An energy efficiency intervention

Steam and hot process water

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An energy efficiency intervention carried out by Renovis involves the recovery of exhaust gases exiting a paper drying oven. The innovative intervention developed in Italy, at the Ahlstrom- Munksjö plant in Turin.

In nature, symbiosis is often discussed. Symbiosis between systems, whether organic or else, occurs when one bonds with the other to mutually benefit from the resources provided by each other or the environment. Symbiosis, in this sense, is nature’s way of spontaneously generating efficiency. «This concept is of course actively and consciously applied by humans in the many realities that constitute civil society, in different productive segments and in the industrial sector» explaned at Congresso Aticelca 2023 Alan Carnevali, sales engineer at Renovis Rational Energy Solutions.

Renovis is an energy service company (Esco) which proposes and manufactures solutions for energy efficiency of industrial production processes.

«Even the paper industry presents a variety of opportunities for virtuous symbiosis or integration with other industries or services» he said. «For instance, consider the fact that, since paper is often a recyclable commodity, the paper mill can receive raw material from outside through the separate waste collection service. Consequently, integration between systems is often an opportunity to increase efficiency in many highly energy-intensive industrial processes, and paper mills, absorbing nearly 10 percent of the energy produced globally, represent the ideal environment for transforming such opportunities into truly efficient systems».

An emblematic and representative case of this is the plant set up by Renovis at one of the largest special-purpose paper manufacturers in Italy: Ahlstrom-Munksjö.

The heat recovery system

In Ahlstrom plant in Mathi (Turin) Renovis realized a heat recovery system that exploits the heat contained in the exhausted fumes coming out of the two chimneys of the paper drying oven. The heat recovered from the plant is now used to produce 2-barg process steam (recovered heat power 630 kWt) and preheat a flow of process water (recovered heat power 430 kWt).

«The realized plant consists of two stages in series» said the engineer «which exploit the heat contained in the fumes coming out of two chimneys of a drying oven. The recovered heat is used to produce steam and hot water, both of which are necessary for the paper mill’s production process. Thus, there is constant use throughout the year, resulting in natural gas energy savings and economic savings».

The first stage uses a Recovery Steam Generator (RSG) to produce steam averaging 2 barg. «The RSG has a vacuum tube heat-pipe technology with respect to which the flue gas passes transversely through the lower section of the RSG, while the water intended for evaporation feeds the upper section; the vacuum tubes pass through a diaphragm that separates the upper section to which they transfer the heat recovered from the flue gas into the lower section, thus making it possible for low-enthalpy evaporation of the feed water. The steam produced exits the upper section of the RSG and is fed into the plant manifold, serving the utilities connected to it. The thermal energy recovered in this first stage, obtained by cooling about 34,700 Nm3/h of flue gas from 190 °C to 140 °C, enables the conversion of 1,030 kg/h of feed water at 95 °C to steam at 2 barg, recovering an average of 630 kWt».

The economizer

At the outlet of the RSG, the flue gas passes through an additional heat recovery unit, namely the Economizer, a heat exchanger used to preheat the feedwater entering the Recovery Steam Generator. «This step allows the RSG to increase its output by obtaining warmer feed water.

Circulation of feed water, taken from condensate storage tanks and sent in series to the economizer and RSG, is achieved by a centrifugal pump equipped with an inverter.

Flue gas leaving the RSG is directed to the second heat recovery stage, called RC2, which consists of a flue gas/water heat exchanger and is used to preheat a process water flow rate. Thermal energy is in this case used to heat 21,600 lt/h of water from 74 °C to 91 °C, with subsequent cooling of flue gas from 140 °C to about 105 °C; the heat exchange is countercurrent».

The process water is recirculated between the exchanger and the plant’s tines, where it is stored, by means of an inverter-equipped centrifugal pump.

Results

The system allows to obtained a lot of results. The total natural gas savings from the solution is about 1,181,000 Sm3/year. The plant also allows the generation of Energy Efficiency Certificates (EEEs), proportionally to the fuel savings: «in this case, up to 810 EEEs/year can be obtained, for an economic countervalue that exceeds 200,000.00 euros/year» continues

Carnevali. Lastly, through heat recovery from the furnace’s combustion fumes, the amount of CO2 released into the atmosphere is reduced by about 2,340 tons/year. «The plant built therefore contributes concretely to a considerable reduction in climate-altering gas emissions generated by the Turin production site».

Therefore, the heat recovery system realized in Mathi achieves considerable energy and economic savings as well as significant environmental benefits and reduction of the ecological footprint of plant processes.

The new system’s benefits

The system allows the following results and economic benefits to be obtained.

– Natural gas saving: 1,181,000 Sm3/year and 1,181,000 €/year (1.00 €/Sm3)

– Primary energy saving: 810 Toe/year and 202,500 €/year (250 €/Toe)

– CO2 not released in the atmosphere: 2,340 tons/year and 163,800 €/year (70 €/ton)

– Intervention payback: less than 2 years.