Generating power for use on site from ‘waste’ local heat is particularly advantageous. A waste heat recovery plant based on the Organic Rankine Cycle can now work with heat at lower temperatures than ever before, writes Jason Gold.
Industrial companies have pursued waste heat recovery projects for many decades. Waste heat boilers, air preheaters and boiler preheater systems are common in most US industrial facilities and help improve energy efficiencies throughout the process. These systems make the most effective use of high-quality heat (above 430°C) to generate incremental power and – where relevant – also use lower-quality heat (200–430°C) for air or water preheating.
But a new adaptation of a decades-old technology called the Organic Rankine Cycle (ORC) is turning this paradigm upside down and allowing industrial companies to convert their low-quality waste heat into clean, emission-free electricity. Since electricity often carries three times the economic benefit of process heating, the economic justification for retrofitting and designing new facilities to include ORC-based solutions is compelling.
This new approach to waste heat recovery could vastly improve industrial efficiencies, cut companies’ power bills and create many jobs. Industrial companies across the US and throughout the world are now realizing this ‘hidden gem’ of an energy resource and installations of ORC-based waste heat recovery systems are accelerating.
A CHANGE IN PACKAGING
ORC systems are the industry standard for low-temperature geothermal projects, and have been successfully deployed on simple-cycle gas turbines, biomass systems, cement plants, district heating systems, solar thermal systems, sawmills, gas plants, landfill gas, glass plants and reciprocating engine exhaust. Many of these projects were self-developed using standard ORC modules.
In recent years, some manufacturers of ORC systems have recognized that modular, customized packages with precisely sized turbines, pumps and evaporators generate higher physical and financial efficiencies than monolithic, standard-sized systems. Using these new, tailored solutions developers have generated meaningful cost savings and project returns for their clients – even on projects for which previous economic evaluations suggested only marginal returns.
With proper equipment sizing, projects can leverage turbines that run ‘on design’ more frequently and where there is no financial waste on the evaporating or cooling surfaces.
A TIME-TESTED AND TRUSTED SOLUTION
Production facilities are historically cautious about implementing new technologies – particularly those technologies that interface directly with their sensitive process equipment. But Organic Rankine Cycle systems are gaining approval from engineers around the world as they realize these units are not only field-proven but also quite similar to traditional steam Rankine cycle systems – albeit with meaningfully simpler designs and lower maintenance costs.
Figure 1. ORC cycle diagram Source: KGRA Energy
The ORC plant produces electricity (and where relevant and/or needed low-temperature heat) through a closed-loop cycle that uses a low vapour-point, environmentally benign refrigerant as the working fluid instead of water. The by-product – low-temperature heat not consumed in the electricity generation process – is discharged to the atmosphere through air- or water-based condensers.
ORC customers can derive further economic benefits from that rejected heat by using it to meet their process heat needs. This solution significantly improves a facility’s energy efficiency and reduces its consumption of fossil fuel-based electricity.
The working fluid is pre-heated in a recuperator/heat exchanger (see Figure 1, Area 2) and then vapourized in the evaporator heat exchanger through contact with heat from the original heat source. In flue gas situations, the process uses an interposing hot oil loop that delivers thermal energy from the hot gas to the ORC (Area 1). The high-temperature, high-pressure vapour generated from this process is expanded in a turbine that drives a generator (Areas 3 and 4).
After leaving the turbine, the organic working medium (still in the vapour phase) passes through the recuperator (Area 5) that serves two functions, both of which improve total system efficiency:
- it pre-heats the incoming organic liquid prior to vapourization;
- and this ‘heat dump’ lowers the parasitic duty of the cooling circuit (Area 6).
After exiting the cooling/condensing circuit, the working medium is pumped back to the appropriate pressure (Area 7) and is then preheated by internal heat exchange in the recuperator (Area 5).
SOURCES OF USEFUL WASTE HEAT
Relying on the same thermodynamic principles that govern traditional waste heat recovery analysis, ORC-based waste heat recovery projects convert as much as 25% of the incoming energy they receive into usable electricity. The quantity of energy such a system receives is as important as the quality of energy it receives in determining a project’s feasibility and financial rate of return. But the definition of ‘high-quality’ heat to an ORC can be as low as 200°C whereas ‘high-quality’ heat to a traditional Rankine cycle system is 430°C.
ORC systems have been successfully deployed on liquid heat sources as cool as 80°C, on steam sources as cool as 120°C, and on gaseous sources as cool as 180°C. Generally speaking, conversion efficiencies (the rate at which incoming BTUs are converted into electricity) rise to 25% and level off at incoming heat source temperatures of 260°C. This lower-temperature ‘sweet spot’ represents a very fruitful source of useful heat energy that often remains untapped at industrial sites.
A 750 kW ORC being set at the Weyerhaeuser lumber mill in North Carolina Photo: KGRA Energy
Although the types, sizes and temperatures of heat sources vary across industries, a few industrial processes are widespread and provide substantial opportunities for profitable waste heat recovery:
- turbine exhaust (several sites along US and Canadian pipelines have implemented ORC-based waste heat recovery);
- reciprocating engine exhaust (a recently announced project with Chesapeake Energy will harvest heat from five reciprocating engines to produce nearly 2 MW);
- process furnace exhaust;
- boiler exhaust (several projects throughout the world are attached to existing boilers);
- line heater exhaust;
- process steam (a developer has recently received a $2 million grant for a process steam project at a pulp and paper mill);
- kilns (Weyerhaeuser is pursuing a project adjacent to one of its drying kilns at a lumber mill in Ayden, North Carolina);
- coolers (projects in India and Morocco have made efficacious use of clinker coolers for waste heat recovery);
- hot liquids;
- hot sides of water cooling loops/jacket water.
GOOD PROJECT QUALITIES
Projects with the highest chance of succeeding tend to share some similar characteristics. Not all these attributes are essential for an installation to be successful and each project is evaluated individually. But these features contribute to the success of a project and attention should be directed to those projects that have the characteristics to give them the highest likelihood of success.
These project attributes are:
- high capacity factor/runtime of the heat source;
- large flow of useful energy from the heat source;
- high temperature of useful energy (recognizing that ‘high’ is only 230°C).
Today’s ORC packages are custom-assembled in climate-controlled factories and are shipped to customer sites on flatbed trucks. These modular systems can be stacked or arranged to suit customer space constraints and on-site installation times are as short as four or five weeks.
Importantly, the general construction of these projects requires no customer downtime – the heat source needs to be taken out of service only at the time of interconnection to the customer’s primary process. This is typically co-ordinated with routine or scheduled maintenance activities to eliminate disruption.
A BURGEONING OPPORTUNITY
As ORC project implementation accelerates, companies at the forefront of this trend are enjoying lower energy costs (the recent Weyerhaeuser project is saving a ‘double-digit percentage’ on the energy costs the ORC displaces) along with increased competitive positioning and myriad public relations benefits.
The adoption of an ORC system by one firm is, in some instances, spurring other players in the industry to similarly implement ORC projects – or to risk falling behind their peers with respect to cost competitiveness and energy efficiency.
Further, to help encourage ORC adoption, project developers have begun to offer ‘build-own-operate’ models that relieve the end-user of the capital expenditure burden associated with implementing a system. This business model instead offers customers the ability to save on their energy costs by locking in below-market power prices under long-term power purchase agreements.
The full responsibility for evaluating, designing, procuring, constructing, owning, and operating the facility remains with the developer, and the end customer is left only to enjoy the benefits of inexpensive electricity sourced from waste heat.
With carbon legislation looming, industrial companies are realizing this green solution may be a well-considered hedge and are acting quickly to implement these solutions throughout their portfolios.
Jason Gold is the Chief Executive Officer of KGRA Energy, New York and Chicago, US. Email: firstname.lastname@example.org