Reduction greenhouse gas emissions in hot water 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”

Currently, the world pays great attention to combating global warming caused by anthropogenic emissions of greenhouse gases. To combat climate change and its negative impacts, 197 countries adopted the Paris Agreement in 2015. The agreement, which came into force less than a year later, aims to significantly reduce global greenhouse gas emissions and limit the rise in global temperatures this century to 2 degrees Celsius. In 2016, the Russian Federation signed the Paris Agreement, in accordance with which it assumed obligations to reduce greenhouse gas emissions into the atmosphere. At the climate summit in New York, 66 countries have pledged to become carbon neutral, that is, to take measures for emissions of total compensation CO ₂ .

The main greenhouse gases from fossil fuels are carbon dioxide, methane, water vapor and nitrogen oxides. At the same time, European countries have focused on reducing carbon dioxide emissions into the atmosphere.

In the production of heat energy at CHPPs and in boiler houses, the following ways of reducing carbon dioxide emissions are possible:

Increase in efficiency;
Increasing the heat utilization rate of the fuel;
Conversion from coal to natural gas combustion.

Switching from coal to gas can reduce carbon dioxide emissions by about 50%.

When one kilogram of methane (CH ₄ ), the main component of natural gas, is burned, 2.75 kg of carbon dioxide (CO ₂ ) is emitted into the atmosphere.

Burning one kilogram of carbon – the main component of coal – releases 3.7 kg of carbon dioxide into the atmosphere.

Taking into account that the net calorific value of carbon is approximately 1.5 times less than the net calorific value of methane, carbon dioxide emissions into the atmosphere when burning an equivalent mass of carbon will amount to 5.5 kg.

Therefore, the conversion of power and heating boilers to natural gas leads to a significant reduction in carbon dioxide emissions into the atmosphere.

It should be noted that when natural gas is burned into the atmosphere, water vapor is formed along with carbon dioxide.

As noted in [2,3], a number of foreign and Russian researchers in their articles reported that the greenhouse gas No. 1 is water vapor.

This article proposes a concept for reducing emissions into the atmosphere, both water vapor and carbon dioxide when burning natural gas in steam and hot water boilers.

One of the ways to reduce emissions of water vapor and carbon dioxide into the atmosphere when burning natural gas is the condensation of water vapor from flue gases.

The higher heat of combustion of natural gas exceeds the lower heat of natural gas, depending on its composition, by 10-11%. Accordingly, the use of the heat generated during the condensation of the water vapor of the flue gases will lead to an increase in the thermal power of the power plant or to a decrease in the consumption of natural gas while maintaining the heat load.

Reducing the consumption of natural gas while maintaining the thermal capacity of the installation leads to a decrease in carbon dioxide emissions into the atmosphere.

A further decrease in carbon dioxide emissions into the atmosphere during the combustion of natural gas is possible when hydrogen-containing gases (HSG) are mixed with natural gas, including the addition of pure hydrogen.

At present, both abroad and in the Russian Federation, the issues of using excess electricity at nuclear power plants, hydroelectric power plants, wind farms and solar power plants for hydrogen generation are being worked out.

At the same time, in parallel with the development of an independent hydrogen infrastructure, the addition of hydrogen to natural gas in trunk and distribution networks is also being considered.

Foreign studies show that up to 20 or 30% (volume) of hydrogen can be added to the existing system of transportation and distribution of natural gas, which leads to a decrease in carbon dioxide emissions into the atmosphere by 7 … 11%.

The fig. 1 shows how an increase in the volume fraction of hydrogen in natural gas leads to a decrease in carbon dioxide emissions.

Fig. 1

The higher heat of combustion of hydrogen exceeds its lower heat of combustion by 18%. Accordingly, the use of the heat generated during the condensation of water vapor of flue gases will lead to a greater increase in the thermal power of the power plant than during the condensation of water vapor generated during the combustion of natural gas.

Use of condensing heat recovery units

In [4], it is noted that during the combustion of organic fuels in steam and hot water boilers, the largest losses are heat losses with exhaust gases. As a rule, the temperature of the flue gases is 130-200 ° C, that is, it significantly exceeds the dew point temperature of water vapor.

Historically, in the development of boiler units, it was necessary to avoid condensation of water vapor, since the coal and fuel oil that were burned in the boilers contained sulfur. Condensation of water vapor in the presence of sulfur leads to the formation of sulfuric acid, which led to accelerated corrosion of low-temperature boiler heating surfaces.

When burning natural gas, in the composition of which the concentration of sulfur compounds does not exceed the standard indicators, it is possible to obtain additional heat energy by condensation of water vapor, which is formed during the combustion of natural gas. If the temperature of the flue gases is reduced to 30-50 ° C, then significant fuel savings can be obtained. Additional heat is obtained due to the latent heat of vaporization during condensation of water vapor.

Utilization of the heat of exhaust flue gases is widespread in the EU countries. Most of these installations are operated on hot water and steam boilers of small capacity (5-50 MW) [5].

When hydrogen is mixed with natural gas, the efficiency of the condensation heat exchanger will increase due to an increase in the volume of condensed water vapor.

Using the design of a condensing heat exchanger with preheating and humidification of the air supplied for combustion will reduce the formation of thermal nitrogen oxides, which are a very strong greenhouse gas, by reducing the combustion temperature of the fuel.

Also, with an increase in the proportion of hydrogen in the fuel gas, the amount of condensate obtained in the condensation heat exchanger also increases, which can be further used for various purposes, for example, feeding the heating network, humidifying the air supplied for combustion, producing hydrogen-containing gas by steam reforming or obtaining hydrogen by the method electrolysis.

Comprehensive solution to reduce greenhouse gas emissions
in hot water and steam boilers

To obtain the greatest economic and climatic effect, it is advisable to equip boilers with condensing heat recovery units when combusting WASH.

The following are the main measures that need to be taken to switch to combustion of hydrogen-containing fuels in existing boilers.

When burning HSG with a high concentration of hydrogen, it will be necessary to replace or modernize the burner.
As noted above, a burner device adapted for combustion of hydrogen must be equipped not only with a back-flame detection system, but also with protection against upstream flame propagation.
The WASH preparation system must have a special system for monitoring the condition of equipment and gas pipelines.
Gas pipelines and shut-off and control valves should be made of materials to avoid hydrogen embrittlement.
The fuel gas preparation system must be redesigned, in particular, the development of a device for mixing hydrogen and natural gas will be required.
The development of an automated gas pipeline purging system will be required.
Modernization of the automated control system of the boiler will be required.

Since the volumetric heat of combustion of hydrogen is significantly lower than the volumetric heat of combustion of natural gas, the flow sections of gas pipelines and equipment for the fuel gas treatment system (filters, shut-off and control valves, heat exchangers) should be increased in relation to the flow area of gas pipelines designed to operate on natural gas. gas.

Design and construction of new hydrogen fuel boilers

When designing new power plants using natural gas, it makes sense to lay in technical solutions that in the future will make it possible to use hydrogen-containing gas as a fuel.

In addition to the measures listed above, which are necessary when modernizing boilers, it is necessary to take into account that with an increase in the hydrogen concentration in the fuel gas in the flue gases, the proportion of water vapor will increase.

Therefore, the calculation of boilers must be carried out taking into account possible changes in the composition of combustion products.

In [5] it is reported that the efficiency of the condensation process of water vapor is significantly affected by the excess air ratio, with an increase in which the dew point temperature decreases. Therefore, to increase the efficiency of the condensation heat exchanger (KTU), optimization of fuel combustion is required.

Сonclusions

Condensation of water vapors of flue gases from boiler units can solve several problems:

reduce emissions of carbon dioxide and water vapor into the atmosphere;
reduce fuel consumption by 10-11% when burning natural gas;
to obtain condensate, which can be used to feed the heating system, humidify the air supplied for combustion, and produce hydrogen.

Adding hydrogen to natural gas will further reduce carbon dioxide emissions into the atmosphere, as well as increase condensate consumption.

Reduction greenhouse gas emissions in hot water and steam boilers