The use of condensing technology in steam and hot water systems and its potential for reducing running costs and CO2 emissions – part 1
Bosch Industrial
Complete solutions for steam, hot water, heating, decentralised energy supply, as well as air conditioning & ventilation
Many commercial and industrial applications with heat and process heat demands lose considerable amounts of heat via chimneys. One option for reducing flue losses from steam and hot water generators caused by physical processes is to use condensing technology. This technology, which has been standard in domestic heating systems for many years, is now starting to be adopted by industry. While it involves additional costs, they are often amortised within two to three years. Using condensing technology allows companies to improve their energy efficiency, conserve resources in their production processes and sustainably protect the environment.
Net and gross calorific values and condensation heat
A good analogy for explaining how condensing technology works is to imagine a cooking pot. If you pour boiling water into a pot without a lid, it takes a lot of time and energy to make the water completely evaporate and leave the pot empty. In contrast, with a lid placed on top, the water vapour will condense. This amount of energy is then released when the vapour changes back into a liquid.
The following brief explanations illustrate the distinction between the indicators of net calorific value and gross calorific value. The net calorific value ("lower heating value", Hi or Hu) is the maximum energy that can be used and which is released during complete combustion. During this process, the flue gas cools down to the reference temperature at a constant pressure. The water vapour created during combustion remains gaseous. The indicator of net calorific value therefore specifies the amount of heat contained in the flue gas (sensible heat).
The gross calorific value ("upper heating value", Hs or Ho) contains the sensible heat and the condensation heat (or "latent heat") in the flue gas. This means that the water vapour in the flue gas condenses and releases additional heat after combustion and cooling.
When calculating efficiencies, you should refer to a fuel's net calorific value. Previously, it was essential to leave the water vapour in the flue gas in a gaseous state in order to prevent potential corrosion damage. Nowadays, a wide range of modern condensing systems that use flue gas condensation are available on the market. The net calorific value has remained as a reference value and results in condensing systems having efficiencies of over 100%, i.e. gross calorific value = net calorific value + condensation heat. Nevertheless, it goes without saying that the "real" degree of primary energy efficiency can never be exceeded.
Basis for using condensing technology
Condensing technology harnesses the energy content in the water vapour of flue gases. In order to make full use of this potential, the flue gases from combustion must cooled below the dew point until the onset of condensation. This requires corrosion-resistant materials, such as heat exchangers made of stainless steel, and moisture-resistant flue systems and chimneys.
Which fuels are suitable for condensing technology?
When hydrocarbons are burnt, both the carbon and the hydrogen are combusted. The longer the chains are in the hydrocarbons, the higher the relative proportion of carbon will be. Natural gas largely consists of methane, which only contains one carbon atom in each molecule (CH4) and therefore forms the highest proportion of water in the flue gas and the least CO2. Hydrogen is the only element that combusts without producing any CO2.
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When comparing the relevant key data for established fuels, natural gas currently offers the highest potential use for condensing technology (see table of key data for various fuels).
Natural gas has:
Compared to fuel oil EL, natural gas offers more condensation heat at a higher condensation temperature level. This means that flue gas condensation begins at higher flue gas temperatures. The flue gases produced by combustion are virtually free of soot and sulphur, making contaminated heating surfaces very easy to clean. This maintains the effectiveness of the condensing technology and prevents operating faults. Moreover, the pH value of the flue gas condensate is higher than for fuel oil EL, reducing the effort involved in disposing of the flue gas condensate. Nevertheless, fuel oil is also suitable for use with condensing technology and allows boilers to be operated more economically.
It is expected that alternative and carbon-neutral fuels, such as hydrogen, will be increasingly used in the future. The same established implementation rules and technologies for natural gas can be used for hydrogen combustion. In addition, the potential for using condensing technology with hydrogen is even higher than it is for natural gas.
Condensing technology achieves an efficiency of over 100% in relation to net calorific value Hi
If suitable heat exchangers and the coldest possible circulating water are used, it is possible to cool the flue gas down to below its dew point. Figure 1 shows the influence of the flue gas dew point and the temperature of the return water on the quantity of condensing water vapour and the achievable boiler efficiency. Figure 2 illustrates examples of efficiency curves which show the potential of condensing technology. Using this technology considerably increases the operational and economic benefits in hot water and steam generation and achieves an efficiency of over 100% in relation to the net calorific value Hi. Compared to using conventional systems with standard flue gas heat exchangers, condensing technology reduces the fuel quantity, fuel costs and emission of pollutants by over 8%. This makes a significant contribution towards environmental protection and the sustainable reduction of CO2 emissions.