XYLENE POWER LTD.

SYNTHETIC LIQUID FUEL

By Charles Rhodes, Xylene Power Ltd.

GLOSSARY OF TERMS

INTRODUCTION:
This web page examines the production of synthetic liquid fuel without emitting CO2 to the atmosphere. Sooner or later governments will have to face the realities that fossil fuels are increasingly expensive depleting resources and that CO2 emissions from combustion of fossil fuels have extremely expensive environmental consequences.

Liquid hydrocarbon fuels are integral to modern society. During the year 2004 residents of the Province of Ontario consumed about 15.7 billion litres of liquid hydrocarbon fuels. Liquid hydrocarbon fuels are used because they are convenient and because in combination with oxygen from the air their stored thermal energy is very high (~ 36 MJ / lit, ~ 45 MJ / kg). The high energy storage capacity allows most automobiles to travel ~ 600 km between fuel tank refills.

In 2010 over 90% of the liquid fuel came from fossil feedstock. If mankind is to significantly reduce fossil CO2 emissions to the atmosphere the use of fossil carbon for prime energy generation must be halted. However, there are various applications, such as fuelling: aircraft, ships, surface vehicles going to remote locations, electricity generators at remote locations and remote heating, where there is no practical substitute for liquid hydrocarbon fuels. Hence society must address large scale synthesis of liquid hydrocarbon fuels from non-fossil feedstock. Hence liquid hydrocarbon fuel must be synthesized using only biomass carbohydrate to provide the required carbon and only water to provide the required additional hydrogen.

BIOCARBON CONSERVATION:
There are a multiplicity of processes for converting carbohydrate into alcohols and oils. However, most of these processes use biochemical steps that generate carbon dioxide (CO2) and hence do not incorporate all the available biocarbon into the synthetic liquid fuel. As synthetic liquid fuel usage increases the demand for carbohydrate for liquid fuel production will increasingly affect the supply of carbohydrate for food. To minimize the impact of synthetic liquid fuel production on food production it is essential to incorporate all the available biocarbon into the synthetic liquid fuel.

The requirement for maximizing utilization of available biocarbon effectively rules out large scale use of bioethanol and biobutanol processes, both of which emit large quantities of CO2. The requirement for maximizing use of available biocarbon also rules out simple destructive distillation of carbohydrate. The only fully biocarbon conserving process that produces only liquid fuels is hydrogenation of carbohydrate to form methanol followed by reforming of methanol to form oil (methylcyclopentane, cyclohexane) and water. After methanol production the biomass feedstock residue is removed and recycled as a fertilizer. After oil (methylcyclopentane, cyclohexane) production the reaction product water is recycled for hydrogen production.

HYDROGEN PRODUCTION:
This synthetic fuel production process requires a large energy input for splitting water to provide the required hydrogen. If we assume that to minimize biomass feedstock transportation costs the synthetic fuel production plants must be located close to where the biomass feedstock is grown, then the only proven practical source of hydrogen that does not involve fossil fuel is electrolysis of water.

There is a possibility of obtaining the required hydrogen by splitting water using the copper chlorine process and nuclear heat, but the extra costs associated with implementing the copper chloride process along with the required liquid metal cooled nuclear reactors and hydrogen distribution piping may exceed the cost savings realized by avoiding electricity generation, electricity transmission/distribution and electrolysis.

SYNTHESIS CHEMISTRY:
The details of liquid hydrocarbon synthesis from biomass are quite complex. However, from an energy perspective the overall thermal-chemical reactions can be simplified to four steps:
Step #1 - Electrolysis of Water:
(water) + (electricity) = (hydrogen gas) + (oxygen gas) + (heat)
or
6H2O + 6(electricity) = 6H2 + 3(O2) + (heat)

Step #1 is done at a high temperature/pressure with conductivity control to minimize the electrical energy requirement. The heat output from Step #1 is an input to Step #4. The hydrogen gas output from Step #1 is an input to Step #3.

Step #2 - Drying of Biomass:
(biomass) + (heat) = (carbohydrate) + (water vapor)
or
C6H12O6 + liquid H2O + (heat) = C6H12O6 + H2O vapor

Step #2 is done at about 120 C, low pressure (partial vacuum) to extract the water vapor from the biomass. The required heat is obtained from condensation of the reaction products in Step #4. The liquid input to Step #2 is typically 10% to 25% of the biomass by weight. The dry C6H12O6 output from Step #2 is an input to Step #3. The H2O vapor output from Step #2 is rejected to the atmosphere.

Step #3 - Hydrogenation of Carbohydrate:
(hydrogen gas) + (plant carbohydrate) = (methanol)
or:
6(H2) + C6H12O6 = 6(CH3OH) vapor
= 6(CH3OH) liquid + latent heat

Step #3 is done at about 300 C to realize the desired reaction rate and reaction product. The liquid methanol (CH3OH) output from Step #3 is an input to Step #4. The latent heat output from Step #3 may assist with Step #2 before being rejected to the atmosphere via a cooling tower. Note that Step #3 is an exothermic chemical reaction.

Step #4 - Reforming of Methanol:
(methanol) + (heat) + (catalyst) = (cyclohexane) + (water)
= (methylcyclopentane) + (water)
or
6(CH3OH) + (heat) + (catalyst) = C6H12 vapor + 6(H2O) vapor
= CH3C5H9 liquid + 6H2O liquid + (latent heat)

Step #4 is done at a high temperature (600 C), high pressure and with appropriate catalysts to realize the desired reaction products. Vaporizing the liquid methanol (CH3OH) input to Step #4 requires heat that can be obtained from Step #1. Step #4 also requires further input energy that must be externally provided to raise the methanol vapor to about 600 C. The reaction products of Step #4 are condensed and density separated. The high pressure liquid hot water output from Step #4 is an input to Step #1. The heat output from Step #4 obtained by condensation of the reaction products is an input to Step #2. Note that the operating pressure in Step #4 is set high enough that the reaction products of Step #4 will condense above the operating temperature of Step #2 so that heat flows from the output of Step #4 to the input of Step #2.

The biomass input to Step #2 is a net process input. The methylcyclopentane and cyclohexane outputs from Step #4 are net outputs. The oxygen gas output from Step #1 is a byproduct that can be sold or vented to the atmosphere.

For some applications methanol is a satisfactory fuel so Step #4 is unnecessary.

The practical safety issues in Steps #2 and #3 related to loading biomass, evacuating air and water vapor from the reaction chamber, loading compressed hydrogen, running the reaction, removal of remaining hydrogen and removal of biomass residue indicate that Steps #2 and #3 should be physically separated from Step #1 and Step #4.

The above chemical equations allow estimation of the energy and feedstock inputs necessary to make synthetic fuel (methanol or cyclohexane) without net emission of CO2 to the atmosphere.

MATERIAL DATA:
The material data is as follows:
H2:
Name = hydrogen
Enthalpy of combustion = 141.86 MJ / kg, Atomic Weight = 1.008, Molecular Weight = 2.016
C:
Atomic Weight = 12.011
O2:
Name = oxygen
Atomic Weight = 15.9999, Molecular Weight = 31.9998
H2O:
Name = water
Enthalpy of formation = -285.83 kJ / mole, MP = 0 C, BP = 100 C,
Molecular Weight = 2(1.008) + 15.9999 = 18.0159
C6H12O6:
Name = carbohydrate
Enthalpy of formation = -1271 kJ / mole, Enthalpy of combustion = -2805 kJ / mole
CH3OH:
Name = methanol
Enthalpy of formation = ___, Enthalpy of combustion = 19.7 MJ / kg, MP = -97 C. BP = 64.7 C
C6H12:
Name = cyclohexane
Molecular weight = 6(12.011) + 12(1.008) = 84.162, Enthalpy of formation = -156 kJ / mole,
Enthalpy of combustion = -3920 kJ / mole = 46.577 MJ / kg, MP = 7 C, BP = 80.74 C
Density = .779 kg / lit
Above 275 C cyclohexane can rearrange itself to form the isomer methylcyclopentane
CH3C5H9:
Name = methylcyclopentane
MP = -142.4 C, BP = 71.8 C, Density = .749 kg / lit, insoluble in water

HEAT RELEASE:
The heat released in forming 1 mole of cyclohexane (C6H12) from hydrogen gas and carbohydrate is given by:
-1271 kJ / mole + 156 kJ / mole + 6(285.83 kJ / mole) = 600 kJ / mole
Converting this heat into kWh / kg C6H12 gives:
(600 kJ / mole) X (1 mole / 84.162 gm) X (1000 gm / kg) X (1000 J / kJ) X (1 W-s / J)
X (1 h / 3600 s) X (1 kW / 1000 W)
= 1.980 kWh / kg

FUEL COMPARISON:
Methanol (CH3OH) is not an ideal fuel because it corrodes aluminum. Methanol also attacks certain rubbers. Methanol also does not have the high energy / kg of methylcyclopentane (CH3C5H9) and cyclohexane (C6H12). When burned in a boiler methanol loses significant latent heat via stack loss due to the high water content of the combustion exhaust. However, producing methanol is relatively simple. From an environmental perspective methanol is relatively safe because most life forms on planet Earth have evolved to be tolerant to the small concentrations of methanol that naturally occur in the environment.

Producing methylcyclopentane and cyclohexane requires a high temperature reforming process. Safe combustion of methylcyclopentane and cyclohexane requires a high quality combustion process to prevent release of incompletely combusted aromatic hydrocarbons to the atmosphere. Some aromatic hydrocarbons such as benzene (C6H6) are known carcinogens. In nature aromatic (closed ring) hydrocarbons are usually only found in fossil fuels that, due to geological processes, have been subject to high underground temperatures and pressures.

Pure cyclohexane is not a good liquid fuel because when cooled it starts to form a solid phase at 7 degrees C. For many practical uses cyclohexane should be mixed with another liquid hydrocarbon such as methylcyclopentane to lower the mixture melting point. Such hydrocarbon mixtures are a normal result of reforming chemical reactions. Note however that, if the reaction product mixture includes either methanol or ethanol, precautions must be taken to prevent corrosion of aluminum fuel system components.

Note that this issue of liquid fuel solidification at low ambient temperatures is a problem shared with other biodiesel fuel production processes. For many practical applications it is important to keep the fraction of methylcyclopentane high.

HYDROGEN:
The atomic weights of hydrogen (1.008) and carbon (12.011) in the above synthesis equations show that to make 84.162 gms of cyclohexane requires 12(1.008) = 12.096 gms of hydrogen gas. Hence to make 1 kg = 1000 gm of cyclohexane requires:
(12.096 / 84.162) X 1000 gms = 143.72 gm of hydrogen gas.
The thermal energy content of hydrogen gas is:
141.86 MJ / kg. Hence if hydrogen gas could be obtained by electrolysis of water with no thermal losses, to make 1 kg of cyclohexane would require:
(143.72 gm H2) X (141.86 X 10^6 J / 1000 gm H2) X 1 W-s / J X 1kw / 1000 W X 1 h / 3600 s
= 5.663 kWh of electrical energy.

However, industrial electrolysis processes are not 100% efficient. Typical efficiencies are in the range of 56% to 73%, so the actual electrical energy requirement for providing sufficient hydrogen to make 1 kg of cyclohexane is in the range:
5.663 kWh / .73 = 7.758 kWh to 5.663 kWh / .56 = 10.113 kWh

HEAT LOSS DURING ELECTROLYSIS:
The heat loss during electrolysis of sufficient water to produce sufficient hydrogen to make 1 kg of cyclohexane is in the range:
7.758 kWh - 5.663 kWh = 2.095 kWh / kg
to
10.113 kwh - 5.663 kWh = 4.450 kWh

PROCESS HEAT DISSIPATION:
Thus the total heat that must be dissipated at the synthetic fuel production site per kg of C6H12 produced is in the range:
2.095 kWh / kg + 1.980 kWh / kg = 4.075 kWh / kg
to
4.450 kWh / kg + 1.980 kWh / kg = 6.430 kWh / kg

In reality the process energy dissipation is significantly larger than the above calculations indicate because practical production of cyclohexane involves first making methanol at a modest temperature and pressure and then making cyclohexane at a much higher temperature and pressure. The two process steps occur at substantially different reaction conditions. Generally the methanol vapor is condensed to form a liquid and then pumped into the high temperature high pressure methylcyclopentane/cyclohexane production chamber. During condensation of methanol (CH3OH) its latent heat of vaporization is removed at a temperature that is too low for most practical uses. This heat has no commercial value except to the extent that it can assist in drying the carbohydrate feedstock. However, in the above chemical equations the carbohydrate feedstock is already assumed to be dry. Hence the net available heat is less than the above equations seem to indicate.

THERMAL ENERGY CONTENT OF C6H12:
The thermal energy content of C6H12 is given by:
3920 kJ / mole X (1 mole / 84.162 gm) X (1000 gm / kg) X (1000 J / kJ) X (1 W-s / J)
X (1 h / 3600 s) X (1 kW / 1000 W)
= 12.938 kWh / kg

Note that part of the fuel thermal energy content is provided by the electricity and part is provided by the carbohydrate feedstock.

CARBOHYDRATE:
The atomic weights of hydrogen (1.008), carbon (12.011) and oxygen (15.999) in the above synthesis equations show that the amount of carbohydrate required to make 84.162 gm of cyclohexane is given by:
[6(12.011) + 12(1.008) + 6(15.999)] = 180.156 gm of carbohydrate.
Hence the amount of carbohydrate required to make 1 kg of cyclohexane is about:
(180.156 / 84.162) X 1000 gm = 2140.6 gm.

However, as biomass naturally grows only about 70% of its mass is usable carbohydrate. The balance is lignin that in practice is unusable for supplying biocarbon to the synthesis reaction. Hence the minimum amount of biomass required to make 1 kg of methylcyclopentane/cyclohexane mixture is:
2140.6 gm / 0.7 = 3058.0 gm.

There are substantial costs involved in growing biomass, harvesting it, debarking it, and in lignin removal. The remaining material, which typicallycontains about 10% water, costs about $450 / tonne (eg. white rice). Hence the cost of the dry carbohydrate from wood is about:
($450 / tonne) X (1 tonne / 0.9 tonne carbohydrate) = $0.50 / kg

A key issue in biofuel production is use of more efficient agricultural techniques to reduce the cost of the dry carbohydrate feedstock. In biofuel production, unlike lumber or paper production, there is little concern about cellulose fiber quality. In biofuel production the dominant issue is the cost of cellulose. In biofuel production a greater fraction of the biomass can be used. If the biofuel production facility is located close to where the biomass is grown a larger fraction of the biomass can be used without concern about rot, fungus formation, etc. Possibly the dry carbohydrate cost can be reduced to about $0.30 / kg.

COSTS OF FEEDSTOCKS:
Assume that the synthetic fuel production facility uses only off-peak electricity and the average delivered cost of this electricity is $0.07 / kWh. Then the cost of feedstock electricity for making 1 kg of methylcyclopentane/cyclohexane is in the range:
7.758 kWh X $0.07 / kWh = $0.54305
to
10.113 kWh X $0.07 / kWh = $0.7079

Assume carbohydrate purchased in large quantities and delivered to the synthetic fuel production facility costs $0.50 / kg. Then the cost of biomass feedstock for making 1 kg of methylcyclopentane/cyclohexane is about:
2.1406 kg X $0.50 / kg = $1.0703

TOTAL COSTS:
Hence the combined cost of energy and biomass feedstock for synthesizing 1 kg of methylcyclopentane/cyclohexane via the above mentioned synthesis reactions is typically in the range:
$0.54305 / kg + $1.0703 / kg = $1.61335 / kg
to
$0.7079 / kg + $1.0703 / kg = $1.7782 / kg

Clearly at the above assumed costs for electricity and carbohydrate, in 2011 these combined costs are not competitive with fossil fuels.

POTENTIAL WASTE HEAT SALE:
If the carbohydrate feedstock is bone dry and if the waste heat can be sold at $.04 / kWh there are additional potential cost savings in the range:
(4.075 kWh / kg) X ($0.04 / kWh) = $0.1630 / kg
to
(6.430 kWh / kg) X ($0.04 / kWh) = $0.2572 / kg

TOTAL REDUCED COSTS:
Hence the combined cost of energy and biomass feedstock for synthesizing 1 kg of methylcyclopentane/cyclohexane via the above described synthesis reactions under relatively favorable circumstances is in the range:
$1.61335 / kg - $0.1630 / kg = $1.45035 / kg
to
$1.7782 / kg - $0.2572 / kg = $1.521 / kg

By comparison the price of methanol made from fossil fuels varies from:
($227.99 / 55 US gal methanol) X (.3326 US gal methanol / kg methanol) = $1.38 / kg methanol in a single drum shipment
to as low as $0.46 / kg in multi-tonne shipments. In order to compare methanol to cyclohexane on an energy content basis it is necessary to compensate for the ratio of the energy content / kg. Thus the cost of sufficient methanol from fossil feedstock to provide the same thermal energy as 1 kg of cyclohexane varies from:
$1.38 X (46.66 MJ / kg) / (19.7 MJ / kg) = $3.27 in a single drum shipment
to
$0.46 X (46.66 MJ / kg) / (19.7 MJ / kg) = $1.09 in multi-tonne shipments.

On a per unit of thermal energy basis methanol containing fossil carbon, when purchased in large quantities, costs about the same as furnace oil containing fossil carbon. This data seems to indicate that use of agricultural methanol might soon make ecomomic sense in remote markets where the cost of delivery of oil or methanol containing fossil carbon is relatively high.

The density of cyclohexane (C6H12) is about .779 kg / lit. Hence the net combined energy and feedstock costs per litre of cyclohexane formed from biomass feedstock are in the range:
$1.45035 / kg X .779 kg / lit = $1.13 / lit
to
$1.7782 / kg X .779 kg / lit = $1.385 / lit

This cost does not include any of the costs related to financing the methanol/methylcyclopentane/cyclohexane production equipment, operating the equipment, maintaining the equipment and storing, distributing and selling the synthetic liquid fuel product. Hence it is reasonable to conclude that the methylcyclopentane/cyclohexane selling price without taxes will likely have to exceed $3.00 / litre before the above described liquid fuel synthesis process is economically viable.

Presently the above described synthesis reactions are not used because existing biofuel producers believe that it is less expensive to sacrifice over 1/2 of the available biocarbon to remove oxygen from the remaining carbohydrate than it is to use purchased electricity to remove oxygen from the carbohydrate. This belief may be in part a result of agricultural subsidies and electricity rates that do not properly reflect electricity system costs. There is a distinct risk that future demand for carbohydrate for biofuel production might impact on the cost of food, which is also a carbohydrate.

ENERGY CONTENT:
The energy density of cyclohexane is given by:
(3920 kJ / mole) X (1 mole / .084162 kg) X (.779 kg / lit)
= 36283.4 kJ / lit
= 36.283 MJ / lit
The energy density of methylcyclopentane is similar. The mass density of methylcyclopentane (.749 kg / lit) is less than the mass density of cyclohexane (.779 kg / lit). However, the chemical energy per unit mass for methylcyclopentane is larger than for cyclohexane.
By comparison the thermal energy content of diesel oil and furnace oil is about:
38.2 MJ / lit

ENERGY REQUIREMENT IN ONTARIO:
The above analysis shows that the electricity requirement for displacing 2004 liquid fossil fuel use in Ontario is in the range:
15.7 X 10^9 litres / year X 7.758 kWh / kg X .779 kg / lit X (38.2 / 36.283)
= 99.90 X 10^9 kWh / year
to
15.7 X 10^9 litres / year X 10.113 kWh / kg X .779 kg / lit X (38.2 / 36.283)
= 130.22 X 10^9 kWh / year

By comparison, total annual electricity production in Ontario in 2004 was about 151 X 10^9 kWh / year. The total amount of nuclear and renewable electricity generation has to be about doubled to produce sufficient hydrogen to minimize the future consumption of carbohydrate for liquid fuel production. In constructing this new electricity generation it is essential to keep the cost of the resulting electricity as low as possible. Thus the only economically viable non-fossil fuel prime energy sources are on-shore wind, hydro and well managed nuclear. Each synthetic fuel plant must be located close to a large supply of biomass, close to a major electricity transmission line and close to a supply of cooling water.

Once the population at large comes to understand that nonavailability of liquid fossil fuels will cause at least a tripling of liquid hydrocarbon fuel prices, the merits of electic vehicles, wherever they can be used, will be better appreciated.

This web page last updated September 1, 2011