Such different gas motor fuels. Why do cars run on liquefied gas and not compressed gas?
I once happened to read an article where it was written that everything is fine with gas stations for cars, and it will get even better.
What is written sounds bravura:
A set of equipment for a CNG filling station (gas filling station for cars) with a capacity of 1000 m3/h for 6 hoses costs 33 million rubles with VAT. If not [швыряться деньгами]construction and installation will cost the same amount (or twice as much if there are problems with connections and gas). We are not saving much, so let the total capital expenditure be around 100 million rubles. 1. The cost of a cubic meter of gas consists of the cost of 1 cubic meter from the pipe (let it be 9 rubles) and the electricity spent on compression (1/3 kW*h, let it be 3 rubles). Total – 12 rubles. 2. The selling price of a cubic meter of gas is 20-25 rubles in Russia, in areas with high demand the price reaches 26-27. Let's take the average of 23 rubles. 3. Total margin per cubic meter is 11 rubles. 4. Fixed costs for wages and other things – about 500 thousand rubles per month. 5. The average load of CNG filling stations in Russia now is about 7 thousand cubic meters per day.
But in reality, the number of CNG filling stations in Russia is growing slowly. In 2016, the number of such stations was 270, and at the end of 2021 – 422. Now their total number is 667, which, you must agree, is not enough for such a country. And they are still owned by Gazprom, not private individuals. Which suggests that not everything is as beautiful, smooth and profitable as written in the letter.
And this is despite the fact that there is a program for the development of gas motor fuel, which was first started by Gazprom, and then the Ministry of Energy joined in. At the same time, another type of gas filling stations, AGZS, has been developing for a long time and very rapidly without any federal programs. Without Gazprom, but by ordinary private individuals. As an example, two maps of the Orenburg region, the first one shows AGNKS, there are seven of them
And at this far from complete one – 44 gas stations (red signs)
Before talking about why this is happening and why the program for using natural gas as a motor fuel is seriously lacking, let's sort out the terminology.
Gas motor fuel refers to various types of gas that can be used as fuel for internal combustion engines, both gasoline and diesel. There are three different types of gas motor fuel:
Liquefied natural gas (LNG) – this refers to liquefied natural gas, almost pure methane. Its storage requires very low temperatures (boiling point -158…-163 °C) and high pressure. Such conditions are impossible (or very expensive and low-tech) to create in road transport, so liquefied natural gas is used as fuel in gas tankers.
Compressed natural gas (CNG) – this is the same natural gas, which is based on methane, only it is highly compressed (200-250 bar) at normal temperature, so it does not turn into a liquid state. It is this gas that is so actively promoted as a gas motor fuel, it is compressed gas that is refueled at CNG stations – automobile gas filling compressor stations.
Liquefied petroleum gas or propane-butane mixture (LPG, PBS) – this is also a type of liquefied gas. Only it is not based on methane, but on heavier gases of the alkane series – propane and butane. They are capable of liquefying at positive temperatures under low pressure. Propane and butane are usually part of associated petroleum gases extracted together with oil, but there are many of them in ordinary natural gas, depending on the deposit, and their number is determined by a funny slang term fat content of gas.
It is LPG that is the most popular gas motor fuel at the present time, used in motor transport. It is used for refueling at AGZS – automobile gas filling stations.
Is it profitable to use gas as fuel?
Of course, yes. At least because there is much more gas than oil. But not everything is so clear-cut. Let's look at an interesting table, it reflects the specific heat of combustion of some types of the most commonly used fuels.
If you look at the first two lines, you can immediately see the most profitable fuel – hydrogen. It has a simply enormous calorific value. But we do not consider hydrogen as a gas motor fuel, and from the available options we are interested in methane, propane, and butane (46.8 MJ/kg)
But if you look at the table as a whole, you can track several dependencies:
The less oxygen in the fuel, the more calorific it is. It is obvious that oxygen itself does not burn, but is an oxidizer.
If the fuel contains carbon and hydrogen, the higher the percentage of carbon, the less calorific it is. And here everything is clear, the heat of combustion of carbon is much lower than that of hydrogen.
But the third dependence has the most direct relation to the questions posed in the title. Despite the impressive indicators of the calorific value of fuels by weight with a high hydrogen content, these fuels are very inferior to fuels with a higher carbon content in the calorific value of one liter of such fuel, i.e. by volume.
So, despite the fact that a kilogram of propane contains 117% of the energy of a kilogram of diesel fuel, its energy value per liter is only 63% of the energy value of diesel fuel. And all because the higher the hydrogen content, the lower the density of the fuel.
Therefore, measuring the calorific value of fuel based on volume is completely wrong. One liter of kerosene gives twice as much energy as ethanol. A liter of hydrogen, when burned, gives the least energy of all possible fuels, but a kilogram of hydrogen gives the most energy. That is why they came up with an exact unit for measuring energy value – accounting is carried out in tons of equivalent fuel (t.e.f.). 1 t.e.f. is equal in calorific value to 1 ton of high-quality coal, or 29.308 MJ or 7000 (seven thousand) kilocalories per kilogram.
If we take the table, then the most profitable gas motor fuel from the point of view of energy value will not be methane, even if it is liquid, but propane, which is slightly inferior to butane. But if we take gaseous methane, everything will be even worse. After all, it is in the liquid state that the maximum possible mass fits into the same volume (if we are talking about the same substance in a liquid and gaseous aggregate state). Compressed methane will be even less in mass than liquid.
I looked at the cylinder for my car, and it’s not that small.
This is the lightest composite, metal is cheaper, but it also weighs 60 kilograms. Moreover, composite is safer.
But 50 liters is the volume of the cylinder, not the gas that fits in it. This volume depends primarily on temperature and pressure.
That is, if the total pressure is 200 bar, and the air temperature is from -30 to +40 °C, then according to calculations, a 50-liter container can hold approximately 11.63 to 14.93 cubic meters of natural gas. At 20 degrees Celsius, this cylinder can hold only 12.5 m3 of methane. On average, the mileage on a cubic meter of methane is the same as on 1.1 liters of gasoline. That is, if I put such a fool in the trunk of a length of 90 centimeters and a diameter of 32.7 centimeters, then even according to the passport consumption in the combined cycle I will travel an incredible 182.6 kilometers!!! At the level of some miserable electric train like a Nissan Leaf.
And if you consider that CNG filling stations are installed outside residential areas, on bypass roads, for safety reasons, then everything will be even sadder. I looked at the map, I have to drive 17 kilometers from my daughter's apartment to the CNG filling station!!! That is, having gone to the gas station and back, my mileage will be 148.6 kilometers. Taxi drivers are simply crying with delight and signing up for a queue for methane equipment. Of course, you can install more cylinders, but where to put things in the trunk? And the car will drag around an extra 70-120 kilograms, essentially a hefty man or a fat woman, which simply cannot but affect the consumption.
Now let's take gas cylinder equipment for PBS. And choose a cylinder for the same 50 liters
It is a little smaller and much cheaper. But, most importantly, it holds 50 liters of propane-butane mixture. Even taking into account that it has less calorific value and one liter of the mixture can drive as much as 0.85 liters of gasoline, then on one filling I can drive 566 kilometers in a combined cycle. This is quite a good result, especially considering that the gas station is several times larger than the CNG filling station.
Let's also compare the cost of equipment. In our Orenburg, the price of LPG for PBS on a turnkey basis is from 28 thousand rubles. But the price of LPG for methane with cylinders of the first and second type, and not composite, is not less than 65 thousand and this is a promotional price.
Now let's talk about the cost of gas motor fuel. The price of one cubic meter of methane in Orenburg is now 25 rubles. But the cost of LPG is still the same 25 rubles. If you take the difference in mileage, then on methane you can drive 25 percent more. That is, the cost of one kilometer of mileage on my car is 3.03 rubles. And on LPG – 3.78 rubles. These are good indicators, on gasoline it is much more expensive – 6.66 rubles.
But to simply recoup the difference in equipment, I need to drive almost fifty thousand kilometers. That's how much I drove on my previous car in five years. But the most important thing is that the mileage is very low. 188 kilometers is very little even for the city, especially considering that there are few gas stations and they are located far outside the residential area.
Conclusions:
There are several types of gas motor fuel – LNG, LPG and CNG
The most profitable type of gas motor fuel in terms of cost per kilometer will be compressed natural gas
The cost of 1 kilometer of mileage on liquefied petroleum gas is higher than on CNG. But due to cheaper and simpler equipment, lighter cylinders and, most importantly, the mileage on one filling, installing LPG equipment is often more profitable and provides greater autonomy
Compressed natural gas is more suitable for commercial vehicles, since the size and weight of the equipment are less critical for it.
Due to this, the fleet of CNG vehicles will be significantly inferior to the fleet of LPG vehicles.
Well, what alternative could there be, you ask me? Are we really not going to drive on methane en masse?
I don't know, I'm not a prophet. But as for me, the only alternative is if they learn to use liquefied natural gas as a gas engine fuel. Even if we take into account that its density is very low and is 0.39 kg/liter (based on the fact that methane is pure, while the average density of LNG is 0.41-0.5 kg/liter), then on fifty liters of LNG you can travel 440 kilometers, which already provides more or less good autonomy. But everything comes down to price and technology.
Afterword
There is another way to create liquid fuel – to make synthetic oil. This method is called GTL – Gas to liquids, gas-to-liquid conversion, but about this some other time.
Author: Sigurd Sigurdovich
