The hot streams of the steel plant sites, representing an energy quantity of 1’297 GWh/a, are summarised in Tab. 4.1. The selection of the process stream types and the characterisation of their physical properties are based on Tarres-Font [213], who assessed the site of Differdange in detail. The author mentioned that waste heat recovery from the Electric Arc Furnace (EAF) off-gases after post-combustion and from the water jacket is technically difficult due to the dust separation system limiting the formation of dioxin but also constraining the temperature levels of the hot streams. These heat sources are nevertheless considered here, as they present extremely high load and temperature levels, thus potentially generating high enough revenues to justify a modification of the processes. The inlet temperature levels were set to a maximum of 400°C. An initial cooling end temperature of 40°C is considered, to which a dTmin of 10K is applied to obtain a final system temperature of 50°C. The loads of the site of Belval were proportionally adapted as a function of the production mass flow estimated from nominal data of the Integrated Pollution and Prevention Control (IPPC) authorisation, considering a
24 hours, 6 days/week production schedule and a correction factor of 75% to reflect reduced production activity. All sources are assumed to be located at the same place in the plants.
Table 4.1: Heat source data
Site Stream Load [kW] Temp.Tin
[°C]
Temp. Tout[°C]
Steel_ Differdange
EAF off gases after post combustion
43’750 400 50
EAF off gases end of water jacket 40’625 400 50 EAF off gases end after
quenching
7’500 400 50
WBF 1 off gases 3’250 400 50
WBF 2 off gases 4’125 400 50
Steel_Belval
EAF off gases after post combustion
24’306 400 50
EAF off gases end of water jacket 13’542 400 50 EAF off gases end after
quenching
4’167 400 50
WBF 1 off gases 1’806 400 50
WBF 2 off gases 2’292 400 50
The main thermal streams of the three industrial sink sites, totaling 74 GWh/a of energy demand, are described in Tab. 4.2. According to the heat cascade approach, the temperatures were corrected by a temperature difference of 10K to consider the temperature difference from the heat exchanger, except for the streams of the non-ferrous manufacturing plant, which are set to 400°C. The temperatures and specific heat demands are taken from Stadler [198], based on data from Brown et al. [32], Peinado et al. [164] for the asphalt industry, Vlachopoulos and Wagner [232], Wen [238] for the plastic manufacturing industry and Zawrah [243], NIST [149] for the hard metal manufacturing plants. The mass flow data is based on information from the IPPC authorisation of the asphalt plant and estimated as 1’000kg/h and 5’000 kg/h for the two other industries, considering a 12 hours, 5 days/week production schedule.
The residential heating demand data (Tab. 4.3) is generated using the regression analysis information of Bertrand et al. [19], combined with georeferenced data indicating residential building type and location (Entreprise des Postes et Télécommunications [56], Service des travaux municipaux [194]) as well as averaged building age, inhabitant and household number data from the 2011 population census (Statec [199]). The total energy demand amounts to 455 GWh/a. The temperature levels are those of the district heating systems of the city of Luxembourg (70/95°C), to which a correction temperature difference of 10K is added. Non- residential buildings are not included, as the necessary data is not available.
4.3. Case study
Table 4.2: Industrial heat sink data
Site Stream Load [kW] Temp.Tin
[°C] Temp. Tout[°C] Asphalt manufacturing Aggregate heating 5’079 30.0 185.0 Evaporation 2’521 109.9 110.1 Plastic manufacturing Heating 65 30.0 174.9 Melting 65 174.9 175.1 Superheating 26 175.1 220.0 Hard metal manufacturing
Oxide reduction heating 417 30.0 400.0
Carbonisation heating 102 30.0 400.0
Table 4.3: Urban heat sink data
Site Stream Load [kW],
t1/t2&t3/t4&t5
Temp.Tin
[°C]
Temp. Tout[°C]
Belvaux Space heating and DHW
23’380 / 7’329 / 503 80 105
Differdange Space heating and DHW
34’499 / 11’006 / 1’015 80 105
Ehlerange Space heating and DHW
3’850 / 1’196 / 67 80 105
Esch-sur- Alzette
Space heating and DHW
81’981 / 26’333 / 2’714 80 105
Mondercange Space heating and DHW
15’072 / 4’688 / 272 80 105
Niederkorn Space heating and DHW
19’023 / 6’022 / 493 80 105
Oberkorn Space heating and DHW
12’734 / 4’014 / 305 80 105
Sanem Space heating and
DHW
11’203 / 3’487 / 206 80 105
Soleuvre Space heating and DHW
21’456 / 6’703 / 430 80 105
The CHP units are composed of an internal combustion engine and a peak boiler. The ICE load corresponds to the sink load at period t2and t3, while the boiler load make up the remaining
power demand at t1. With a thermal efficiency of 47% and an electrical efficiency of 39%
Eq. 4.6 is set to 26% fort1and 83% for the other periods, thus reflecting the use of the ICE as
base-load unit producing electricity, while the remaining heating demand is covered by the natural gas boiler. The efficiency of the electricity production turbine on the steel plant sites is considered to reach 24% (Tarres-Font [213]). The boiler is attributed an efficiency of 90%. The efficiency of the heat pumps are calculated considering a Carnot efficiency of 55% (Becker et al. [13]), an average outdoor temperature of -1°C and the uncorrected process temperatures indicated in Tab. 4.2 and Tab. 4.3.
The pipe internal and external diameters, maximum mass flows per size and heat transfer rate ki(0.0255 W/m*K) are taken from the design manual of a pipe manufacturer (Isoplus [105]
- Appendix B.2). To cover the full range of temperature intervals of this case study, the heat transfer fluid considered is Dowtherm A thermal oil, which can be used at temperatures as low as 12°C and up to 425°C which makes it an acceptable fluid for this study. The heat capacity data, given in Appendix B.3, is taken from Dow Chemical Company [50].
The expected lifetime of heating utilities and heat exchangers are assumed to be 15 years and 30 years for the pipes.