A guide to waste heat recovery – Part Three

This is part three of a three-part article written by Peter Beleznay, Heatcatcher,for World Cement’s August issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch upon-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the August 2015 issue here.

Expander technologies for electricity generation

The recovery of waste heat and generating electric power using steam turbines expanders within a Rankine Cycle is a well proven and widely used technology, being mandatory for cement plants in China in order to have a WHR captive power plant on site. An increasing number of larger plants across Asia with waste heat temperatures higher than 300°C can achieve higher returns on investment with steam generator WHR systems. The increasing technological and commercial advances in Organic Rankine Cycle (ORC) systems provide an alternative WHR technology to traditional Rankine Cycle steam based systems, when temperatures available are below 300 °C.

The temperature difference between the waste heat source and the minimum operating temperature of the chosen expander technology determines the efficiency of electricity generation. The larger the drop between the input and output temperatures of the expander, the more efficient the generator.The available temperature range to be recovered will determine the use of expander technology.

Turbine Expanders (either axial or radial type) have relatively high efficiency of between 12-18% depending on available input temperature and the type of refrigerant, but there is little flexibility in the temperature range to the lower region. Heat source temperature for recovery could be between 150°C and 300°C. Turbines are very efficient, but the refrigerant has to be at a superheated state before it enters the high speed turbine.

For lower input temperatures, screw expander ORCs can be used instead of turbines. The screw technology allows a certain amount of liquid refrigerant to enter the expander without damaging it. This allows the recovery of heat sources that are prone to temperature fluctuation to lower ranges. The efficiency of this system is lower than that of the turbine expander (ranging around 6-9%), but it is more robust and less sensitive to changes in operational circumstances. Screw expanders are also suitable for smaller steam systems, providing more flexibility in the steam/liquid ratio of such a system, allowing high volumes of water droplets to flow through the expander, which would otherwise damage a steam turbine.

Capex versus Opex costs

The control of WHR systems is automated, with all major components controlled through the distributed control system (DCS). However, unlike the closed working fluid loop of an ORC generator, steam WHR systems need regular water quality checks - up to three times per day. This would include checks on water contaminants level, PH level and silica level, in addition to the regular maintenance activities and checks. A steam WHR system of 5 MW or above would require a minimum of seven site-based staff for operation and maintenance purposes. This would typically include a shift engineer, two boiler operators and two turbine operators (one of each for both DCS operation and field operation). The operating and maintenance costs of an ORC system are expected to be a quarter of that of a steam system, depending on the working fluids used in the primary energy loop of the exhaust gas heat exchanger and the condensing loop cooling method.

Steam systems have a lower Capex cost per MW than the ORC systems above a minimum size of generating capacity, however the Opex costs are much higher. The difference in these Opex costs is magnified when comparing the staff costs of skilled engineering labour in China and Asia to those in Europe.

Return on Investment

To date the operational challenges of integration, and a return on investment rates lower than competing projects for capital expenditure, has resulted in a slow adoption of WHR technology across Europe. The return on investment of a WHR system will depend on many variables. With the size of the majority of kilns across Europe being less than 3000 tpd – without the benefits of economies of scale and lower cost base seen with large steam generator WHR systems rolled out across China – returns on investment are typically up to 5 years without incentives or grants.

The variables having the greatest effect on the return on investment are the delivered cost of electricity and the plant’s annual operating hours. Setting these variables against the WHR system’s annual net electrical generation and other associated electrical savings, versus the full turnkey project Capex and Opex costs, determines the return on investment rate. With lower return on investment rates the kiln operator can consider the option of a Power Purchase Agreement (PPA), also known as an Energy Services Contract (ESCo). Under this finance option the Capex and Opex costs are met by a third party green investment fund. This funding option allows the kiln operator to purchase electricity at a discounted cost, and receive the carbon emission reduction benefit without Capex or Opex costs and associated technology risk, in return for providing an agreed minimum amount of waste thermal energy per year.

Conclusion

As more cement kiln operators consider WHR as a solution to reduce their electrical consumption and carbon emissions, the methods of integration and generator technology options are continuously developing. Ensuring that as a kiln operator you are considering the best available technologies and techniques is key to the lowest risk of integration to the production process, whilst achieving the highest return on investment for the operating life of the plant.

Read the article online at: https://www.worldcement.com/special-reports/07082015/a-guide-to-waste-heat-recovery-part-two-289/