Opportunities for solar thermal process heat integration in South African sugar mills

Beukes, H. T. (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The sugar milling sector is one of the major agro-processing industries in the South African economy. This sector, however, is under pressure to remain profitable under strenuous economic conditions. In order to enhance the competitive advantage of the industry, stakeholders are investigating opportunities to reduce the input costs of raw sugar production as well as alternative income streams, such as the production of bagasse by-products or the cogeneration of electricity. Since the production of raw sugar is characterised by a significant demand for thermal energy, this study has been conducted to identify opportunities for the integration of solar thermal process heat into this process. Potential solar heat integration points have been identified by considering all of the heat sinks and input streams within a generic raw sugar factory. The suitability of each of the integration points have been assessed in terms of the heat demand and expected impact of solar heat integration. Integration opportunities that conserve bagasse and coal or enhance the potential for electricity cogeneration have been prioritised. The sugar production process consists of various processes, such as sugarcane preparation and juice extraction, clarification, evaporation, crystallisation and drying of the raw sugar. Although there are numerous potential solar heat integration points within these processes, only six have been found to be potentially feasible in terms of the abovementioned criteria. The major opportunities for solar process heat integration into the sugar production process have been found to be the parallel production of live and exhaust steam, the drying of bagasse and sugar, the preheating of boiler feed water and, to a lesser extent, the heating of mixed juice. Basic integration concepts have been developed for the abovementioned integration points in order to assess the potential solar gains. Rudimentary energy yield simulations have been used to estimate the expected solar gains of the proposed concepts and the collector fields have been pre-dimensioned according to the mean thermal loads of the processes. According to this preliminary study, solar thermal process heat can potentially supply between 10 and 27 % of the respective processes’ heat demand without thermal storage. According to a basic economic assessment, the levelised cost of heat (LCOH) of the particular integration concepts is expected to be between R 0.43 and R 1.72 /kWh1. Although this study is only a preliminary evaluation of the potential of solar heat integration into the sugar milling industry, it has been shown that there are feasible integration points within the production process and that solar process heat integration can be considered as technically and financially feasible. However, owing to the intricacies of the heat supply and distribution network of a typical sugar factory, detailed studies should be conducted to optimise the integration of solar heat into the industry.

AFRIKAANSE OPSOMMING: Die suikermeulsektor is een van die vernaamste agro-verwerkingsnywerhede in die Suid-Afrikaanse ekonomie. Hierdie sektor is egter onder druk om in die huidige strawwe ekonomiese toestande winsgewend te bly. Ten einde die mededingende voordeel van die bedryf te verbeter, ondersoek belanghebbendes geleenthede om die insetkoste van die ru suiker produksieproses te verlaag, sowel as alternatiewe inkomstestrome, soos die produksie van bagasse-byprodukte of die opwekking van elektrisiteit. Aangesien die verwerking van ru suiker gekenmerk word deur 'n beduidende vraag na termiese energie, is hierdie studie uitgevoer om geleenthede te identifiseer vir die integrasie van sonenergie as 'n vorm van proseshitte. Potensiële integrasiepunte van sonenergie is geïdentifiseer deur alle hitte-verbruikers en insetstrome in 'n generiese ru suiker fabriek in ag te neem. Die geskiktheid van elk van hierdie integrasiepunte is beoordeel met betrekking tot die vraag na hitte en die verwagte impak wat die integrasie op die produksieproses sal hê. Voorkeur is verleen aan integrasiegeleenthede wat moontlik die verbruik van bagasse en steenkool kan verminder, of die potensiaal vir elektrisiteitopwekking verbeter. Die produksieproses van ru suiker bestaan uit verskeie prosesse, soos die voorbereiding van die suikerriet en sap-onttrekking, suiwering, verdamping, kristallisering en droging. Alhoewel daar talle potensiële integrasiepunte vir sonenergie in hierdie prosesse is, blyk dit dat slegs ses hiervan potensieël lewensvatbaar mag wees. Die mees gepaste geleenthede vir die integrasie van sonenergie, is die gelyklopende produksie van hoë druk sowel as uitlaatstoom, die droging van bagasse en suiker, asook die voorverhitting van toevoerwater na die stoomketel en die verhitting van gemengde sap. Basiese integrasie-konsepte is ontwikkel vir die bogenoemde integrasiepunte, om sodoende die potensiële energie-opbrengs te evalueer. Basiese energie-opbrengsimulasies is gebruik om die verwagte jaarlikse opwekking van die voorgestelde konsepte te bepaal, terwyl die versamelaarsvelde gegrond is op die gemiddelde termiese ladings van die prosesse. Die voorlopige ondersoek het getoon dat sonenergie as proseshitte potensieel tussen 10 en 27 % van die hitte-aanvraag in die onderskeie prosesse kan voorsien, sonder termiese stoorkapasiteit. Volgens 'n ekonomiese evaluasie is die gebalanseerde koste van die hitte van die betrokke integrasie-konsepte na verwagting tussen R 0.43 en R 1.72 / kWh2. Alhoewel hierdie studie slegs 'n voorlopige evaluasie van die potensiaal van die integrasie van sonhitte in die suikermeulbedryf is, is dit 'n bewys dat daar lewensvatbare moontlikhede bestaan. Daar behoort egter meer gedetailleerde studies uitgevoer word om die integrasie van sonenergie in die industrie te optimeer

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