Liquid Slurry Fuels

[shynung]

I recently read about a liquid fuel obtained by finely crushing coal and suspending it in water, resulting in something called a coal-water slurry:

CWS can be used in place of oil and gas in any size of heating and power station. CWS is suitable for existing gas, oil, and coal fired boilers. During the last 30 years the US Department Of Energy (DOE) has been researching the use of coal/water fuels in boilers, gas turbines and diesel engines. When used in low speed diesels CWF provides a thermal efficiency rating that rivals combined cycle gas turbines that burn natural gas as their primary fuel.

Considering that:

A turbojet has a remarkably undiscriminating appetite, and will run, or can be made to run, on just about anything that will burn and can be made to flow, from coal dust to hydrogen.

Is it possible for a form of coal/charcoal-water slurry to be used as fuel in an (1) aircraft-mounted turbine gas engines, or (2) liquid-fueled rocket, using either LOX, N₂O, or N₂O₄ as oxidizer?

If so, what are the fuel mixture composition likely to be used in each scenario?

[RuBisCO]

Sure but getting over the inhabition of combustion by the water is difficult, energy is lost boiling all that water, ISP would be decreased. I once heard of the idea of aluminum particles in liquid hydrogen as a way to increase LH2 volumentric energy density without seriously decreasing the ISP, so I see no reason coal can't be mixed with a little bit of hydrocarbon instead of water.

[Ralathon]

Turbojet would probably be doable. Although I'm worried about the burn rate which effects the efficiency.

Rocket engine I'm doubtful. Liquid fueled engines are very picky about their fuel. You can't run them on just anything, they need very consistent burns.

[shynung]

Burn rate can be adjusted through different particle sizes. It boils down to the mesh screen used when manufacturing the powdered coal to be slurried. Smaller particles usually means faster combustion, but negatively affects the production capacity.

Though, I'm also concerned about the injector design needed for this kind of fuel. Since it's a particulate colloid, it could be rather abrasive.

[Tarrow]

Is it possible for a form of coal/charcoal-water slurry to be used as fuel in an (1) aircraft-mounted turbine gas engines, or (2) liquid-fueled rocket, using either LOX, N₂O, or N₂O₄ as oxidizer?

Possible? Yes. Efficient? No.

Grade I coal / water slurry (the highest energy content by mass of the slurry variants) releases about 4,700kcal/kg. Hydrogen releases 33,900 kcal/kg. You'd need to carry 7 times the mass of fuel in comparison to a hydrogen fuelled rocket.

[shynung]

Coal-water slurry is much denser than liquid hydrogen (more fuel can be stored in the same tank capacity; water's density alone is 1000 kg/m³ compared to LH₂'s 70, CWS would be higher), and much safer to handle (not only liquid hydrogen is a deeply-cryogenic liquid, it also forms dangerous explosives in the gaseous form at a wide range of concentrations). The latter also has a peculiar property: it boils, molecule by molecule, off the tank it's in, and cannot be stored indefinitely. Plus, slurrying the coal with a hydrocarbon rather than fuel should kick up it's total energy density.

[K^2]

Weight is still very important for airliners. Short regional hops aren't usually so bad. But long range flights operate effectively on rocket formula, and a considerable fraction of their takeoff weight is fuel.

[shynung]

In that case, higher density fuels is at an advantage. For the same fuel mass, less tankage is required, freeing space for payloads or other systems.

[phoenix_ca]

Call me crazy, but wouldn't this have more-or-less the exact same problems coal power plants did before we slapped limestone filters on them? Acid rain for everyone...

[Northstar1989]

Call me crazy, but wouldn't this have more-or-less the exact same problems coal power plants did before we slapped limestone filters on them? Acid rain for everyone...

Yeah, pretty much.

If you want a higher energy-density for rocket fuels, why would you both powdering a solid fuel and creating a slurry?

Why not use a LIQUID fuel, such as Kerosene/LOX? (which is already one of the main fuels for rockets- and has a much higher energy density than LH2/LOX despite lower ISP) We already know how to turn coal into petroleum products- the Germans have been doing it since WWII when they used it to fuel their tanks after western nations cut off their oil supply...

Regards,

Northstar

[shynung]

A slurry fuel has a higher density than kerosene, but retains the controllability of liquid fuel rockets. It's also cheaper to make than a liquid hydrocarbon from coal, through something like the Bergius process (which was used by the Germans in WWII). Rocket-grade kerosene, though has a much tighter tolerance than jet-fuel (in fact, no two barrels of JP-4 fuel were ever alike), so I'd doubt whether a coal-derived hydrocarbon would still work in a rocket.

Turbojet, though, would probably run on them just fine.

Though, that was beside my point. I was exploring the idea of using slurry fuels in gas turbine engines, to which I think turbopumps are a close relative.

That gives me an idea though; Is it possible to make a usable monopropellant by slurrying a 20-80 powdered aluminium-HTPB in nitrous oxide? The combination is a common hybrid rocket fuel, as far as I know.

[Armchair Rocket Scientist]

Coal-water slurry is much denser than liquid hydrogen (more fuel can be stored in the same tank capacity; water's density alone is 1000 kg/m³ compared to LH₂'s 70, CWS would be higher), and much safer to handle (not only liquid hydrogen is a deeply-cryogenic liquid, it also forms dangerous explosives in the gaseous form at a wide range of concentrations). The latter also has a peculiar property: it boils, molecule by molecule, off the tank it's in, and cannot be stored indefinitely. Plus, slurrying the coal with a hydrocarbon rather than fuel should kick up it's total energy density.

As others have said, liquid hydrocarbons would be better fuels. Kerosene is common, but even propane can be stored as a liquid at room temperature, and although methane is cryogenic it's no worse than liquid oxygen.

A "practical" slurry fuel would probably be something like solid methane in liquid hydrogen. On the one hand it would have higher energy density than pure LH2. On the other hand, it would have the same boiloff problems as hydrogen while getting lower ISP.

Airbreathing engines could probably make better use of fuels with high energy density but low "rocket mode" ISP. Even on a turbojet, a lot of the reaction mass is air heated by the combustion. On a high bypass turbofan like the ones used by nearly all modern jet airliners and cargo planes, most of the thrust is actually coming from the fan. On a turboprop or turboshaft, the thrust from the exhaust is as close to zero as the designers can make it.

From what I know, most jet airliners store their fuel in the wings. This means they need to use high-density fuels to keep a good airfoil, and the tanks are awkward shapes which make storing fuel at high pressures rather difficult. Fuel costs are a large portion of total operating costs, so a good airplane fuel also has to have a lot of energy per dollar - which means not being a pain in the ass to manufacture or store.

Non-cryogenic hydrocarbons generally seem to be the best solution to this, but it's possible that some slurries would be effective.

[Tarrow]

water's density alone is 1000 kg/m³ compared to LH₂'s 70, CWS would be higher)

Coal-water slurry density is less than that of water. Coal itself is less dense than water (the densest form of pure anthracite coal reaches 929kg/m3), and as it's a suspension not a solution the volume of the coal is still a factor. Mixes of CWS go up to 70% coal and 30% water, which'd give you 650kg of coal and 300kg of water per m3 of slurry - overall density 950kg/m3

A 1m3 tank of coal slurry contains 950 x 4,700 = 4,465,000 kcal

A 1m3 tank of hydrogen contains 70.8 x 33,900 = 2,400,120 kcal

In terms of energy per unit volume the numbers are in favour of CWS, by a factor of 1.86. I won't argue with that. But in real world terms the fuel mass for a given energy output is very important,.

I'll give you a practical example of why it's important.

We know the space shuttle external fuel tank (I've got the SLWT specs, so will use those those) contains a mass of 106,261kg of H2. That's a net energy content of 106,261 x 33,900 = 3,602,247,900 kcal.

The mass of CWS to reach the same overall energy content is 3,602,247,900 / 4900 = 735,152kg.

The total empty mass of the the SLWT tank is a near-insignificant 26,500kg.

You may be able to halve the empty mass of the tank due to the increased fuel density, saving 13 tonnes or so, but you do so at a cost of adding more than 600 tonnes to the takeoff weight of the shuttle.

The same applies to pretty much any other fuel in comparison to CWS - there're some where there'd be a volume saving, but the mass difference is an absolute killer.

[shynung]

I see. The rockets that would use CWS as fuel would be something that needs a very high density of fuel, in order to fit a specified maximum dimension, while also remaining throttleable in flight. The closest thing to those would be anti-aircraft missiles, and they're using solids already. Or, the lower stage of a rocket that has to be kept in a silo, like ballistic missiles.

Jet fuel from non-cryogenic hydrocarbons is common because their specifications were made when oil was cheap (somewhere in the 1950's). However, the gas turbine itself is flexible on the fuel (the same Honeywell gas turbine in an M1 Abrams can run on Jet-A or diesel fuel), so I don't see why a form of slurry fuel can't be used as jet fuel.

[mrfox]

Jet engines are pretty inefficient thermally, the reason they are used in aircraft is because they are low weight. Using a low specific energy (high weight) fuel defeats the purpose.

[Aghanim]

You really do not want to have any ashes in the turbojet engine, it will mess up the turbine blades and increasing maintanance cost

[shynung]

That would be true if the fuel is a low-grade coal, like lignite. If the fuel itself produces almost no ash at all (charcoal, or maybe coke), the turbine would run fine. Of course, that would raise fuel costs for the additional processing stage.

[Armchair Rocket Scientist]

Jet engines are pretty inefficient thermally, the reason they are used in aircraft is because they are low weight. Using a low specific energy (high weight) fuel defeats the purpose.

That's only sort of true. Gas turbine engines are less efficient than piston engines. Most helicopters indeed seem to use turbine engines for the extra power-weight ratio, probably because helicopters need more power to stay airborne than airplanes, and aren't that long-range anyway. The same may be true for turboprops, I'm not sure.

For actual jet engines, the story is different. The engine itself may be less efficient, but most jet aircraft operate at speeds where the blade tips of a propeller would have to be moving faster than the speed of sound. This makes the propeller's efficiency drop precipitously, so jet engines actually end up being more efficient once you factor in the losses from turning shaft horsepower into forward thrust.

[mrfox]

That's only sort of true. Gas turbine engines are less efficient than piston engines. Most helicopters indeed seem to use turbine engines for the extra power-weight ratio, probably because helicopters need more power to stay airborne than airplanes, and aren't that long-range anyway. The same may be true for turboprops, I'm not sure.

For actual jet engines, the story is different. The engine itself may be less efficient, but most jet aircraft operate at speeds where the blade tips of a propeller would have to be moving faster than the speed of sound. This makes the propeller's efficiency drop precipitously, so jet engines actually end up being more efficient once you factor in the losses from turning shaft horsepower into forward thrust.

This then becomes a chicken/egg problem. The reason jet aircraft cruise at such speeds is because they are powered by jets. The cost of a jets inefficiencies are offset by the cost of time it saves from high speed travel. This balance is not static - Notice in the past decade the resurgence of turboprops for short/medium range flights because of the increase of the price of fuel. If fuel cost continue to increase it would not be a surprise if ideas such as unducted fans with higher propusive efficiencies are reintroduced, bringing the long range cruise speeds down from where it is today.

Edited August 10, 2014 by mrfox

[shynung]

That, or the bypass ratios on newer turbofans become higher and higher, as the engineers try to squeeze out more efficiency from the gas generator.

[mrfox]

Hence you are aware of the razor thin performance margins of current aerospace tech, and thus, the issue of using high weight/low energy fuel in this application.

There are specific applications which you can justify such a system - something where there is a high weight cost for transporting the power plant, but a zero to nil transport cost for getting the fuel - generators for remote mining camps for instance, where everything needs to be flown in, but low grade fuel is avlb locally.