The Dutch process industry uses around seven billion m3 of natural gas annually to produce steam for its manufacturing processes. Heating costs are therefore a very significant expense. During heat exchange approximately 20% of the condensate formed disappears unused down sewage or evaporates as vapour steam. This loss of heat is equivalent to approximately 420 million m3 of natural gas, an enormous loss.
Steam boilers are hardly ever optimised
A fully functioning steam boiler has an efficiency rate of approximately 85%, that value is often not achieved due to:
The latter causes internal contamination of the boiler: pollution is deposited on the heated surface. As a result flues and flue tubes are covered with an insulating layer. Therefore considerably more energy is required to reach the desired temperature.
Conclusion: insufficient purging decreases efficiency and increases costs. Boiler pollution can be prevented through automatic purging. It is of crucial importance that the quality of the boiler water - especially the salinity - is properly monitored. Salt thickens the water in the boiler and then settles on the flues and the flue tube. Since salinity determines conductivity it is possible to measure and monitor the salinity of the water very accurately, by measuring conductivity. By properly pre-treating boiler supply water and by regularly monitoring the water quality you can minimise purging loss.
Optimisation of the steam process requires an integrated approach
In order to optimise the efficiency of your steam process the situation in the boiler house and beyond can be evaluated, as well as the process in which the steam is used. In theory yields can be gained in each of these aspects.
Let us first look at the combustion process in your boiler house. Approximately 10 m3 flue gases are released for each 1 m3 of natural gas combusted. Flue gases consist mainly of water vapour released during combustion, which is very hot: these gases leave the boiler at a temperature that is generally approximately 40°C above the steam temperature. In a steam boiler of 10 bar the steam temperature is approximately 180°C and the flue gas temperature therefore is approximately 220°C. These gases also contain a lot of heat energy. You can use this (free) energy to preheat the boiler supply water with a heat economiser before it is fed into the boiler. The economiser reduces the flue gas temperature to approximately 120°C, which provides an additional yield of 5%. The performance improvement is about 1% per 20°C temperature reduction. The economiser is possible if your steam boiler has a capacity of at least 5 tonnes of steam per hour and if sufficient boiler supply water is available for heating.
Flue gas condenser
If you should add a flue gas condenser on top of an economiser, the flue gas temperature can be further reduced to the dew point (approximately 58°C). That provides an additional 3% yield, which already provides a total efficiency improvement of around 8%!
Depending on your system type you can further optimise boiler efficiency by reclaiming flash tank heat from the boiler purging water. The steam released here can be used in the degasser while the warm condensate can be used to preheat the supply water. If your boiler is equipped with automatic purger this option is certainly worth exploring. Depending on the type of boiler and the type of company a further 0.5% efficiency improvement can be achieved, which brings your total potential savings to 8.5%. This option is only applicable if your boiler has a capacity of at least 5 tonnes of steam per hour.
If the capacity of your steam boiler is less than 5 tonnes it can still be useful to optimise the settings of the burner - in particular the percentage of excess air. 10% of excess air is ideal. More is not meaningful since the additional volume of air does not participate in the combustion and heating and is therefore meaningless. This would naturally also cost additional energy.
Because generated steam is transported to consumers through a pipe it is also possible to achieve savings outside of the boiler room. Firstly this can be done by checking whether the steam pipes are properly laid out and whether your system is properly insulated. A well-insulated pipe emits 80% less heat than an uninsulated pipe. It is also important to also insulate the valves and flanges. The latter is often neglected, which is a shame because 1 kW of energy loss costs in a continuous operation amounts to burning approximately 1,243 m3 extra natural gas per year.
Adequate condensate drainage
The capacity of the system must also be guaranteed by including a steam trap every 30 metres for adequate drainage of the condensate. This condensate is preferably reused.
Identify leaky steam traps
It should also be regularly checked if the steam traps are working well. It has been shown in practice that in a steam system that has not been checked for three to five years, between 15% to 30% of the steam traps are defective and an average of 25% of the steam traps are leaking steam. And that leads to a considerable loss of steam. A leaky steam trap - regardless of the make or model - can produce 45 tonnes of CO2 emissions per year and waste up to € 3,000.00 in steam! Detecting leaks through annual steam trap inspection - or even better: continuous leak detection - pays itself back within a few months.
Finally, processes that use steam for indirect heating through heat exchangers provide opportunities for cost savings. Here steam transfers heat energy through a metal wall to heat the product. The condensate which is thereby released is discharged through steam traps and returned via a condensate return pipe to the boiler room. Just after the heat exchanger the residual heat in the condensate is used to make flash steam to pre-heat the product. In the boiler room the condensate can then be reused as boiler supply water without requiring pre-treatment. With the residual heat still present in the condensate, much less energy is required to bring the condensate back to the desired temperature.