It is almost a tradition to use steam powered mechanical energy in the chemical and petro-chemical industry (i.e. turbo-powered compressors or pumps). But is it the right thing to do? Many industrial professionals attribute this choice to the viability of the electrical supply vs. steam availability. But is this statement still applicable in the 21st century developed countries? And moreover is it applicable in a context where backup decentralized power generation is a must in every oil or petro-chemical industry (for process safety issues)?
So how to get out of the age of steam?
- STEP 1: What type of turbine are you using?
Steam turbines come in all shapes and sizes, but we are interested here in whether the turbine is of the back-pressure type or of the condensing type. In fact, back-pressure turbines have a steam outlet of low to medium pressure, which can be reused in the plant as thermal energy, whereas condensing turbines expand the steam to a negative gauge pressure, making it impossible to reuse the heat, thus losing it in the condenser at very low temperatures (30 to 40 °C). Condensing turbines are therefore much less efficient than their back-pressure counterpart (the overall system efficiency ranges between 10 and 20% for condensing turbines). This statement is however true only in the case where the outlet steam is reused as thermal energy. A quick calculation shows that in the great majority of developed countries the condensing steam turbine is more costly than buying electric energy from the grid.
- STEP 2 : Are you using all the medium and low pressure steam coming out of the back-pressure turbine?
If all steam coming out of the steam turbine is used in the plant for thermal needs, than we are in a cogeneration configuration. In that case go to step 3
If surplus low or medium pressure steam exists, this means that thermal energy is wasted (either by direct steam release or by condensers). In this case, all should be done to reduce and eliminate this waste. The key, again, is the steam turbine load. Different options are possible:
- Eliminate one or more steam turbines (by replacing them with electric motors). The choice should be directed in a way to eliminate surplus low and medium pressure steam.
- Make one or more turbines work in parallel with electric motors (like a parallel circuit of a turbo-pump and an electric-pump). This choice should also serve to eliminate all surplus low and medium pressure steam.
Again, calculations show that electric energy is much more cost-effective when used to eliminate the steam waste.
- STEP 3 : Is your overall energy system more efficient than the grid electric power system?
This is a purely economic analysis and comparison. The whole system should be analyzed in order to identify which is best. The analysis should take into consideration the following data:
- Steam production cost (i.e. the cost of fuel and boiler system efficiency)
- Steam turbine efficiency or specific consumption (i.e. tons of steam per kWh mechanical energy)
- Electric cost from the grid or from auto-production
- Electric motor efficiency
In conclusion, using steam turbines to power mechanical equipment like compressors and pumps is not always a bad idea, but can sometimes be the worst. An economical optimum should be defined using technical and economic analysis to reduce the cost of running the machines. As an example, among the last 3 sites, audited for energy efficiency, only one was using steam turbines as mechanical energy in the economically correct way. For the other 2, overall yearly economic savings that could be achieved amount to a total of €20M, representing respectively 13 and 23% of the total energy bill of each site.