Heat Recovery - A Key Issue
The main purpose of the turbine is to drive the generator converting steam to electricity.
Energy recovery as related to energy optimization is one of the key issues of our times. This involves both optimizing efficiency and ensuring environmental protection by reducing CO2 emissions. For instance, the glass production process - similar to many other continuous heated processes - creates tremendous amounts of waste heat that are too valuable to simply be released through a stack. To efficiently use this waste heat, steam turbines are used in some plants to convert it into electricity via waste heat boilers. However, the key to success in this application is the ability to accommodate partial- or full-load operation in a harsh environment.
The product being manufactured is always the focal point of any operation, which is why the waste heat recovery system must not have any negative impact on the production process. Reliability and uptime of the waste heat recovery system typically are more important than the peak amount of energy recovered. Finding the right compromise between rugged design and highest theoretical efficiency will result in a system recovering the most energy over the plant’s life cycle.
The waste heat recovery system consists of these core components:
Heat recovery boiler.
Steam turbine with generator.
In order to recover the most heat, the waste heat recovery system boiler should be installed between the furnace and the electrostatic precipitator (ESP), exposing the boiler to the furnace’s contaminated flue gas. Particles that could clog up the boiler and the composition of the flue gas are factors that should be considered during the boiler design.
The temperature drop in the boiler, which is recoverable energy, is limited by the input temperature (oven or furnace’s output temperature) as well as the minimum temperature allowed to be passed on to the ESP and, finally, to the stack. Most production processes provide a changing flow stream of waste heat that can vary from 50 to 100 percent (partial- to full-load operation) - a range the boiler must handle.
The turbine is the second core component and the heart of the system. Its main purpose is to drive the generator converting steam to electricity. One of the turbine’s important characteristics is flexible construction. This flexibility ensures that a proven standard product can be used in a range of applications without customizing on a plant-by-plant basis. Customizations typically lead to higher initial and maintenance costs.
Another important aspect is the turbine’s high efficiency from partial- to full-load operation. For instance, in the glass industry, the volume of the extracted air from the glass production process increases over the furnace’s lifetime. In other industries, the amount of available waste heat can vary based on the quantity and type of product being produced. The turbine must be capable of processing the corresponding changing steam volume with a load range from 50 to 100 percent, ideally without impacting the turbine’s efficiency.
In practice, it is beneficial to have a twin turbine with one high-pressure stage and one low-pressure stage, driving the same generator via a gear box. This approach can increase the efficiency and the energy that can be recovered, and steam can be tapped between the two stages for process heating or space/building purposes. A two-stage system can qualify as a combined heat and power (CHP) system and provide specific advantages.
Because high temperatures and pressures are involved in the waste heat recovery system steam cycle, the monitoring and control system needs to be carried out as a fail-safe system (safety integrity level [SIL] classification) with the capability to be integrated into the overall control system, typically a distributed control system (DCS). This allows the waste heat recovery system to be monitored and controlled from both the central control room and on-site from a local operating station. If a manufacturing execution system (MES) is involved, key performance indicators like energy consumption and energy recovery can be tracked at any given time and contribute to the overall plant efficiency.
Reliability and uptime of the waste heat recovery system typically are more important than the peak amount of energy recovered. Finding the right compromise between rugged design and highest theoretical efficiency will result in a system recovering the most energy over the plant’s life cycle.
Return on Investment
More than 90 percent efficiency can be achieved if the steam is used not only for generating electricity but also for other processes via CHP units, as well as for heating water or for air-conditioning with absorption-type refrigerating systems. Up to 60 percent of the electrical energy used in some manufacturing plants such as glass processing can be covered with the electricity generated in the waste heat recovery system.
Depending on the infrastructure, the heat energy also can be fed back into the municipal district heating grid or supplied to nearby companies. This shows that many factors must be considered that go beyond the confines of the site, and that is the reason why full installation, from working out the overall concept all the way to implementation, can take two to three years. Many experienced specialists must pool their knowledge and the system must be designed as a whole, always keeping the manufacturing process, and any negative consequences, in mind.
The break-even point for a waste heat recovery system can vary. For instance, at a medium-sized float-glass plant, it is typically just a few years, depending on the design of the overall plant, the production volume and local energy prices. The fees paid for electricity and heat fed into municipal utility grids, subventions for CHP plants, bonus payments for renewable energy, and rebates on mineral oil taxes can help attain return on investment sooner.