Yes it can if there is first-in, first-out processing. In a standard hopper design this implies a live bottom (auger of some sort). In a strictly batch mode, the bottom of the charge will be torrified (at least!) while the top layer will be just getting within moisture spec. From a materials point of use this promises to be an inefficient design. Plus having heterogeneous fuel qualities can only make control more difficult.
It's hard for me to see the analogy with atomizing petrol. Mark -----Original Message----- From: Henri Naths [mailto:[email protected]] Sent: Sunday, December 19, 2010 12:21 PM To: [email protected]; Discussion of biomass pyrolysis and gasification Subject: Re: [Gasification] Drying fuel with IC exhaust and otherpleasures... Hi Mark you wrote <Saturated, but cooled, exhaust gas and some <evaporant would exit the hopper but it would not be a good design because <the upper layers of fuel would experience very little heat transfer. The <stratum where actual drying would be occurring would slowly move upwards in <the fuel bed. <I am just trying to sort-out the heat exchange mechanisms <involved in order to make suitable design decisions The feed stock drying and supply can be calibrated to the demand. Much like the atomizing of petrol in an ic... Henri ----- Original Message ----- From: "Mark Ludlow" <[email protected]> To: "'Ken Calvert'" <[email protected]>; "'Discussion of biomass pyrolysis and gasification'" <[email protected]> Sent: Sunday, December 19, 2010 3:40 AM Subject: Re: [Gasification] Drying fuel with IC exhaust and otherpleasures... > Got it, Ken! But I'd like to point out that without a pressure > differential > (as in a multiple-effect evaporation system) that the daisy-chain that you > describe is impossible. Saturated steam exiting a nozzle, for instance, > will > create a local zone where the steam is superheated, but once it is fully > expanded it becomes saturated. One can't generate a pound of steam, which, > in turn, creates another pound of steam ad infinitum. I think that's a > Second Law violation. > > Water is virtually the only condensable gas in an exhaust stream, as far > as > I am aware. Certainly there is no restriction on the exhaust gas > transferring its sensible heat to the wood and to the water vapor, but > there's plenty of room to 'expand' in a hopper that is not a pressure > vessel. The example that you give of a teakettle is not apt. Water vapor > becomes visible at the spout of a teakettle because the vapor is cooled by > the surrounding air and condenses into visible droplets. Once it is mixed > with the surrounding air, the vapor disappears, as you will observe, but > only if the surrounding air is has capacity for more water. Entropy > increases and the combination of water vapor and air that results has much > less utility. > > I am not asserting that woodgas IC exhaust will not dry wood. I am > claiming > that it will do so only until the point where the partial pressure of the > water in the exhaust gas does not exceed that at the surface of the wet > fuel. If one had a very, very deep hopper, the moisture from lower levels > of > fuel would condense in upper levels and migrate back down toward the > drying > zone, essentially putting the dryer in reflux mode (operating much the > same > as a packed column evaporator). Saturated, but cooled, exhaust gas and > some > evaporant would exit the hopper but it would not be a good design because > the upper layers of fuel would experience very little heat transfer. The > stratum where actual drying would be occurring would slowly move upwards > in > the fuel bed. > > We are all aware of the sensitivity of woodgas quality to moisture content > of fuel. When setting out to design the 'perfect' woodgas IC system, > recuperating exhaust heat for use in fuel drying is obviously very > desirable. I am just trying to sort-out the heat exchange mechanisms > involved in order to make suitable design decisions. Most decisions hinge > on > the characteristics of the exhaust gas itself, as it exits the IC engine > under normal operating conditions. > > Thank you for your insights. > > Best, Mark > > -----Original Message----- > From: Ken Calvert [mailto:[email protected]] > Sent: Sunday, December 19, 2010 1:28 AM > To: [email protected] > Subject: Re: [Gasification] Drying fuel with IC exhaust and other > pleasures... > > No Mark I am saying that running an engine on producer gas has a lot less > moisture than on petrol. And I am saying that hot exhaust gas has so much > excess heat that it will boil off water as steam and and as long as that > steam is moving through the wood it will boil off more steam until the > temperature drops to the point where psychrometrics apply, and generally, > if you have ever seen the steam condensing above a hopper of wet wood, or > watched the spout of a boiling kettle, that only happens after it has left > the confined drying zone and had room to expand. Ken C. > > > --- Original Message ----- > From: "Mark Ludlow" <[email protected]> > To: "'Ken Calvert'" <[email protected]>; "'Discussion of biomass > pyrolysis and gasification'" <[email protected]> > Sent: Sunday, December 19, 2010 9:48 PM > Subject: RE: [Gasification] Drying fuel with IC exhaust and other > pleasures... > > >> Hi Ken, >> Not to flog this horse(power) to death, but are you saying that the >> combustion chemistry (and composition of the exhaust gas) is >> substantially >> different at idle than when an engine is running higher up its horsepower >> curve? I fully realize that the heat flux would be greater. But >> intuitively, >> it seems as if when a load were placed on a normally aspirated, running >> engine, disproportionately more fuel would be required and thus there >> would >> be more products of combustion--including water--by design. Carbureted >> engines used to run pretty rich at idle, but ECMs have addressed that >> issue >> for the most part on fuel injected powerplants. I realize that this may >> have >> little to do with fueling with biogas! >> >> Modern industrial tunnel dryers are often counter-flow; the driest >> product >> comes in contact with the driest (lowest RH) air and this air, in turn, >> 'preheats' the new product coming into the dryer, principally through >> surface condensation, i.e. latent heat transfer. Efficiency is at its >> highest when the exhaust is saturated. This is the principle behind 93%+ >> eff. Furnaces and water heaters. This is a quite different scenario from >> a >> packed-bed arrangement where the feedstock closest to the engine dries >> much >> sooner than higher layers. Once dry, it serves no useful function in the >> feed bed and is responsible for much of the pressure drop through the >> dryer. >> A live-bottom hopper/dryer would address this, I suppose. >> >> If anyone has any dewpoint measurements of IC exhaust gases under various >> load conditions in a woodgas-fueled engine, I would be very interested in >> having these data. I am too lazy to crank the numbers on this one! >> >> Very best regards, Mark >> >> -----Original Message----- >> From: Ken Calvert [mailto:[email protected]] >> Sent: Saturday, December 18, 2010 11:29 PM >> To: [email protected]; Discussion of biomass pyrolysis and gasification >> Subject: Re: [Gasification] Drying fuel with IC exhause and >> otherpleasures... >> >> Mark, sure you can watch the condensation from an engine ticking over, >> and >> >> which you have probably just started, but I challenge you to bore a hole >> in >> the floor of your car and point a IR beam thermometer down on the exhaust >> pipe when you are doing >80mph. You wouldn't be worrying about >> relative >> humidity then. >> Its apples with apples, not iceblocks out of the frig! >> Ken C. >> ----- Original Message ----- >> From: "Mark Ludlow" <[email protected]> >> To: "'Guag Meister'" <[email protected]>; "'Discussion of biomass >> pyrolysis and gasification'" <[email protected]> >> Sent: Sunday, December 19, 2010 3:26 PM >> Subject: Re: [Gasification] Drying fuel with IC exhause and >> otherpleasures... >> >> >>> You are right in a way, Peter. But the water has to leave the wood, yes? >>> Where will it go if the surrounding atmosphere is saturated with water? >>> >>> Let me illustrate with an extreme example: Try drying wood at, say, 110C >>> in >>> a pressure cooker, with the wood covered with water. >>> >>> If the exhaust gas has room for the vapor phase of water, the water will >>> be >>> carried away; if it does not (if it is already near its dewpoint), the >>> gas >>> will become supersaturated and nucleate condensation will occur. There >>> will >>> be rain in the forecast. >>> >>> I held my hand to the exhaust pipe of my 3.6-l automobile while it was >>> idling. It was not scorchingly hot but it was palpably humid. Not >>> exactly >>> Science, I admit. >>> >>> Best, Mark >>> >>> -----Original Message----- >>> From: Guag Meister [mailto:[email protected]] >>> Sent: Saturday, December 18, 2010 4:48 PM >>> To: 'doug.williams'; 'Discussion of biomass pyrolysis and gasification'; >>> [email protected] >>> Subject: Re: [Gasification] Drying fuel with IC exhause and other >>> pleasures... >>> >>> Hi Mark ; >>> >>> This is true, but 100C is a special case because the water can exist as >>> both >>> a liquid and gas. If we discuss a slightly higher temperature, it may >>> be >>> clearer. If you heat wet wood to 105C by any means, all the water >>> present >>> in the wood will be a gas. Yes it will be in equilibrium, but since >>> water >>> as a gas occupies 100x more volume than water as a liquid, you will have >>> succeeded in driving off 99% of the water. You could even do the >>> heating >>> with saturated steam and it would still work. >>> >>> Please correct me if there are any errors in the logic. >>> >>> Best Regards, >>> Peter G. (armchair gasifierist) >>> Thailand >>> www.gac-seeds.com >>> >>> >>> --- On Sat, 12/18/10, Mark Ludlow <[email protected]> wrote: >>>> If the IC exhaust is saturated at >>>> 100C, the best the wood can do is reach an >>>> equilibrium moisture content with respect to this >>>> environment. >>> >>> >>> >>> >>> >>> >>> >>> >>> _______________________________________________ >>> The Gasification list has moved to >>> [email protected] - please update your email contacts to >>> reflect the change. >>> Please visit http://info.bioenergylists.org for more news on the list >>> move. >>> Thank you, >>> Gasification Administrator >> >> >> > > > > > > _______________________________________________ > The Gasification list has moved to > [email protected] - please update your email contacts to > reflect the change. > Please visit http://info.bioenergylists.org for more news on the list > move. > Thank you, > Gasification Administrator > > __________ Information from ESET NOD32 Antivirus, version of virus > signature database 5716 (20101219) __________ > > The message was checked by ESET NOD32 Antivirus. > > http://www.eset.com > > > _______________________________________________ The Gasification list has moved to [email protected] - please update your email contacts to reflect the change. Please visit http://info.bioenergylists.org for more news on the list move. Thank you, Gasification Administrator
