Energy sources

With primary energy efficiency ratings of over 99.6 % (e.g. Bosch ELHB), boiler systems are among the most efficient thermodynamic systems available. Nevertheless, the choice of energy source is crucial for economic efficiency, environmental impact and security of supply. Due to how readily available they are, the following conventional fuels are currently still used in the majority of industrial and district heating thermal systems:

• Natural gas, in various forms and also as liquid gas (e.g. LNG)
• Fuel oil in various grades
• In particularly high-capacity facilities (e.g. power stations, ships, water-tube boilers), fuels that are particularly harmful to the climate, such as coal and heavy fuel oil, are still in use today

However, due to the carbon footprint of these fossil energy sources, many countries are increasingly rethinking their approach. The following energy sources are therefore used to generate heat, some of which have a better environmental balance than others:

• Other combustion gases (e.g. hydrogen)
• Biofuels (e.g. low-calorific gases, sewage and biogases, wood, other biomass)
• Electricity
• Contaminated by-products from the chemical industry (e.g. styrene, toluene)
• By-products from other industries (e.g. animal fat, fish oil)

The choice of fuel depends primarily on its availability at the planned installation site. Oil is usually delivered by tanker lorry, while gas requires a gas transfer station connected to the gas network. Some biofuels, hydrogen and (green) electricity are often generated locally on the company premises or at least nearby.

When there are high demands on security of supply, two or more fuels can also be used in a single boiler. In most cases, natural gas is used as the main fuel, with fuel oil or alternative energy sources used as a supplement or emergency alternative. The background to this is greater flexibility in availability, particularly for biogas and green electricity.

Another important criterion for selecting the fuel is economic efficiency. In this context, care must be taken to ensure that the cost can be precisely compared. For natural gas as a fuel, the comparative price can be taken directly from the gas bill or requested from the gas supplier. Heating oil delivery prices are published on the Internet. Energy procurement prices often vary greatly from company to company or region to region. This is particularly evident in the case of electricity: Some companies can purchase it at significantly reduced rates, comparable to those for natural gas. For many companies, electricity is not an option as the main energy source for thermal applications, because it is around three to four times more expensive per kilowatt hour than natural gas in many places. Even when prices on the electricity exchange are very low or even negative, grid fees usually still have to be taken into account. The situation is different in the case of green electricity surpluses, e.g. from own wind or PV systems, or in the event of fuel supply bottlenecks, where electricity is also a good alternative.

In the case of biogenic or self-generated energy sources on site, the cost analysis is significantly more complex, as larger investments and running costs usually have to be taken into account. The same also applies to wood in its various forms of use. In addition to the higher costs for the actual system, there are usually additional costs for production (biogas), storage (wood/biomass) and backup supply (second boiler, multi-fuel burner, etc.).

In addition to fuel costs, ancillary costs for the operation, maintenance and inspection of boiler systems and, in some cases, the cost of space for ancillary buildings must also be taken into account. In terms of total cost of ownership (TCO), gas-fired boilers are generally slightly more cost-effective, unless the costs of CO₂ and the environmental impact are taken into account.

In addition to costs, environmental impacts are also relevant in terms of approval: When burning the fuel used, the emissions permitted at the respective installation site of the boiler system play a particularly important role. Natural gas as a fuel produces lower levels of particulate matter and pollutant emissions, such as CO₂, NO_x and SO₂ emissions, than fuel oil or most biofuels. When using these, further restrictions may apply or additional flue gas purification measures may be necessary.