Heat recovery solutions for mine ventilation systems

Recently, Boliden Mineral AB acquired the Finnish copper mine Kylylahti. In connection with that, potential improvements, for instance in the ventilation system, is investigated. The supply air going into the mine must be heated during the colder months of the year to prevent icing that can damage the supply air shaft and adjacent structures. Currently, the heating is done with LPG, but there is a lot of available energy to possibly make use of since the return air is saturated. A heat exchanger could possibly decrease the operating cost, the carbon dioxide emissions and the noise that spreads over the area and disturbs the neighbours. Recently, the mine Zinkgruvan made a very successful investment in a plate heat exchanger, and a Canadian study shows that also battery heat exchangers can be a good alternative. In this work, both kinds of heat exchangers were evaluated. Calculations were made for the total number of hours spanning between February 2014 and January 2015, which were later normal year corrected. Tabulated values and given data, including airflows and ambient air temperature, were used to calculate enthalpy in the airflows. Available energy, energy demand and transferable energy with heat exchanging was calculated through changes in enthalpy. To calculate the payback period, the investment cost for the two systems were estimated and the change in operating cost were calculated. The change in carbon dioxide emissions was also calculated. All calculations were made for eight different cases, where factors such as required supply air temperature, return air temperature and LPG-price was varied within reasonable ranges. The investment cost for the battery heat exchanger system is approximately 500 000-700 000 euros and for the plate heat exchanger system approximately 1-1,1 million euros. Both systems reduce the LPGdemand, but the electricity demand is increased since the fans have to overcome larger pressure drops. Both systems have a lower operating cost than the current system in all tested cases, and in most cases the plate heat exchanger system has the lowest. For the plate heat exchanger system, the shortest possible payback period is 3,2 years, and the longest possible is over 72 years. With the current values the payback period is 12,5 years. For the battery heat exchanger system, the shortest possible payback period is 1,9 years, the longest possible is 9,4 years and 6,3 years with current values. But in the future, the LPG-price will probably increase and the payback period then becomes 6,4 years for the plate heat exchanger system and 3,4 years for the battery heat exchanger system. The plate heat exchanger is more efficient than the battery heat exchanger, but it does not have the same short payback period since the investment cost for a plate heat exchanger system is significantly higher. Compared to the current heating method, both the evaluated systems reduce the carbon dioxide emissions. With a margin electricity perspective, the reduction is low, only 10-13% of the current emissions. With the method Boliden uses for similar assessments the reduction is greater and the plate heat exchanger system gives the greatest reduction, around 60% in comparison to 55% for the battery heat exchanger system. With other possible improvements, the conditions for a heat exchanger system may change, very likely so that the payback period becomes longer. The payback period must be compared to the expected life of the mine, which currently is around 6 years. But before a decision can be made, an estimation of how much a heat exchanger can reduce the noise level must be made. Sammanfattning Nyligen kopte Boliden Mineral AB den finska koppargruvan Kylylahti och i samband med det undersoks potentiella forbattringsmojligheter, bland annat i ventilationssystemet. Tilluften som gar ned i gruvan maste under vinterhalvaret varmas upp for att forhindra isbildning som kan skada tilluftsschaktet och angransande strukturer. For nuvarande gors uppvarmningen med gasol men det finns mycket energi att ta tillvara pa eftersom franluften ar mattad. En varmevaxlare skulle forhoppningsvis kunna sanka driftkostnaden, koldioxidutslappen och aven minska bullret som i nulaget sprids over omradet och stor boende. Nyligen gjorde gruvan Zinkgruvan en mycket lyckad investering i en plattvarmevaxlare och en kanadensisk studie som gjorts visar att aven batterivarmevaxlare kan vara ett bra alternativ. I detta arbete gjordes berakningar pa bada. Berakningar har gjorts for alla timmar under perioden februari 2014 till januari 2015, vilket sedan normalarskorrigerades. Det anvandes tabellvarden och givna data for bland annat luftfloden och utomhustemperatur for att berakna entalpier i luftflodena. Med hjalp av entalpierna har sedan tillganglig energi, energibehov och vad som kan overforas med varmevaxlare beraknats. Investeringskostnad for de tva systemen uppskattades och andring i driftkostnad beraknades for att slutligen kunna berakna aterbetalningstid. Aven andringen i koldioxidutslapp har beraknats. Berakningarna har gjorts for atta olika fall, dar faktorer som bland annat uppvarmningstemperatur, franluftstempertur och gasolpris varierats inom rimliga intervall. Investeringskostnaden for batterivarmevaxlarsystemet ar ungefar 500 000-700 000 € och for plattvarmevaxlarsystemet ungefar 1 000 000-1 100 000 €. Bada varmevaxlarsystemen minskar behovet av gasol, men istallet okar elbehovet eftersom flaktarna maste jobba mot ett hogre tryckfall. Bada systemen har i alla testade fall lagre driftkostnad an nuvarande system. I de flesta fall har plattvarmevaxlarsystemet lagre driftkostnad an batterivarmevaxlarsystemet. For plattvarmevaxlaren ar den kortast mojliga aterbetalningstiden 3,2 ar och i varsta fall ar den over 72 ar. Med de varden som galler just nu ar aterbetalningstiden 12,5 ar. For batterivarmevaxlaren ar den kortast mojliga aterbetalningstiden 1,9 ar, i varsta fall ar den 9,4 ar och med de varden som galler just nu 6,3 ar. Men i framtiden vantas gasolpriset stiga och aterbetalningstiden blir da istallet 6,4 ar for plattvarmevaxlarsystemet och 3,4 ar for batterivarmevaxlarsystemet. Plattvarmevaxlaren ar effektivare an batterivarmevaxlaren, men far aldrig lika kort aterbetalningstid eftersom ett plattvarmevaxlarsystem ar mycket dyrare att investera i. Bada losningarna sanker koldioxidutslappen. Med ett marginalelsperspektiv ar sankningen ganska lag, bara 10-13% av de nuvarande utslappen. Med den metod Boliden anvander for liknande bedomningar ar sankningen storre och plattvarmevaxlaren ger storst sankning, cirka 60%, jamfort med 55% for batterivarmevaxlaren. Med andra mojliga forbattringar kan forutsattningarna for ett varmevaxlarsystem forandras, mycket troligt sa att aterbetalningstiden blir langre. Aterbetalningstiden maste jamforas med gruvans forvantade livslangd, som for nuvarande ar cirka 6 ar. Men innan beslut maste aven uppskattning av hur en varmevaxlarlosning kan sanka bullernivan goras vilket ligger utanfor detta arbete.

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