Theoretical analysis of steam generation methods – energy, CO2 emission, and cost analysis

Panel: 3. Energy management: the nuts and bolts

Authors:
Frédéric Bless, NTB University of Applied Sciences of Technology Buchs, Switzerland
Cordin Arpagaus, NTB University of Applied Sciences of Technology Buchs, Switzerland
Stefan Bertsch, NTB University of Applied Sciences of Technology Buchs, Switzerland
Jürg Schiffmann, Ecole Polytechnique Fédérale de Lausanne
(Laboratory for Applied Mechanical Design), Switzerland

Abstract

Steam is used in many industrial sectors for distillation, sterilisation, heating, drying, etc. Water vapor generation is producing carbon dioxide, in particular when a gas-fired boiler or an electrical heater produces the steam. This study analyses different alternative methods (vapor recompression, direct electrical heating, heat pump, use of waste heat, and a mixture of thereof) to produce steam. The steam generated has a pressure around 3 bar which is common for an industrial low pressure steam network. Simulations using EES have been performed to calculate the energy (and exergy) needed to generate 1 kg/s of steam. The emission of CO2 and an approximation of the operating and capital cost for each method is also determined. The study also looks at the amount of waste heat needed to minimise the energy consumption. The gain of using waste heat at different temperatures is evaluated. From an energy point of view, a gas-fired boiler is the least efficient method. However, when natural gas boilers are used, the CO2 production is lower than with other methods, such as a 4-stage water vapor recompression system using the US electricity mix. Depending on the availability and the quality of waste heat, steam generation by vapor recompression reduces the energy consumption by a factor of 3 to 5. The CO2 emission of the electricity used is crucial to determine the best method in terms of global warming potential. Cost analysis shows that the usage of a natural gas-fired boiler reduces the operating cost. Vapor compression methods can be cost-competitive when the available waste heat is at high temperature and when the electricity is not too expensive.