Solar live steam generation and solar bagasse drying for South African sugar mills.

Krog, Willem (2018-03)

Thesis (MEng)--Stellebosch University, 2018.

Thesis

ENGLISH ABSTRACT: Two solar thermal integration concepts have been identified as promising options for implementation in South African sugar mills as a result of work within the Sugarcane Technology Enabling Programme for Bio-Energy. The two integration options are the drying of bagasse using solar heated air and the generation of live steam using concentrating solar thermal collectors. This study further develops and evaluates the two integration options. Solar integration into the bagasse drying concept will help to save exhaust steam, which can be used to dry bagasse. An evacuated tube air collector field was simulated to assess the impact it could make on the bagasse drying system. It was calculated that 3 140 ton of bagasse or 1020 ton coal can be saved through solar thermal integration. If only exhaust steam is used in the bagasse drying system, bagasse usage can be reduced with 5 %, but if the solar system is integrated it can be decreased with 7.05 %. The System Advisor Model was used to simulate two parabolic trough fields for the live steam generation integration point, one for a normal sugar mill and one for a mill with a back-end refinery. The simulations showed that the solar systems have low capacity factors, ranging from 13 - 14.9 % depending on the mill and time of operation. This was due to the low amount of direct normal irradiance received in Durban, which severely hampers the performance of the solar system. Three different solar live steam configurations were evaluated for the two mills, each of which can save bagasse or coal and/or generate extra electricity. By using the simulation results, it was determined that Configuration 1 can save 2 459.7 ton coal for a normal mill and 3 248 ton coal for a mill with a refinery. Configuration 2 can save 2 241 ton of coal and increase electricity exports with 257 % for a normal mill. For a mill with a refinery 3 072 ton coal can be saved and electricity exports can be increased by 102 %. Configuration 3 can generate the most extra electricity and would enable the mill to increase its electricity exports with 297 % for the normal mill and 111 % for the mill with the refinery. The simulation results were also used in an economic assessment for both of the integration options. The assessment determined that none of the integration options are financially feasible under current conditions. As none of the integration points could achieve a levelised cost of heat lower than that of coal (4.03 Euro-ct/kWh) or an internal rate of return higher than 10 %. There is, however, a possibility that the integration points can become more financially rewarding in the future, as the cost of solar thermal technology is set to reduce significantly over the next 10 years. Furthermore, the possible carbon tax which is to be implemented in South Africa will increase the cost of using coal, making solar energy the cheaper option to supply thermal power.

AFRIKAANSE OPSOMMING: Twee son termiese integrasi konsepte is as belowende opsies vir implementering in Suid-Afrikaanse suikermeulens deur ‘n vorige studie van die Suikerriet Tengologie Instaatstellings Program vir Bio-Energie geïdentifiseer. Die twee integrasie konsepte is die uitdroog van bagasse deur son verhitte lug en die opwekking van hoë drukstoom deur gebruik temaak van gekonsintreerde sonkrag. Hierdie studie ontwikkel die integrasie opsies verder en evalueer die impak wat dit kan maak op ‘n suikermeule. Son termiese energie kan die gebruik van uitlaatstoom in die verdrogings sisteem verminder. ‘n Veld van vakuumbuis sonkollektors is gesimuleer om die potensiële energie opbrengs wat dit kan bied te bepaal, asook die impak wat dit kan maak op die verdrogingssisteem. Dit was bereken dat 3 140.62 ton bagasse of 1020.38 ton steenkool gespaar kan word deur die sonkragsisteem. As net uitlaatstoom gebruik sou word vir die verdrogings proses dan sal die gebruik van bagasse met 5 % kan afneem, maar as son termiese energie ook gebruik word dan kan dit met 7.05 % verminder. Die System Advisor Model is gebruikom twee parabolise trog sisteme te simuleer vir die hoë druk stoom opwekking stelsel, een sisteem vir ‘n gewone suiker meule en een vir ‘n meule met ‘n suikerraffinadery. Die simulasies wys dat die sonkragsisteme lae jaarlikse kapasiteitsfaktore het wat wissel tussen 13 – 14.9 %. Die lae kapasiteitsfaktore in die simulasies is veroorsaak deur die lae jaarlikse direkte normale sonsbestraling in Durban, wat die sonkragsisteme se prestasies ernstig benadeel. Drie verskillende konfigurasies vir die hoë druk stoom opwekking stelsel is evalueer, die konfigurasies kan bagasse of steennkool besparaar en/of ekstra elektrisiteit opwek. Die simulasie resulate was gebruik om te bereken dat Konfigurasie 1, 2 459.7 ton steenkool kan spaar vir die gewone suikermeule en 3 248.5 ton steenkool vir die meule met die raffinadery. Konfigurasie 2 kan 2 241 ton steenkool spaar en die elektrisiteits uitvoere met 257 % vermeerder vir die gewonemeule, terwyl dit 3072 ton steenkool kan spaar en die elektrisiteits uitvoere met 102 % kan vermeerder vir ‘n meule met ‘n raffinadery. Konfigurasie 3 kan die meeste ekstra elektrisiteit opwek, dit kan lei tot ‘n 297% toename in elektrisiteit suitvoere vir ‘n normale meule en ‘n toename van 111 % vir ‘n meule met ‘n raffinadery. Die ekonomiese evaluering toon dat nie een van die integrasiekonsepte finasieël lonend sal wees onder huidgi ekondisies nie. Dit is as gevolg van die feit dat nie een van die twee ‘n laer gebalanseerde koste van hitte kan bied as steenkool nie, wat staan op 4.03 Euro-ct/kWh. Die integrasiekonsepte kan ook nie ‘n interne oprengskoers hoër as 10 % behaal nie. Daar is egter hoop dat die integrasiekonsepte meer finansieel lonend sa lwees in die toekoms as gevolg van die potensiële koste vermindering van sonkragsisteme in die volgende 10 jaar. Suid-Afrika beplan ook om ‘n koolstofbelasting in te stel, wat die gebruik van steenkool beboet en sodoende son termiese energie die goedkoper opsie kan maak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103768
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