simply tried to pull out the chemistry card and got shot down, so now you're changing your story. I'm not here to engage in a pissing match, but I will certainly correct your misinformation as I see fit. For example, you previously said that "...when people are talking about the efficiency we are talking about the percent of the energy stored in the hydrocarbon bonds that is converted to a useful form like electricity," which is factually incorrect for this system. The chemical potential energy doesn't factor in to this efficiency calculation at all unless you are burning the biomass. The efficiency of interest in this fuel cell system (as with any energy production scheme) is the output energy divided by the input energy. Since the input energy here is extremely small, the efficiency is actually very high. If you wanted to investigate the rates of energy production, then this technology might not compare favorably with burning biomass. However, this technology isn't for broad-based energy production as I stated in my first post. It's for powering the plant that is utilizing it, and it is extremely effective for that purpose.
EDIT: also FWIW, methane produced from waste water is already cofired with coal in some locations to produce "clean" energy, incidentally, of all the "alternative" methods of energy; solar, wind, biomass ,etc.. this is the ONLY one so far that is currently economically viable with current sources of electricity, however it can only be utilized in a small number of locations where wastewater treatment facilities and coal or natural gas plants are located very close to each other and is therefore not very available. Also hindering the development is the fact most econuts don't seem to consider fuel made from shit to be "clean" even though in terms of reducing the greenhouse effect it is ~100 times more cost effective than solar.
Yes, I'm fairly familiar with cogeneration engines. The city of Dayton, OH (which happens to be where I did my masters work) was the first plant to have cogen engines that were fired entirely by methane output from their process. They were a nightmare to maintain because they had to run at a constant frequency with variable power outputs, which is hardly a trivial feat of mechanical engineering. Throwing the biomass in with coal doesn't address the same need that this technology is targeting, so your comparison is futile. No one (at least, not me) is saying that this technology is a viable large-scale energy source. However, it is an excellent supplemental technology when applied correctly in powering wastewater plants, which often generate a small energy surplus using this method. And nothing is ~100 times more cost-effective than solar at this point or we would all be using it.
