Thermal optimization study at Prince George: Pinch analysis identified substantial energy savings

• Published in: Pulp & paper Canada Vol. 104; no. 9; pp. 24 - 26
• Main Authors: DYLKE, E, FOLKESTAD, C, RETSINA, T, SAVVAKIS, I
• Format: Magazine Article
• Language: English
• Published: Don Mills, ON Southam 01.09.2003; Annex Publishing & Printing, Inc
• Subjects: Applied sciences; Energy conservation; Exact sciences and technology; HVAC; Paper, paperboard, non wovens; Polymer industry, paints, wood; Pulp & paper mills; Return on investment; Steam; Wood. Paper. Non wovens; Pinch method; Experimental study; Water vapor; Investment cost; Energy savings; Optimization
• ISSN: 0316-4004; 1923-3515

The hot and cold composite curves and the grand composite curve for the Prince George mill are shown in Figs. 1 and 2. From these curves it can be deduced that the minimum thermal energy required to run the process streams for this mill was 360 GJ/h, for the minimum temperature driving force of [Symbol Not Transcribed] [delta]T[Symbol Not Transcribed] = 10[Symbol Not Transcribed] [degree]C. The existing process thermal consumption was 492 GJ/h. Therefore, there was a scope to reduce process heat demand by up to 132 GJ/h. Table I contains a summary of these results. The process pinch interval temperature identified in the composite curve was 73[Symbol Not Transcribed] [degree]C (68[Symbol Not Transcribed] [degree]C for cold streams and 78[Symbol Not Transcribed] [degree]C for hot streams). The critical pinch stream was the No. 2 evaporator condenser. Bringing cold stream up to 68[Symbol Not Transcribed] [degree]C using the condenser's heat can maximize heat-recovery. Ideally, there should be no cold water used above 73[Symbol Not Transcribed] [degree]C, no steam below 73[Symbol Not Transcribed] [degree]C and no heat transfer across this pinch temperature. In order to determine the true dollar savings, a realistic approach must be taken regarding the cost of steam. The true value of marginal steam is derived from calculating the cost of fuel needed to generate this steam and subtracting from it applicable credits for desuperheating. The calculation also takes into account the utility system needs and costs, i.e., demineralized water, deaerator steam and blowdown. Furthermore, the cost of steam differs depending on whether the steam is used indirectly (condensate is returned), or injected (condensate is not returned), and also on which fuel is used to generate the steam.