Exergy, techno-economic and exergoeconomic analyses for improving energy efficiency of a typical sugar mill and designing integrated biorefineries

Dogbe, Eunice Sefakor (2020-03)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The sugar industry is energy-intensive, consuming about 350–600 kg of steam and 25–32 kWh electricity per ton of sugarcane processed into raw crystalline sugar. Though mostly energy-self-sufficient, improving its energy efficiency is necessary to produce sugar more cost-effectively. Besides, the decreasing trend and fluctuations in the world sugar prices necessitate product diversity to ensure the economic sustainability of the industry. Therefore, improving the current energy status of the sugar industry will make sugarcane resources available for further valorisation. This study aimed to improve the energy efficiency of a typical South African sugar mill towards its economic sustainability and competitiveness. The first objective (objective 1) towards achieving this aim was to identify the locations, magnitudes, and causes of inefficiencies in a typical South African sugar mill through exergy analysis. This analysis was based on rigorous mass and energy balances calculated from an Aspen Plus® simulation of a typical 250 ton per hour sugar mill. The cogeneration system had the highest exergy destruction (90 582 kW) representing 86% of the total sugar mill irreversibility. However, with the lowest exergetic efficiency of 9.6%, the crystallization unit recorded the most inefficient use of energy due to the process complexity. Following the exergy results, objective 2 was to select energy-efficient technologies to improve the sugar mill efficiency while objective 3 involved assessing the economic feasibility of integrating them into the mill. Organic Rankine cycle (ORC) and absorption heat pump (AHP) technologies were selected, which improved the cogeneration exergetic efficiency by 1.7% and minimized overall system irreversibility by 0.14%, saving 0.83% on total bagasse for valorisation, respectively. Though only marginal improvements were achieved, both ORC and AHP integrations were economically feasible and could be optimized to achieve better energy improvements. Furthermore, promising biorefinery products; succinic acid (SA) and short-chain fructooligosaccharides (scFOS) were integrated for the economic competitiveness of the industry in objective 4. Based on the exergy results, the biorefineries were developed to utilise A-molasses for the production of SA and scFOS in seven different scenarios, which were allhighly profitable with internal rates of return (IRRs) between 24 and 62% compared to theminimum required IRR of 9.7%, due to the integration benefits. Moreover, co-utilization of C-molasses and lignocellulose residue as first- and second-generation (1G-2G) feedstocks for the production of SA was also considered to fully valorise the sugarcane plant considering the current crystallization scheme. Objective 5 applied an aggregated system exergoeconomic methodology to assess the holistic performance of the biorefineries and to identify the most cost-effective one. With the lowest cost rate of 1 029 US$/h and exergoeconomic factor of 0.56, the scFOS powder scenario (S-FP) showed a good balance between the irreversibility- and investment-related costs and was considered the most cost-effective biorefinery for integration into the sugar mill. Overall, this study presented a broad spectrum of solutions to the energy and economic challenges of the sugar industry to be explored further for implementation, using exergy/exergoeconomic methodology as better design tools than conventional energy and economic analysis.

AFRIKAANSE OPSOMMING: Die suikerindustrie is energie-intensief, met die verbruik van omtrent 350–600 kg stoom en 25–32 kWh elektrisiteit per ton suikerriet geprosesseer in rou kristalvormige suiker. Al is dit meestal energieselfonderhoudend, is dit nodig om die energiedoeltreffendheid te verbeter om suiker meer koste-effektief te produseer. Buitendien, die afnemende tendens en fluktuasies in die wêreld se suikerpryse maak produkdiversiteit noodsaaklik om die ekonomiese volhoubaarheid van die industrie te verseker. Daarom sal die verbetering van die huidige energiestatus van die suikerindustrie suikerrietbronne beskikbaar maak vir verdere valorisasie. Hierdie studie beoog om die energiedoeltreffendheid van ’n tipiese Suid-Afrikaanse suikermeule te verbeter na ekonomiese volhoubaarheid en mededingendheid. Die eerste doelwit (doelwit 1) om hierdie mikpunt te bereik, was om die ligging, groottes, en oorsake van ondoeltreffendhede in ’n tipiese Suid-Afrikaanse suikermeule te identifiseer deur eksergie-analise. Hierdie analise is gebaseer op streng massa- en energiebalanse bereken uit ’n Aspen Plus®-simulasie van ’n tipiese 250 ton per uur suikermeule. Die kogenerasiestelsel het die hoogste eksergie verwoesting (90 582 kW) gehad, wat 86% van die totale suikermeule onomkeerbaarheid verteenwoordig. Met die laagste eksergieke doeltreffendheid van 9.6%, het die kristallisasie-eenheid egter die mees ondoeltreffende gebruik van energie aangeteken as gevolg van die proseskompleksiteit. Na afleiding van die eksergie resultate, was doelwit 2 om energiedoeltreffende tegnologieë te kies om die suikermeuldoeltreffendheid te verbeter, terwyl doelwit 3 die assessering van die ekonomiese uitvoerbaarheid van die integrasie daarvan in die meule ingehou het. Organiese Rankine siklus (ORC) en absorpsie verhittingspomp (AHP) tegnologieë is gekies, wat die kogenerasie eksergiese doeltreffendheid met 1.7% verbeter het en die algehele stelsel onomkeerbaarheid met 0.14% geminimeer het, wat 0.83% totale bagasse vir valorisasie spaar, onderskeidelik. Al is slegs marginale verbetering bereik, is beide ORC- en AHP-integrasie ekonomies uitvoerbaar en kan geoptimeer word om na energieverbeteringe te lei. Verder is belowende bioraffineerderyprodukte suksiensuur (SA) en kortketting-fruktooliggosakkariedes (scFOS) geïntegreer vir die ekonomiese mededingendheid van die industrie in doelwit 4. Gebaseer op die eksergie resultate, is die bioraffineerderye ontwikkel om A-molasse te gebruik vir die produksie van SA en scFOS in sewe verskillende scenario’s, waarvan almal hoogs winsgewend was met interne opbrengskoerse (iok) van tussen 24 en 62% in vergelyking met die minimun vereiste iok van 9.7% as gevolg van die integrasie voordele. Verder, kogebruik van C-molasse en lignoselluloseresidu as eerste- en tweede-generasie (1G-2G) voermateriaal vir die produksie van SA, is ook oorweeg om die suikerrietplant ten volle te valoriseer met die inagneming van die huidige kristallisasieskema. Doelwit 5 het ’n versamelde stelsel eksergie-ekonomiese metodologie toegepas om die holistiese doeltreffendheid van die bioraffineerderye te assesseer en om die mees koste-effektiewe een te identifiseer. Met die laagste koers van 1 029 US$/h en eksergie-ekonomiese faktor van 0.56, het die scFOS-poeier scenario (S-FP) ’n goeie balans tussen die onomkeerbaarheid- en belegging-verwante kostes en is oorweeg as die mees koste-effektiewe bioraffineerderye vir integrasie in die suikermeule. Oor die algemeen het hierdie studie ’n wye spektrum oplossings vir die energie en ekonomiese uitdagings van die suikerindustrie getoon om verder ondersoek te word vir implementasie, deur eksergie/eksergie-ekonomiese metodologie as beter ontwerphulpmiddels te gebruik eerder as konvensionele energie en ekonomiese analise.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107952
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