Optimising heat recovery from industrial processes with heat pumps

Jan. 16, 2015 | An article from Kenneth Hoffmann, GEA

Waste heat recovery is the most efficient way of using heat pump technology. Many processes in the food and industry are producing large amount of waste heat at the same time as they have large heating demand. By taking the waste heat and boosting the temperature to a useful level a high performance and short payback can be achieved.

GEA have developed a range of high efficient ammonia heat pump for applications between 200 kW – 15,000 kW and heating temperature up to 90⁰C. In this document is presented 3 different applications where our heat pump has been used.

1.     FLUE GAS COOLING

Heat recovery from any fuel combustion process is an excellent technique for lowering global carbon dioxide emissions. Today most of the World’s electricity production is generated without recovering heat from flue gas. Efficient electricity production can achieve efficiencies of 40 to 50 % – but if the heat is recovered, it is possible to obtain energy efficiencies close to 100 %.

When burning fuel with low moisture content such as gas, oil, and coal, most of the heat (over 90 %) can be recovered by direct heat exchange with a hot water circuit. With wet fuels such as biomass and waste, only 80 % of heat recovery from the combustion process is possible by direct heating of water. The final 20 % of heat recovery can be achieved by a wet scrubber system that condenses the flue gases and cools them down to less than 30°C. A wet scrubber system also reduces the toxic emissions from fuel combustion – whether from waste, biomass, gas, or oil. Future restriction on emissions from the EU could make wet scrubber systems a requirement on most mass boiler systems. If a wet scrubber system is installed, the condensate must be cooled – by cooling towers, seawater, absorption heat pumps, or mechanical heat pumps. Cooling towers and seawater cooling would waste the heat, but this technique is commonly used where there is only electricity production without heating demand.

A heating system with a combination of CHP and heat pumps provides the opportunity of taking advantage of changes in fuel and electricity prices. If electricity prices rise, operators can utilize the heat pump less and sell more electricity. If electricity prices fall, operators can use the heat pump more and purchase electricity cheaply. If fuel prices rise, they can use the heat pump more, and if fuel price fall, they use the heat pump less. It is most effective to

install an algorithm that utilizes the CHP and heat pump most efficiently, in accordance with the cost of fuel and electricity. Industrial heat pumps designed specifically for these markets can be controlled with this level of sophistication. All CHP driven by the heat demand can benefit of an electrical driven heat pump to condense the flue gasses.

In February of 2013, a 7200 kW heat pump for a waste incineration plant in Stockholm was installed as the preferred technology. This project is based on cooling flue gas from 50 °C to 25°C by using a single high-efficiency electrical heat pump. The installed systems heat the return heating water from the district heating network from 60°C to 65°C, at a heating COP exceeding 6.50 reducing the CO2 emmisions with 14%.

2.     ADD-ON HEAT PUMP

We often experience that the customer wants to install a heat pump into the existing heating system without the necessary assessment of what temperatures are required. An existing boiler system is likely to be set up to supply water at 95°C to pasteurize milk at 80°C, or to produce steam at 130°C to provide sanitary water at 60°C. Such a configuration is typical because it keeps infrastructure costs down and exerts slight influence on the efficiency of the boiler. These high temperatures make the heat pump uneconomical. If the process heat exchangers can be enlarged in surface area and if the water temperature is lowered, the capital cost increases – but life-cycle costs and carbon savings improve significantly.

Most refrigeration plants are installed with floating head pressure to control the condensing temperature to optimize the energy efficiency. When using the condenser heat as source for the heat pump it is important to take into consideration these changes in condensing temperature. Either design the heat pumps with flexible suction temperature or if possible dedicate some of the refrigeration plant for the supply of heat to the heat pump at constant temperature while the remaining plant still are operating with floating head pressure.

In the UK we installed a 924-kW heat pump in a dairy to recover condenser heat from a refrigeration plant at 24°C, and to heat water for pasteurization from 72°C to 82°C(4), with a COP of 4.9. The benefits include achieving payback after 1.5 years and saving more than 1,000 tons of CO2 per year. Similar to the installation of add-on heat pump for a dairy in the UK, we have experience from similar applications for breweries, chicken abattoirs, leisure centres, food processing, chemical industry and hospitals.

  

3.     WASTE HEAT RECOVERY

In many processes, cooling and heating demands are individually covered without a full production layout of heating and cooling needs. This leads to wasteful energy flows in which hot water is cooled by chilled water to achieve the required temperature. We also see many processes with large heating demand at high temperature (e.g., 80°C for pasteurization) and large cooling demand at low temperature (2°C chilled water) – both of which lead to considerable waste heat at medium temperature (35 - 60°C).

Production regimes that generate waste water at 35 to 60°C often use cooling towers to cool this water before sending it to the sewage system: commonly, legislation limit the maximum temperature (around 30°C) of waste water allowed to enter the sewer. The cost of running these cooling towers can be saved by generating useful heat from the waste heat.

The carbon savings and return of investment on waste heat recovery heat pumps can be very attractive – but this all depends on the efficiency of the heat pump. Below is a graph showing the typical COP of a high-efficiency ammonia heat pump, as a function of the temperature lift it needs to perform.

Figure 2: The heating coefficient of performance (COP) as function of the temperature difference between source and output temperature for high-efficiency ammonia heat pumps

Recovering the waste heat and boosting it to provide useful heating results in good payback, reduction in carbon footprint, and lower costs – with heating COP’s from 4 to more than 14.

Papermills have a large hot water demand, which they use in the processing paper. A papermill in Sweden have seen the potential of cooling their cleaned waste water from 50⁰C to 35⁰C and generating 2.5 MW of usefull heating at 70⁰C. This is possible with a heating COP of 5.8 and as the electricity produced in this area is done mainly by hydroelectric power stations the heating is completely carbon neutral and the customer is saving €650,000 per year.