Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 23/10/2013 15:59:51
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A guy in the industry was telling me that over five years the price of Bunker fuel has almost doubled.... although I've not found any stats to back this up. However I think there is little doubt that the price is now much higher than it was in say the 1960's when ships started being built with low-speed diesel engines that could burn this muck.
The other thing is that Bunker has high levels of sulphur - though it is scrubbed out prior to fuelling in many ports these days. Of course much of the coal now mined also has heavy concentrations of sulphur (though perhaps not as high as bunker per kilo; maybe higher in energy equivalence terms though, I don't know).
But coal is cheap... and unlike even residual oil, will remain cheap for a long time yet.
So, if an efficient means to power shipping - gasifying coal on-board prior to combustion in gas turbines say - then CO2 aside, would it be economic (and of good enough air quality near ports/cities) to return to a solid-fuel approach?
...and there's more.
Drax power station is now co-firing coal with torrefied wood to allow them (maybe unfairly) to continue to operate as ghg emission laws from the EU are tightened. With a similar approach to shipping the industry could be both economic and (relatively) clean.
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I've wondered about that.
Of course for trains, there is always electric power which is popular in Europe, but not so much here in the USA.
There are many places where junk wood is of so little value that it is either burnt in place, or let rot in place, perhaps even chipped and let rot. The problem, of course, is getting it from the source to the market. The advantage of wood, of course, is that it is pretty much universally available.
A pelleted wood, or hog fuel could be easily fed with automation, no more men stoking the Titanic with shovels from the bowels of the ship.
I found a note about the energy density of different fuels. (http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,20041&_dad=portal&_schema=PORTAL)
Looking at energy per ton, coal (Anthracite) has about 3x as much energy per ton as wood, and it is about 10x as much when comparing energy per volume. It does mean that more weight and space would have to be dedicated to the fuel.
Prepossessed wood chips (Torrefied Wood) have a bit higher energy density than the raw hog fuel, but would it be worth the added energy expenditure and expense?
It appears as if the efficiency of steam turbines (http://en.wikipedia.org/wiki/Steam_engine#Efficiency) are running as high as 40% to 50%, and could potentially compete with internal combustion engines.
Many ships are converting to electric/electric coupled engines, so the generator would be relatively independent of the drive engines.
Perhaps one issue with steam is the start-up time. On a ship, one may be able to plan starting a few hours early so it may not be a big issue, and tugs often assist in port too.
People might complain about wood-chip hog fuel floating onto beaches after a shipwreck, but it would certainly be less traumatic than huge oil slicks.
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The problem with solid fuels is not the energy content, its the density, they take up more space for the same energy.
Along side this (as yes powdered or pelletised fuel could be used) reduction in the size of particles increases the chance of fires, throw a lit match into a bucket of oil and you probably wouldnt have a fire, do it to a pile of wood or coal, and it will almost guarantee a fire (the smaller the particles the higher the probability), and the dust is explosive. Oil is actually a fairly safe fuel to carry around with you.
A large part of the contaminants such as sulphur is actually removed prior to burning through fuel purification and settling.. (the ACTUAL steps are.. from bunker(30degC) to settling tank(30degC), through a centrifugal purifier to a service tank (120DegC))
Another factor is the stability of the vessel, it is much easier to calculate the stability of the vessel with a liquid filled tank, there are good calculations for the effect of part filled tanks, however with a solid load there are much more serious concerns such as shifting during transit (which has sunk many a vessel) saying that it does reduce some of the free surface effect that worries so many mariners.
I have sailed on diesel ships and steam ships.. I have yet to fully experience diesel electric, but its currently safe to say.. Big ships will remain with direct drive diesel engines, small ships are going towards diesel electric, steam is going the way of the dinosaurs. (Of course functions of the vessels, such as turn around time, port to port distances etc will dictate the choice of engines, in fact large cruise ships are going D/E which saves space and gives much higher maneuverability (with azipods))
Paperwork efficiency aside, there was VERY little difference in the amount of fuel it took to run Diesel vs Steam (at high seas) its when it was in port that the constant boiler requirements outstripped the diesel. But give the beancounters the paperwork and they will choose diesel over steam everytime (In fact in practice Steam is cheaper to run on the life span of the vessel (spare parts, servicing etc)) The steam turbines (drive) we have are pretty good efficiency wise, extracting pretty much all the pressure and velocity out of the steam (down to vacuums of 720-740mmhg (max vac is 760mmhg)) for the power generation its not so good, but its adequate for our needs. (that is ship based.. land based can use much bigger turbines)
Sorry for boring you, but yes, solid fuel could be used, but its not as simple as energy content and how to get that energy.. the entire ship needs to be considered.
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SimpleEngineer,
You bring up quite a few valid points.
Liquid filled tanks, of course, is a problem with the shifting weight of a tilting ship, but may be able to be compensated for with compartmentalization, which then would allow fine tuning the side to side and fore and aft weight distribution.
One could likely do similar compartmentalization of solid fuel bunkers, but then it would make the fuel distribution much more complex.
Pumps and piping are easy to build, and also easy to incorporate redundant systems. Conveyors for the use of solid fuel would be more complex, and perhaps would also be more labor intensive.
But, considering a container ship might be using 100+ tons of fuel a day, if there was substantial cost savings, it may pay for a couple of extra material handlers in the crew.
Fires are a problem for all fuel types. Liquid fires can be a problem, but fighting all types of fires should be about compartmentalization, oxygen deprivation, removing heat, and perhaps flooding. it should be possible to safely use different fuel types.
Hmmm...
How much fuel?
Say a ship takes 300 tons a day of low grade fuel.
A 20 foot shipping container would carry about 25 tons of wood pellets.
So, it might take 12 shipping containers of wood pellets per day to fuel the ship.
How many days? 30?
So, perhaps just under 400 shipping containers to fuel the ship for an ocean crossing.
It sounds like a lot, but many cargo ships can carry over 10,000 containers.
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Pretty much everyhting in that post is spot on CliffordK..
(Even the figure for 300 tonnes a day fuel consumption for the bigger ships)
However I dont think having shipping containers full of the fuel is such a great idea. (the biggest ships have set the record at 15,000 containers) You have to consider the centre of gravity and centre of buoyancy (low down things are good!
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Low down things are good!
Yes, weight distribution is important as well having access to the fuel. All fuel tanks will tend to be drained during the voyage. Do ships refill with water?
Apparently the allowable weights for a 20ft container and a 40ft container about the same. A wood filled 20ft container comes close to the maximum allowable weight for that container size so the density distribution isn't bad, although I suppose your onboard material handling will have to include access for repairs, and materials movements. It may be that not only is wood less dense than petroleum products, but it may take a lot more "air space" for handling it.
The conversion for use of Vegetable Oil would likely be quite easy for modern internal combustion ships.
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Thank you for your thoughts gentlemen. Most interesting.
The conversion for use of Vegetable Oil would likely be quite easy for modern internal combustion ships.
Bunker fuel is still cheap compared to pretty much all other fuels. I'd have to wonder if veg oil, even unprocessed, would be less expensive than Bunker. Then, of course, there's the moral impact on basic food prices of using cereal crops.
I found some figures:
Heavy fuel oil: 38.9MJ/litre 40.1 MJ/kg
Wood pellets: 10.1 MJ/litre 17.7 MJ/kg
Torrefied pellets:14.9 MJ/litre 20.4 MJ/kg
(These are Low Heating Values)
Anthractite coal: ~28 MJ/kg
I am most interested in the application of torrefied* wood pellets
* oxygen starved 'cooking' at around 300degC
Now there is obviously a question of what sort of efficiencies engines can return on these very different fuels, but for comparison:
Torrefied pellets have 38% the energy by volume of Bunker.
And 51% the energy density by mass.
This is not great admittedly but neither is it a deal breaker imo.
Personally, although it would add complexity, I think on the largest ships it would be worth exploring gasification of a solid fuel onboard (plenty of H2O around) to make syngas. This would then run gas turbines with a steam bottoming cycle.
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Undoubtedly efficiency would be vital.
If you could make woodgas, or something similar, then the existing engines could probably be converted to use it. However, it would only be good if it could compete with the efficiency of a direct steam engine.
I wonder if some of the Large internal combustion ship engines (http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96-C) could be designed to run on a wood powder, or perhaps a powdered solid/liquid/vapor mix.
Again, one would have to consider the entire system efficiency of producing the woodgas/wood powder/Torrefied pellets vs running raw hog fuel.
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Sulphur emissions from bunker fuel cause pollution and acidification (bad), but some scientists have pointed out that it seeds clouds, which reflect away sunlight, keeping the Earth slightly cooler than it would be if we burnt "clean" fuel (temporarily good).
I am not sure that wood qualifies as a "clean" fuel, since it produces a lot of ash. This could violate pollution regulations while the ship is in harbour.... or you could use electrostatic scrubbers to collect it - and then you have an ash disposal problem when you reach harbour.
[Wistfully]: Things were so much easier in the "good old days" when we could consider the Earth an infinite, low-cost source of everything, and the Atmosphere and Oceans an infinite, free sink for anything we wanted to dump.
PS: Does nuclear power count as "solid fuel"?
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I am not sure that wood qualifies as a "clean" fuel, since it produces a lot of ash. This could violate pollution regulations while the ship is in harbour.... or you could use electrostatic scrubbers to collect it - and then you have an ash disposal problem when you reach harbour.
If the ash is kept clean, then it should be able to be sold as fertilizer, or as a fertilizer ingredient. Or, perhaps as a replacement for lime. Or, even sell it to chemical companies. Perhaps biodiesel manufacture?
Certainly, during a voyage, the ash produced will be less than the wood consumed.
If CO2 is acidic, and wood ash is basic, then it might not hurt to dump it in the ocean, as long as it is well diluted and kept away from the coasts.
PS: Does nuclear power count as "solid fuel"?
It depends.
http://en.wikipedia.org/wiki/Molten_salt_reactor
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If you could make woodgas, or something similar, then the existing engines could probably be converted to use it. However, it would only be good if it could compete with the efficiency of a direct steam engine.
It is tricky with these questions of efficiency isn't it. On the one side, internal combustion engines (gas turbines/diesel) as part of a combined cycle electric power plant should be capable of better efficiencies than the best external combustion engines (eg. the steam turbine). But it all depends on what fuel you have to work with.
Clearly solid fuels and internal combustion don't really mix in the real world. .. That said I believe the coal board has even experimented with gas turbine combustors that burnt solid fuel... but I think a heavy price was paid on the maintenance and lifetime of the power turbine.
Does wood have a lower ash content than coal? ...perhaps not for the equivalent energy content (?).
In relation to Low-speed diesels, of the type used in the largest ships, I understand that experiments have been tried by using a slurry fuel - that is particles of solid fuel suspended in either an oil fuel or water. This too may be an option.
Or upgrade the fuel first:
Gasification (syngas[water gas-> H2 and CO], or woodgas[H2, CO, CH4, etc]) has the advantage of being suited to internal combustion, but of course there will be the unavoidable losses of the gasification itself.
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Ships are rarely run without any ballast water at all.. when the level in the fuel tanks drops, there are small ballast changes to maintain the stability of the vessel (I have had the pleasure of doing all the calcs by hand, now they are done using computer programs)
As for slurry fuel... not a practical thing to do in the slightest. part of the efficiency of the engine is the level of atomisation the fuel injectors can achieve, this is where the fuel is pumped at high pressures (120 bar nowadays) through very small nozzles to create the spray.. any non liquids would damage and reduce this efficiency, so whoever tried to do that, weas a very brave man indeed. (plus not worried about spending half his time with his engines down changing injectors)
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As for slurry fuel... not a practical thing to do in the slightest.
Maybe not practical (as yet) but...
Biomass-oil mixture (slurry) (http://www.comb.ritsumei.ac.jp/combust/Paper_Plastic/Paper068.pdf)
These experiments still rely on 72% Heavy Fuel Oil but it opens the door to using slurries on large (marine) diesel engines.
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"To summarize, the new fuel
offers good prospects for combustion but problems with the
stability of the fuel and the injection process have to be
solved."
Not only this, as someone who used to take the same reasons very regularly I would question the equivalent runs, (not saying it is bad but it is 'dressed' up like side by side runs, when its actually a modified cycle as can be seen by the pencil chart)
The report does not go into sustainability, I would really like to see the energy returns on this, and I really think the wear on the injector (and potential blockages) would certainly count against it.
Like you say, it opens the door, (although I would say it starts the discussion on opening the door) but I wouldnt risk my life on it. Or risk thousands of seafarers life on something that has not considered the mechanical impact.
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It also says: 'Particles with diameters of 50 μm or larger are responsible for injector blockages. ...Sieving or further grinding are under consideration at this stage of the research.'
The engine under experiment is only a 1000cc unit, so - as I said - large marine diesels should be better able to work reliably with such fuels. The main problem the experimenters faced was the short-term stability of the fuel in storage.
I don't think there is any suggestion that this process is mature and ready for 'risking thousands of seafarers lives on' ... it clearly needs more research and then rigorous sea trails, etc.
And it would be nice to find a way that substantially cuts the amount of Bunker fuel used at sea (or in ports!) as soon as possible.
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Particles have a tendency to agglomerate, nozzles block up with current bunker fuel for the same reasons (there ARE already particles in bunker fuel, one reason for its cheapness and requirement for purification before use in the engine) increasing the amount of particulates will not only block the nozzles and reduce the efficiency of the engine its the abrasion on the nozzles which would be the killer. Currently an injector would typically last about 18 months before REALLY needing refurbishment. (this is typically done at 12 months)
As for reducing bunker fuel usage in ports.. ships currently use standard (maybe a little subgrade) diesel for power generation whilst in port (this in itself causes significant problems) and the diesel engines are not used. (Steam ships do use bunker fuel but so much less of it) The entire misunderstanding and the grasping at straws to 'help reduce pollution' tends to misunderstand the simple facts. 90% of fuel usage happens at sea, whilst in port the ship will be only producing power, if you wish to reduce this power generation pollution the best bet would be to 'plug' the ship into shore based power. NOT and i stress this NOT tell them to use standard diesel as they do now, or change the fuel type to a less efficient type..
HFO is a very funny substance, it is thick and gloopy when cold (almost solid) and VERY non newtonian shear thinning liquid. It has a high concentration (relative) of solids and carbons which actually add to the energy content, but also assist the sealing of the parts of the engine (by 'crudding it up') normal diesel is a fairly typical newtonian fluid, it removes all the crud and tends to burn a bit smoky to begin with (due to crud) and then a LOT cleaner than HFO.. but heres the thing.. it burns HOTTER.. requireing more energy to keep the engine cool, and forcing shjips to reduce revs on the engine or load on the boiler to prevent overheating. It also produces more Nox's due to the increased temperature.. for which us engineers have to keep monitoring. it makes so many things leak that didnt leak on HFO (typically orings and piston rings) causing quite significant risks to seafarers due to the increased chance of fire.. (I have had to extinguish 4 scavenge space fires due to running on diesel, this is the space beneath the pistons, as the diesel tends to leak through the piston rings and with the presence of hot surfaces and abundant pressurised air)
The IPPC has created MORE issues for pollution than it has cured simply due to the misunderstanding of how ships actually work. I am not saying there is nothing that can be done, or that we shouldnt do it, I am saying it needs to be approached in the right way.. and changing fuels on the same system is definately a step backwards.. changing the entire drive of the ship for a cleaner way overall should be the goal, diesel electric is a significant improvement using the same principles.
I pledge for some way to use fuel cells, or just plain burning of hydrogen and oxygen to achieve an almost perfectly clean means of propulsion would certainly be the way forward.
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Illuminating response SimpleEng. Thanks.
Interesting points you bring up about 'plugging in' in port (known as 'cold ironing', for historical reasons) - Actually this an area I've investigated a little as an aside to my day job.
Drawing electrical power from the quayside is very effective at cutting local air pollution from berthed ships and it also saves on fuel costs as the engines can be shut down completely. Another approach that the industry is beginning to adopt is fitting out ships with Liquefied Natural Gas powered gensets, drastically improving air quality impact locally.
I agree with your analysis that eventually other cleaner fuels should be adopted by shipping, but it's an industry known to be (perhaps rightly) averse to change. - That's partially why I've found myself looking at a more pragmatic approach to looking at ways the industry could be improved. Big, slow plodding engines like those found on ships have much going for them, at least in terms of their thermal efficiency. - And drivetrain electrification of even large container ships is now becoming a reality, bringing better part-load efficiencies with it.
As you point out, the bunker has some strange properties (I enjoyed learning that the 'gunk' actually assists in properly sealing the engine!). So, bunker fuel is by no means a 'ready in the tank' product - as we would think of with say petrol or diesel; full of crap its cheapness has made everything else about it worth dealing with it seems. Being aware of this, I still question whether there is a place (even as a bridging technology) for a fuel that is, at least in part, lowering the (life-cycle) CO2 emissions of shipping even a little. Biomass slurry could be one means of doing this, even with the extra challenges it brings.
Going back to issues of particles (in any oily fuel) - used in large low- or medium- speed marine diesels - the larger particles appear more likely to block the injectors (hence the pre-filtering of bunker) and the (many frequent) smaller particles are responsible for the high wear rates.... ?? this may be a vast oversimplification, of course.
In the wider discussion, the use of biomass in boilers (though it may appear a step backwards technologically speaking) may find a place in the market - the biggest problem is the fuel delivery infrastructure globally is now very much focused on 'heavy' liquid fuels. Another reason to explore 'like-for-like' fuels...
Not a dissimilar argument behind why I (for one) am so cynical about the great promise of the fuel cell revolution... Even in road use they are decades away from being practical, not so much because of the FCs themselves; instead that the infrastructure is not going to built at anything like the rate needed to make motorists buy them.
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Well, I sailed on the vessels that carried LNG and in the past there was a lot of development into using the offgas from the tanks (allow the gas to evaporate to keep the liquid cool) I saw it fully in use in the boiler of the steam driven ships, but the investigations showed that there was no benefit to use it in either the power generation or drive of the diesel ships (for steam it is as the power generation are turbo alternators).. The work was done and it is more valuable to sell than it is to burn.. so now we have significant reliquification units on top of the ship.. driven by diesel generators (at no small cost mind you)
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Can we use tnt as a source of power ? it releases tremendous energy on burning and also tnt can be converted to a solid form .