IZAYDAŞ to Utilize Medium and Low Pressure Steam Thermal Capacity from Incineration Plant

In thermal systems, the temperature of flue gases generated after combustion is reduced in the boiler to produce steam and electricity. High-pressure superheated steam supplied to the turbine exits with reduced temperature and pressure, is sent to an air-cooled condenser or cooling tower to be condensed, and is then pumped back to the boiler. In this article, an evaluation was conducted on energy recovery from medium-pressure boiler steam and turbine outlet steam using the energy production system of the İZAYDAŞ waste incineration facility as an example.

Energy Recovery from Incineration Facility Turbine Outlet Steam

İZAYDAŞ Hazardous Waste Incineration Facility is Turkey's first licensed waste disposal facility in which hazardous waste is incinerated at high temperatures. The facility, with a capacity of 35,000 tonnes/year, was commissioned in 1997 and continues to operate. Hazardous waste is incinerated in a rotary kiln and secondary combustion chamber at temperatures between 1000-1200°C. Before the flue gases enter treatment units, they pass through a water-tube boiler to reduce temperature and recover energy. The heat energy produced from waste incineration is 55 GJ/hour in the rotary kiln and 31 GJ/hour in the secondary combustion chamber. In the boiler, waste gas temperature is reduced to 200°C to obtain saturated steam. The produced saturated steam is superheated to 40 bar and 350°C in the superheater section and sent to the turbine. Energy is obtained by supplying the superheated steam to a multi-stage, back-pressure steam turbine. Steam leaving the turbine is now called exhaust steam with reduced pressure. The pressure averages 78 mbar and the temperature is between 45-50°C. Exhaust steam at the turbine outlet is condensed in the condenser unit and reused as boiler feed water. The incineration facility steam energy cycle operates according to the Rankine cycle. A schematic view of the Rankine cycle is shown in Figure 1, and thermal calculations are provided below. [caption id="attachment_123448" align="aligncenter"] Figure 1. Rankine cycle[/caption]   Calculations determined that turbine efficiency is 35%, the thermal capacity of exhaust steam leaving the turbine is 171.68 kJ/kg and totals 600,000 kcal/h. How much of this calculated thermal capacity can be recovered and what design should be applied?

Energy Recovery from Low-Pressure Steam Using ORC or Heat Pump

The energy of low-pressure steam entering the condenser can be recovered using a heat pump or ORC (Organic Rankine Cycle) system. Organic Rankine Cycle (ORC) systems operate on principles similar to conventional Rankine systems. The ORC is a highly efficient form of energy production that converts heat to electricity. [caption id="attachment_123452" align="aligncenter"] Figure 2. Incineration furnaces and boiler[/caption]   Heat energy recovered from waste heat is transferred to the organic fluid circulating in the system. The working principle generally involves the turbo-generator converting thermal energy first to mechanical energy, then through an electric generator to electrical energy. While conventional steam turbines use water vapor as the working fluid, ORC systems use a fluid with higher molecular weight than water. After heating the organic fluid using an appropriate heat exchanger system, it is vaporized and supplied to the turbine, where mechanical energy is produced by expansion of this vapor at constant pressure. Mechanical energy is converted to electrical energy through a generator. Subsequently, the organic fluid circulating in the closed loop is cooled and condensed using air or water, pumped to the regenerator within the closed loop, and the cycle continues. Applying ORC systems achieves high energy efficiency in waste heat recovery. Total thermal loss of 2% occurs in the system, 72%-78% of the remaining thermal power is used to produce mechanical energy in the turbine, and 20%-26% is converted to electrical energy in the generator. [caption id="attachment_123454" align="aligncenter"] Figure 3: Example of Organic Rankine cycle[/caption]   A second option is recovery of the thermal energy of steam at 45-50°C and/or condensate water temperature through a heat pump. Within the scope of heat recovery, both technologies can be used to recover energy from exhaust steam going to the condenser. However, the investment costs of both systems are very high, while the thermal capacity to be used is very low. Another important consideration is that even if a system could technically be established from turbine outlet exhaust steam, it would require significant design modifications. Sending turbine outlet steam to air-cooled or water-cooled towers and reusing it as boiler feed water is a conventional method, while ORC system designs can only be achieved using flue gases or produced pressurized steam. Another method could be to install a heat exchanger system at the turbine outlet. However, this design would create resistance in front of turbine outlet vacuums, negatively affecting the operation of the boiler-turbine-condenser system. [caption id="attachment_123455" align="aligncenter"] Figure 4: Air and water-cooled condenser units[/caption]   Condenser and Cooling Tower: Steam exiting the turbine—in other words, steam with reduced pressure and temperature—is cooled in the condenser and converted to water. The resulting water is returned to the facility's steam generation boiler for reuse. In cooling towers, water from seas, lakes, or rivers is used to provide the cooling effect. In regions far from water sources, air-cooled fans are preferred. Since it will not be possible to recover all of the calculated 600,000 kcal/h heat, calculating the useful energy taken from the condenser based on the energy consumption of the cooling fans used should be the correct approach. It has been calculated that the air fans in the condenser consume an average of 150 kW/h during actual operating time, with equivalent thermal capacity of approximately 130,000 kcal/h. This thermal capacity is quite low, and considering losses that could result from transferring it elsewhere, it is evident that there is no significant heat available.

Use of Medium-Pressure Boiler Steam in Central Heating System

At the incineration facility boiler, the pressure of high-pressure steam is reduced to obtain saturated steam at 6 bar pressure and 180°C temperature. Medium-pressure steam is used in the flue gas dioxin unit heat exchanger, hazardous liquid waste tank heating, and various steam stations at the facility site. Medium-pressure steam is provided by reducing the temperature and pressure of superheated steam. Medium-pressure steam is supplied from the steam flow going to the turbine, and turbine energy production decreases by the amount used. Medium-pressure steam temperature: 180°C, Pressure: 6 bar. Central heating boiler capacity: 500,000 kcal/h. Based on this information, for the central boiler system: Q= m *(hsteam-hcondensate). 500,000 = m * 4.18 * (2600-400); m = 950 kg/h of medium-pressure steam is required. If steam is used instead of natural gas in the heating system, this amount of steam will not go to the turbine. Its electrical energy equivalent is approximately 100 kW/h. İZAYDAŞ facilities used a total of TRY 80,000 worth of natural gas for administrative building heating in 2020. To use steam instead of natural gas in heating the circulating water of the heating system: medium-pressure steam at 6 bar must be drawn to the central heating system area 250 meters away, and using a plate-type heat exchanger suitable for 5 bar - 950 kg/h capacity, the circulating water in the system must be heated.

• Underground 50 mm diameter 250 meter insulated steam line.
• 2" plate-type heat exchanger and condensate piping.
• Pump and valve piping.
• 50 mm diameter 200 meter condensate return line.

The investment cost of such a system would be approximately TRY 130,000. Although the investment payback period is less than 2 years, because steam is drawn from steam going to the turbine, 100 kW/h equivalent energy cannot be produced in the turbine. If calculated only for winter months: 100*24*30*6= 432 MW energy, equivalent value: TRY 142,560. Producing electrical energy by sending steam to the turbine instead of using it in the central heating system is more profitable. Electricity price: average TRY 330/MW.

Conclusion and Evaluation

This article evaluated energy recovery from exhaust steam at the turbine outlet of the İZAYDAŞ incineration facility and the use of medium-pressure steam in the central heating system. In the evaluation conducted: the thermal capacity of turbine outlet steam is low, energy can be utilized with an ORC (Organic Rankine Cycle) turbine system or heat pump, and although technically possible with certain limitations, it requires significant design modifications and investment; it was determined that it is therefore not economical and would negatively affect turbine operation through the turbine-condenser vacuum system. Boiler feed water is heated to 130°C and returning condensate water at 45°C contributes to this, and if energy is extracted from the condenser side, energy must be spent again to heat the boiler feed water; consequently, it was concluded that usable energy available in low-pressure steam at the turbine outlet will remain low. By placing a simple heat exchanger at the turbine outlet, thermal capacity of steam can be used with lower investment; however, this would create resistance in front of the turbine outlet line, and by changing the turbine-condenser vacuum pressure, would negatively affect turbine operation. It was calculated that the useful thermal capacity available in the condenser is 130,000 kcal/h. Considering losses that could result from transferring low thermal capacity elsewhere, it was determined that the investment would not be economical and would not provide operational readiness. Condensing turbine outlet exhaust steam by sending it to an air-cooled condenser or water-cooled tower unit is a conventional design, and any additions that would change this design would reduce the efficiency of the existing system. When medium-pressure steam is used for heating, turbine energy production would decrease by 100 kW/h. Producing electrical energy by sending steam to the turbine instead of using it for heating is more appropriate.     Dr. Şahan Dede Operations Manager İZAYDAŞ