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By Charles Rhodes, Xylene Power Ltd.

Liquid hydrocarbons are a proven means of storing energy. This web page examines the production of synthetic liquid hydrocarbons without emitting CO2 to the atmosphere.

Presently liquid hydrocarbon fuels obtained from fossil sources are integral to North American society. During the year 2010 residents of the Province of Ontario consumed about 16 billion litres of liquid hydrocarbon fuels. These fuels are used because they are competitively priced and because their energy density is very high. The high energy density allows most vehicles to travel 500 km to 600 km between fuel tank refills.

If mankind is going to significantly reduce CO2 emissions to the atmosphere the use of liquid fossil fuels must be halted. However, there are various applications such as fuelling: aircraft, ships, vehicles going to remote locations and electricity generators at remote locations, where there is no practical substitute for liquid hydrocarbon fuels. Hence society must address large scale synthesis of liquid hydrocarbon fuels using only biomass carbohydrate to provide the required carbon.

Heat Use and Storage, 75 Slides

Liquid hydrocarbon fuels are oil or alcohol mixtures. Typical fuel oils have an average density of about 0.9. Hence one litre of fuel oil has a mass of about 900 gm. A fuel oil consists of hydrocarbon chains with about 2 hydrogen atoms per carbon atom, so the hydrogen mass per litre is about:
(2 / 14) X 900 gm = 128.6 gm hydrogen,
while the carbon mass per litre is about:
(12/14) X 900 gm = 771.4 gm carbon.

The details of liquid hydrocarbon synthesis are quite complex. However, from an energy perspective the overall reforming chemical reactions can be simplified to:
(hydrogen gas) + (plant sugar carbohydrate) = (methanol) = (cyclohexane) + (water)
or expressed as a balanced chemical reaction:
6(H2) + C6H12O6 = 6(CH3OH) = C6H12 + 6(H2O)
(water) + (electricity) = (hydrogen gas) + (oxygen gas)
H2O + electricity = H2 + .5(O2)
The first reaction actually involves multiple steps that must operate at a high pressure with appropriate temperatures, catalysts and stoichiometry. The water vapor formed must be continuously removed. The water (H2O) output from the 1st chemical reaction is an input into the 2nd chemical reaction and the hydrogen (H2) output from the 2nd chemical reaction is an input to the 1st chemical reaction. The oxygen (O2) output from the 2nd reaction is rejected to the atmosphere or sold as a byproduct.

These 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. There are ethanol and butanol production processes that superficially seem to require much less energy, but the missing energy is provided by consumption of much more carbohydrate (C6H12O6), which could otherwise be burned to generate electricity, so the net energy balance remains the same. The problem with ethanol and butanol biofuel production processes is that they more heavily impact food production.

Methanol (CH3OH) is not an ideal fuel because like other alcohols it corrodes aluminum. It also attacks certain rubbers. Methanol also does not have the energy density of cyclohexane (C6H12). However, producing cyclohexane requires a higher temperature reforming process.

Reference: On the sources of hydrogen for the global replacement of hydrocarbons

The atomic weights of hydrogen (1) and carbon (12) in the above synthesis equations show that to make 84 gms of cyclohexane requires 12 gms of hydrogen gas. Hence to make 1 kg of cyclohexane requires:
(12 / 84) X 1000 gms = 142.86 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:
(142.86 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
= 5.629 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:
5.629 kWh / .73 = 7.711 kWh to 5.629 kWh / .56 = 10.052 kWh

The atomic weights of hydrogen (1) and carbon (12) in the above synthesis equations show that 180 gm of carbohydrate are required to make 84 gm of cyclohexane. Hence the amount of carbohydrate required to make 1 kg of cyclohexane is about:
(180 / 84) X 1000 gm = 2142.9 gm.

However, as biomass (wood) 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 amount of biomass required to make 1 kg of cyclohexane is:
2142.9 gm / 0.7 = 3061.2 gm.

Wood is normally sold by stacked volume rather than by mass. Howeverfor wood with a material density of 0.6, we can make an approximate conversion from mass to stacked volume as follows:
(3061.2 gm) X (1 kg / 1000 gm) X (1 m^3 / 600 kg) X (1 inch / .0254 m)^3
X (1 ft^3 / 1728 inch^3) X (4 / 3.14) X (1 cord / 128 ft^3) = .001793 cord

Assume that the average cost of delivered electricity consumed by the cyclohexane production plant is $0.14 / kWh. Then the cost of feedstock electricity for making 1 kg of cyclohexane is in the range:
7.711 kWh X $0.14 / kWh = $1.0795
10.052 kWh X $0.14 / kWh = $1.4073

Assume that wood purchased in large quantities and delivered to the cyclohexane production plant costs $33 / cord. Note that this cost of wood can easily double if there are significant transportation distances involved. Then the minimum cost of biomass feedstock for making 1 kg of cyclohexane is about:
.001793 cord X $33 / cord = $.0592

Hence the combined costs of energy and biomass feedstock for synthesizing 1 kg of cyclohexane via the above mentioned synthesis reactions are typically in the range:
$1.0795 + $.0592 = $1.139
$1.4073 + $.0592 = $1.466

The density of cyclohexane (C6H12) is about .779 kg / lit. Hence the feedstock costs per litre are in the range:
$1.139 / kg X .779 kg / lit = $.887 / lit
$1.466 / kg X .779 kg / lit = $1.142 / lit

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

Presently the above synthesis reactions are not used because it is less expensive for biofuel producers to sacrifice 2/3 of the available biocarbon to remove oxygen from the carbohydrate than it is for these producers to use purchased electricity to remove oxygen from the carbohydrate. It appears that the cost of carbohydrate has to increase about five fold with respect to the cost of electricity before the limited supply of carbohydrate is no longer wasted in this manner. This issue has tremendous implications on the future cost of food, which is primarily carbohydrate. If the free market system is allowed to prevail without suitable regulation, much of the world will simply starve. This issue also suggests that the price of liquid fuels will likely reach a plateau in the $3.00 / litre to $4.00 / litre range at which point the supply of synthetic non-fossil liquid fuels will increase about three fold due to more efficient use of the available supply of carbohydrate.

The energy density of cyclohexane is given by:
(3920 kJ / mole) X (1 mole / .084 kg) X (.779 kg / lit) = 36353.3 kJ / lit

The above analysis shows that the electricity requirement for displacing existing (year 2010) liquid fossil fuel use in Ontario is in the range:
16 X 10^9 litres / year X 7.711 kWh / kg X .779 kg / lit = 96.1 X 10^9 kWh / year
16 X 10^9 litres / year X 10.052 kWh / kg X .779 kg / lit = 125.28 X 10^9 kWh / year

By comparison, total annual electricity production in Ontario in 2004 was about 151 X 10^9 kWh / year, so the total amount of nuclear and renewable electricity generation has to be about doubled to minimize the consumption of carbohydrate for liquid fuel production. In constructing this new electricity generation it is essential to keep the cost of the resulting electricity low, typically about $.14 / kWh. Thus the only economically viable energy sources are on-shore wind, hydro and well managed nuclear. Each synthetic fuel plant will have to be located close to a large supply of biomass, close to a major electicity transmission line and must be adjacent to a supply of cooling water.

Cellulosc Hydrocarbon Biofuels Futures

This web page last updated October 20, 2023

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