Now that we are seeing significant increases in gas and electricity costs across the board, re-using heat energy has become hugely important in reducing running costs and carbon footprints. Offering healthy returns on investment, heat exchangers are a very cost-effective way to capture and re-use the waste heat.
Waste heat recovery (WHR) using heat exchangers, can assist in any application where exhaust gas, steam or waste hot water is produced.
There are many ways in which heat can be saved from being wasted to the atmosphere or the drain and captured and re-used. This article seeks to explore some of them.
The current factors contributing to the growing popularity of Waste Heat Recovery
WHR is the process of capturing residual heat that would have otherwise dissipated into the atmosphere, whether inside or outside a facility, and putting it to use in another area of the plant that requires heat generation.
WHR can be used in many applications from capturing heat in coal burning power stations using feed water heaters and re-using it downstream, to a simple heat exchanger that takes heat out of a car’s engine radiator to provide heat for a camp shower.
The WHR market in Australia has increased at a rate of 6% per year between 2016 and 2019, and is predicted to grow at a higher rate in the near future. The main drivers for this are:
- Rising energy prices
Electricity and gas prices for manufacturing businesses have increased sharply in recent years and indications are that prices will continue to increase.
WHR technologies help in reducing the costs of operating facilities by enhancing their energy productivity. The energy recovered during this process is widely used in two major applications, namely, steam/electricity generation and preheating.
- Government regulations & incentives
The requirement to adhere to stringent government regulations regarding energy efficiencies and emissions, as well as incentives provided by the governments to do so, including tax rebates.
- Growing environmental concerns
Global climate change is the greatest economic, political, developmental, and environmental challenge for counties worldwide. WHR technologies are one of the most effective methods of minimising greenhouse gas emissions. The primary benefit of lower
emissions is of course the positive effects on our environment, but it can also offer monetary savings.
- Need for energy efficient equipment and processes
Enhancing energy efficiency is the cheapest and the most reliable method of curbing carbon emissions, thus saving costs.
- Local Council and EPA requirements
Local Councils and the EPA are increasingly concerned about warm contaminated water being flushed down drains without treatment and are demanding that solids are removed along with the heat to reduce algae and bacteria growth.
An easy way to achieve reduction in heat wastage is through a heat exchanger that not only cools waste water to reduce bacterial and algae growth, but can put the recovered heat into usable clean water that can then be reintroduced into the system (a boiler) with demonstrably significant savings. One of Fluid Dynamics’ customers reported a return on investment (ROI) of 2.5 months with a 13.5% reduction in its $100,000 monthly gas bill.
Typical set up where post process residual heat is taken out of the system after use and introduced back into the heat generation source (e.g. Boiler) or the process itself.
Whilst virtually any processing facility can benefit from the recovery of waste heat, Fluid Dynamics has identified some of the industries where heat recovery is most beneficial.
Here are some sectors where facility managers may find that installing Heat Exchangers for waste heat recovery could cut costs and make their processes more efficient.
Some of the top industries that can benefit from WHR
Power Generation (Coal)
Even in regions with abundant coal supply, such as Victoria’s Latrobe Valley, there is still a need to use it efficiently and to implement methods to reuse and recycle the energy produced as a result of its burning.
In coal fired power stations, heat exchangers recover heat from hot streams or gasses that still contain a relatively high amount of energy. One example of this is the exhaust gases from boilers that would otherwise go into the atmosphere unused.
Waste heat from exhaust gases can be extracted through the use of an economiser, where the waste heat in the exhaust gas leaving the boiler is passed over the economiser’s heating surface (e.g. Finned Tubes), heating the feed water that passes through the tubes before it enters the boiler. This increases boiler efficiency and results in less fuel being required to generate steam. The overall thermal efficiency of the cycle can be improved by around 10% using this process.
From other sources such as diesel engines or gas turbines, waste exhaust heat can be reused to pre-heat boiler feed water and generate steam. Waste heat boilers (WHB) or exhaust gas boilers (EGB) can be used to create steam without the need for a separate boiler. Steam from these units is then used in another process. In cases where this steam is superheated, the steam can then be used to drive a steam turbine to generate electricity.
For more information on Feedwater Heaters go to: http://fluiddynamics.com.au/feedwater-heaters-cost-effective-environmentally-friendly-solutions/
Pulp and Paper
One of the main processes in pulp and paper manufacturing is drying. There are various possibilities to recover thermal energy from steam and waste heat in the paper drying process using a heat exchanger. This can include:
- Mechanical vapor recompression and reuse of the superheated steam in the drying process
- Use of heat pumps to recover waste heat
- Recovering heat from the ventilation air of the drying section and using this heat for the heating of the facilities when needed.
Food & Beverage Processing
Food manufacturing processes such as baking involve the use of gas-fired ovens. Only about one-third of the total energy used in these ovens adds value to the final product. The remaining two-thirds is discharged as exhaust gases at 150-250°C and thus represents an opportunity for heat recovery.
Tubular Heat Exchanger used for heat recovery in steam applications.
Though low temperature ranges, fouling and the presence of corrosive materials in the exhaust gases sometimes make heat recovery challenging and uneconomical, heat exchangers can often be used for gas-to-liquid heat transfer to produce hot water for use in other areas of the manufacturing plant.
In processes such as sugar production from sugar cane, coke oven gas refining, and sugar-based ethanol production, combustible by-products are burned to generate process steam and heat. With the introduction of a waste heat recovery system, the need to burn by-products as fuel may be reduced and those by-products can be sold instead.
Petroleum Refining
Producing fuel is incredibly energy-intensive, with processes like distillation, thermal cracking and treatment all requiring high temperatures.
As in coal fired power stations, waste heat from exhaust gases can be extracted through the use of economisers, in which waste heat in the exhaust gas is passed through a heat exchanger and heats the feed water before it enters the boiler. This reduces the energy required to heat the boiler with resultant costs savings, increased boiler efficiency, lessened fuel usage and increased overall thermal efficiency.
For more information on Feedwater Heaters go to: http://fluiddynamics.com.au/feedwater-heaters-cost-effective-environmentally-friendly-solutions/
Abattoirs
Commonly abattoirs generate large volumes of hot wastewater to use in their processing of meat. Clean fresh water is heated in a boiler, used at temperatures up to 85C and eventually disposed of into drains, storage or sewers wasting the heat and potentially creating a biohazard.
By passing both the waste hot water and clean fresh water through a heat exchanger, a significant amount of the heat can be exchanged into the clean water and passed to the boiler reducing the amount of energy/heat needed to bring the boiler water up to the required heat.
For more information on Abattoir Waste go to: http://fluiddynamics.com.au/abattoir-waste/
Commercial Laundries
Another industry using a lot of hot water is commercial laundries. As in abattoirs, hot water is used in the process and the resultant waste water is commonly discharged to waste. Passing that hot waste water through a heat exchanger before discharge to the sewer reduces the water’s temperature, thus retarding algae growth and allowing the heat to be captured by the clean water. This is then fed into the boiler, reducing the energy required to bring the water back to a usable temperature.
Waste Water Treatment
A large amount of organic waste is produced in waste water treatment. In order to reduce the volume of the waste and limit pollution, the residuals, or sludges, are sent to anaerobic digesters where biochemical degradation of the sewage sludge occurs and where bacteriological action takes place in an oxygen free, controlled temperature environment for further processing into biogas.
Anaerobic digestion of sewage sludge allows not only the reduction of discharge but also recovery of the energy produced in the form of biogas which has a high calorific value due to its rich methane composition.
The resultant biogas can be stored in a gasometer and, after treatment to remove polluting or corrosive components, it can be used as a combustible to drive alternators and produce electrical power. This, in turn, can be used either on site or discharged to the electricity grid.
For more information on waste water treatment plants go to: http://fluiddynamics.com.au/case-study-waste-water-treatment-plants/
If you would like more information about waste heat recovery and how it can help you improve your ROI, get in touch with the friendly staff at Fluid Dynamics