Industrial waste heat - sorption heat storage as a solution

Author: Eberhard Lävemann, ZAE Bayern (As of: December 2016) Waste heat from industrial processes is not infrequently released into the environment at temperatures above 100 °C, e.g. as an exhaust gas stream, because it cannot be used on site at the current time. Mobile sorption storage units can be charged with waste heat and later make it available to an industrial process, ideally a drying process, at another location. With the aim of testing whether mobile sorption heat storage is a technical and economic alternative to reduce CO2 emissions, ZAE Bayern in association with its partners Hoffmeier Industrieanlagen in Hamm and Müllverbrennungsanlage Hamm Betreibergesellschaft conducted a project funded by the German Federal Ministry of Economics and Technology, which started in December 2009 and was completed in June 2014 [1].

Sorption heat storage

Zeolites are capable of adsorbing water vapor from an air stream and releasing heat in the process. Based on this principle, a mobile sorption heat storage system was developed. In a thermally insulated cylindrical steel container with a length of 7.5 m and a diameter of 2.5 m, 14,000 kg of zeolite (Köstrolith 13X) were accommodated in such a way that up to 12,000 m³/h of air can flow through it.

Demonstration plant

A demonstration plant was set up by project partner Hoffmeier Industrieanlagen, consisting of two mobile sorption heat storage units (Fig.1), a charging station at the Hamm waste incineration plant (Fig. 2) and an unloading station at the Jäckering Mühlen und Nährmittelwerke company, see Fig. 3. At the charging station, ambient air is heated to approx. 135 °C by means of heat recovery and tapped steam from the medium-pressure rail of the steam cycle of the waste incineration plant and blown into the heat accumulator. There, water vapor is expelled from the zeolite, cooling  the air. At the discharge station, moist air (temperature approx. 60 °C, dew point approx. 40 °C) is taken from the exhaust air stream of a drying process and blown into the heat accumulator. The water vapor from the air is adsorbed there. The adsorption heat released heats the air to approx. 160 °C. The dry hot air is fed to the supply air stream of the drying process and supports a gas burner, which provides the remaining heat to operate the drying process.

Thermal powers

The charge and discharge powers depend on the operating conditions and the state of charge of the storage tank. For storage #1 and #2, 60 and 27 charge and discharge cycles, respectively, were measured and evaluated. The charging power averaged over the charging process and all cycles was 225 kW and  220 kW, respectively. The maximum charging powers reached approximately 500 kW. The discharge power averaged over the discharge process and all cycles was 154 kW and 157 kW, respectively. The maximum discharge powers reached approximately 300 kW. Overall thermal efficiency averaged 78%.

Energy conversion

Heat delivered from the storage tank to the process air stream at about 160 °C was 2384 kWh for storage tank #1 averaged over all cycles. Gas savings averaged 3800 kWh or 3560 kWh per cycle. Auxiliary energy demand for plant operation averaged 6.2% based on gas savings, with 5.4% consumed as electricity for charging and discharging stations, which is primarily required by the fans. The remainder of 0.8% is required as fuel for transport.

The steam for regenerating the sorbent is taken from a cogeneration plant. Their electricity production is thereby reduced. Energetically better would have been the use of the exhaust gas flow. However, connecting the exhaust gas to the  charging station could not be solved structurally. The loss in electricity production is just under 13% relative to the average gas savings. CO2 emissions are reduced by an average of 0.7 t per cycle in line with the gas savings.

Economics

The heat production costs for the demonstration plant and its operation were determined to be 154 €/MWh based on VDI Guideline 2067. From the experience with the demonstration plant, measures for cost optimization of a plant of the same type can be derived, with which the heat production costs can be reduced to approx. 73 €/MWh. The distribution of costs is shown in Fig. 4. If the charging heat can be obtained free of charge as waste heat, the costs for steam are eliminated and the specific heat generation costs are reduced to approx. 67 €/MWh.

Conclusion

The demonstration plant functioned perfectly after commissioning, in particular the coupling of heat into the drying process works without disturbing it. Only minor malfunctions occurred during the one-year regular operation. Under the given conditions, the plant delivered the planned outputs. The logistics concept was tested and optimization potentials have been identified. However, a plant of this type cannot be operated economically even optimized in competition with current gas prices. The heat customer of the demonstration plant purchased its gas for about 38 €/MWh. Operation of the plant was therefore discontinued. In order to come into the range of competitiveness at current gas prices, the costs for plant construction and operation would have to be halved. The project partners currently do not see an option for this.