Combustion of hydrogen-containing gas in hydrogen and steam boilers
M.A. Savitenko , Director of Autonomous Non-commercial Organization “Center for Research and Scientific Development in the field of energy “Hydrogen Technology Solutions”
B.A. Rybakov, Chief Technologist of Autonomous Non-commercial Organization “Center for Research and Scientific Development in the field of energy “Hydrogen Technology Solutions”
In 2015, the Russian Federation signed the Paris Agreement, according to which it assumed obligations to reduce greenhouse gas emissions into the atmosphere.
The main greenhouse gases produced by combusting fossil fuels include water vapor, carbon dioxide, and nitrogen oxides.
Combusting coal produces mainly carbon dioxide and nitrogen oxides, while combusting natural gas produces carbon dioxide, water vapor, and nitrogen oxides.
European countries have emphasized the reduction of carbon dioxide emissions into the atmosphere. The process of reducing CO2 emissions is called decarbonization.
One of the ways to reduce carbon dioxide emissions into the atmosphere is to switch to the combustion of hydrogen-containing gases (HCG), including the combusting of pure hydrogen.
The pic. 1 shows how increasing the proportion of hydrogen in natural gas reduces carbon dioxide emissions.
- Researchers note the following effects associated with hydrogen combustion:
- Flame color changes as the concentration of hydrogen in the fuel gas increases. The color changes from blue to colorless when switching from 90% to 100% hydrogen.
- Combustion rate of hydrogen is higher than that of natural gas. The laminar flame propagation speed of hydrogen is 10 times higher than that of natural gas.
- Explosive concentration limits of hydrogen are higher than those of natural gas.
- Hydrogen has a low activation energy of the flame.
- The density of hydrogen is significantly lower than that of natural gas.
- Hydrogen’s Wobbe index is not significantly different from that of natural gas.
Given the high reactivity of hydrogen and the high flame propagation rate of its combustion, precautionary measures must be taken to eliminate the possibility of flame propagation up the “flashback” stream, as well as the possibility of spontaneous combustion.
Therefore, burner devices adapted for hydrogen combustion must be equipped not only with a “reverse” flame detection system, but also with protection against flame propagation upstream.
One of the significant disadvantages of increasing the share of hydrogen in the combusted fuel gas is the increase in emissions of nitrogen oxides into the atmosphere.
Hydrogen production methods
There are two main ways to produce hydrogen-containing gases from natural gas:
- Steam conversion
During steam conversion water vapor with a temperature above 600°С is mixed with natural gas and a part of methane is converted to hydrogen using a catalyst. A mixture of natural gas, hydrogen, and water vapor is produced at the outlet of the methane steam conversion plant.
Pyrolysis produces hydrogen and solid carbon.
Gas pyrolysis products include ethylene and acetylene.
Acetylene is a combustible gas. Its chemical formula is C2H2. It is lighter than air and has a pungent odor.
In addition to these substances, methane pyrolysis produces:
butane; carbon monoxide; hydrogen; propylene; diacetylene; vinylacetylene
Pure hydrogen can be produced by electrolysis from water.
Main characteristics of methane and hydrogen
Table 1 shows the main characteristics of methane and hydrogen
|Volumetric lowest heat of combustion under normal conditions, MJ/nm3||35,9||10,8|
|Volumetric highest heat of combustion under normal conditions, MJ/nm3||40,0||12,8|
|Density under normal conditions, kg/nm3||0,717||0,09|
|Mass lowest heat of combustion, MJ/kg||50||120|
|Volumetric lowest Wobbe index under normal conditions (WI), MJ/nm3||48,2||40,7|
This table shows:
- the volumetric heat of combustion of hydrogen is 3.4 times less than the volumetric heat of combustion of natural gas,
- The density of hydrogen is 8.3 times less than the density of natural gas,
- the mass heat of combustion of hydrogen is 2.4 times higher than the mass heat of combustion of natural gas,
- The volumetric highest heat of combustion of methane is 11.42% higher than its volumetric lowest heat of combustion,
- The volumetric highest heat of combustion of hydrogen is 18.5 % higher than its volumetric lowest heat of combustion,
- The volumetric Wobbe index of hydrogen is 18% lower than the volumetric Wobbe index of natural gas.
Main characteristics of the methane-hydrogen mixture
The dependences of the density, volumetric heat of combustion, and volumetric Wobbe index of the methane-hydrogen mixture on the volumetric concentration of hydrogen (H2) are presented below.
Since the volumetric Wobbe index of hydrogen-containing gas is lower than the Wobbe index of natural gas (48,1 MJ/nm3), mixing hydrogen with natural gas will reduce the Wobbe index of the fuel gas.
The Wobbe index is the main criteria for the interchangeability of gaseous fuels. Theoretically, if a device designed for natural gas combustion is fed with a different chemical composition that has the same Wobbe index, then the combustion process as well as the concentration of nitrogen oxides in the fuel gases, should not change.
The fact that the Wobbe index of hydrogen-containing gases and pure hydrogen do not differ very much from the Wobbe index values typical for natural gas, gives hope that burner devices designed for natural gas combustion can be used for combustion of hydrogen-containing gases. At the same time, the permissible proportion of hydrogen in hydrogen-containing gases will depend on the specific design of the burner device.
Condensation of water vapors of flue gases
It is noted in /1/ that when combusting organic fuels in steam and hot water boilers, the greatest heat losses are with the outgoing gases. As a rule, the temperature of these flue gases is 130-200°C, i.e. significantly higher than the dew point temperature of water vapor.
If we reduce the temperature of the flue gases to 30-50°C, we can get up to 10-14% of fuel savings when combusting natural gas.
Additional heat is obtained by condensation of water vapors due to the latent heat of vaporization.
When combusting pure hydrogen, its savings due to the heat of condensation of water vapor exceeds 18,5%.
Utilization of the heat of the outgoing flue gases is widespread in the EU countries. Most of these installations are operated on water-heating and steam boilers of small capacity (5-50 MW).
Condensate obtained from water vapor has an increased acidity due to the dissolution of carbon dioxide in water. By adding soda hydroxide to the condensate, it is chemically neutralized.
Humidification of the air supplied to the combustion provides the following positive effects:
- the dew point temperature of the flue gases increases, which increases the temperature of the condensate;
- the combustion temperature decreases, resulting to a 40-60% reduction in nitrogen oxide emissions.
Use of water vapor condensate
Condensate formed from water vapor of flue gases can be used for the following purposes:
- humidification of air supplied for combustion;
- generation of steam which can be used to produce hydrogen-containing gas by steam conversion;
- production of hydrogen by electrolysis.
A comprehensive solution to reduce greenhouse gas emissions in hot water and steam boilers
A side effect of water vapor condensation to increase the fuel heat utilization coefficient is the specific reduction of both carbon dioxide and water vapor emissions into the atmosphere, i.e. the reduction of greenhouse gas emissions per unit of produced thermal energy.
The resulting condensate can be used to produce hydrogen-containing gas, the combustion of which will lead to an additional reduction of carbon dioxide emissions into the atmosphere and an increase in the condensate formed during the condensation of water vapor.
With an increase in the proportion of water vapor in the flue gases, the dew point temperature will change, which may lead to an increase in the volume of condensate produced.
An increase in the volume of water condensate makes it possible to increase the proportion of hydrogen in the fuel gas.
Air humidification can significantly reduce nitrogen oxides emissions into the atmosphere.
Modernization of existing boilers for hydrogen-containing gases combustion
The main measures that need to be taken for transition to combustion of hydrogen-containing fuels in existing boilers are listed below:
When combusting hydrogen-containing gases with high concentration of hydrogen the burner device will require replacement of modernization.
As noted above, the burner device adapted for hydrogen combustion must be equipped not only with a “reverse” flame detection system, but also with protection against flame propagation upstream.
- The hydrogen-containing gases preparation system must have a special system for monitoring the condition of equipment and gas pipelines;
- Gas pipelines and shut-off and control valves must be made of materials that avoid hydrogen embrittlement.
- The fuel gas treatment system must be redesigned, in particular, a hydrogen and natural gas mixing device will have to be developed.
- An automated system of gas pipeline blowing will have to be developed.
- Modernization of the automated boiler control system will be required.
Since the volumetric heat of hydrogen combustion is significantly lower than the volumetric heat of natural gas combustion, the cross-sections of gas pipelines and fuel gas treatment system equipment (filters, shut-off and control valves, heat exchangers) should be increased in relation to the cross-section of gas pipelines designed for natural gas operation.
Design and construction of new hydrogen-fueled boiler units
When designing new natural gas power plants, it makes sense to lay down technical solutions that in the future will allow to use hydrogen-containing gas as fuel.
In addition to measures listed above, which are necessary for the modernization of boilers, it is essential to take into account that with an increase of concentration of hydrogen in fuel gas in flue gases, the proportion of water vapor will increase.
Therefore, the calculation of boilers should be carried out taking into account possible changes in the composition of combustion products.
- Влияние конденсационного утилизатора на работу паровых и водогрейных газовых котлов. Ионкин И.Л., Рагуткин А.В., Росляков П.В., Супранов В.М., Зайченко М.Н., Лунинг Б. Теплоэнергетика, 2015, № 5.