OBSERVATIONS OF THE HARVESTING, TRANSPORTING AND TRIAL CRUSHING OF SWEET SORGHUM IN A SUGAR MILL.

Transcription

1 OBSERVATIONS OF THE HARVESTING, TRANSPORTING AND TRIAL CRUSHING OF SWEET SORGHUM IN A SUGAR MILL By A.J. WEBSTER 1, C.P. HOARE 2, R.F. SUTHERLAND 3 and B.A. KEATING 4 1 CSIRO Sustainable Ecosystems, Mossman, Qld 2 Mossman Central Mill Co Ltd, Mossman, Qld 3 Rondis Global Services, Boyne Island, Qld (Formerly Mossman Central Mill Co Ltd) 4 CSIRO Sustainable Ecosystems, St Lucia, Qld tony.webster@csiro.au KEYWORDS: Sweet Sorghum, Milling, Sugarcane, Extraction. Abstract SWEET sorghum has previously been evaluated as having the potential to contribute fermentable sugars to a bioenergy industry producing ethanol. Sweet sorghum grown and processed complementarily with sugarcane can potentially increase efficiency of production by utilising existing infrastructure to harvest, transport and extract fermentable sweet sorghum sugars. This paper reports observations of a trial of sweet sorghum harvest, transport and crushing at Mossman Central Mill at the conclusion of the 22 sugarcane crushing season. A 15 hectare sweet sorghum plant crop was harvested with a cane harvester in two separate batches: 1. with the harvester s extractor fans operating; and 2. with the harvester s extractor fans turned off. Each batch was transported and crushed with existing sugarcane infrastructure. The first ratoon of sweet sorghum was harvested with a forage harvester. The bulk density of forage harvested sweet sorghum is approximately 4 kg/m 3 compared with approximately 3 kg/m 3 for sugarcane and 2 kg/m 3 for sweet sorghum harvested with a cane harvester. The higher bulk density material is cheaper to transport, placing limits on the geographical area in which cane harvested sweet sorghum could be sourced. A deterioration analysis showed a slight reduction in juice extraction percentage and no reduction in brix of extracted juice of cane harvested sweet sorghum. declined rapidly in forage harvested sweet sorghum over the first 42 hours after harvest. Juice extraction percentage exhibited a similar trend to cane-harvested sweet sorghum. There are significant cut-to-crush limitations of forage harvested sweet sorghum due to the rapid decline in brix of extracted juice. Comparisons of both batches of sweet sorghum were made with sugarcane crushed at the same mill settings for moisture, brix and fibre of fibrated material prior to entering No. 1 mill and after the final mill. extraction percentage was lower for both batches of sweet sorghum than sugarcane. Fibre levels of sweet sorghum are higher and final moisture for sweet sorghum is comparable with sugarcane. Introduction The potential of sweet sorghum (Sorghum bicolor (L.) Moench) to provide fermentable sugars for energy production has been widely studied (Mamma et al., 1996; Türe et al., 1997;

2 Woods, 21). Sweet sorghum, a C 4 type crop, has been shown to be an efficient provider of fermentable sugars, exhibiting favourable radiation use efficiency (Curt et al., 1998), water use efficiency (Curt et al., 1995), and energy use efficiency (Monti and Venturi, 23) when compared with other crops. Ferraris (1988), evaluating sweet sorghum in Australia, concluded the high radiation, temperature and available water of north Queensland cane growing regions would be highly suitable to growing sweet sorghum. The idea of integrating sweet sorghum with sugarcane to produce bioenergy, particularly ethanol, has gained momentum over recent years (Ferraris, 1988; Woods, 2; Keating et al., 22; Sutherland, 22). This can be achieved through construction of an ethanol distillery adjacent to the sugar mill and using molasses supplemented with sweet sorghum fermentables as feedstock. It has been supposed the processing of sweet sorghum could utilise existing sugar mill equipment with potential advantages in increased efficiency of production, land, personnel and other resources by lengthening the crushing season (Woods, 2). Ferraris (1988) attempted to describe some potential cropping systems that could integrate sweet sorghum and sugarcane, concluding sweet sorghum would either complement sugarcane, being processed during traditional out-of-season times, or be supplemented for sugarcane. The ideal scenario is to process sweet sorghum either immediately before or after the sugarcane crushing season, avoiding major maintenance works and reducing possible disruption to the sugarcane crush. At north Queensland sugar mills, this equates to sweet sorghum needing to be available for processing from mid November, or during May and early June. This influences the geographic areas where sweet sorghum could logically be grown. Harvesting sweet sorghum with a conventional sugarcane harvester has been proposed (Ferraris, 1988: Woods, 2) and assumed to be feasible. The authors harvested a small area of sweet sorghum with a sugarcane harvester in 21 with no apparent problems. How well a conventional sugarcane harvester can separate leaves and trash from sweet sorghum stem billets will influence the bulk density of the harvested material. A low bulk density has transport cost implications, due to true cartage charges being on a per bin basis rather than a per tonne basis. Keating et al. (22) made an assumption that sweet sorghum has the same bulk density as sugarcane when constructing financial feasibility statements for sweet sorghum fermentables supplied to an ethanol plant. They found transport distances over 5 km made sweet sorghum feedstock unlikely to be viable at current production costs. The only reference to milling sweet sorghum in a sugarcane mill has been made by Woods (2) who witnessed a small sample of sweet sorghum processed through Triangle Sugar Mill in Zimbabwe in The only observation made from this trial was that sweet sorghum stems were physically capable of being processed. The purpose of this paper is to report on observations made during the trial harvesting, transportation and crushing of sweet sorghum at the Mossman Central Mill at the conclusion of the 22 sugarcane crushing season. The authors are not attempting to detail agronomic performance of sweet sorghum or to make any assumptions of the viability or potential yields of using sweet sorghum as a feedstock for ethanol production.

3 Methods On August 5, 22, approximately 15 hectares of sweet sorghum cv Sugargraze was planted at Biboohra (S16 54 E145 24, 39 m above sea level) for the specific intention of providing sufficient feedstock to conduct a trial crushing of sweet sorghum at Mossman Central Mill. On November 12, 22, the final day of crushing at Mossman Central Mill for 22, 475 tonnes of sweet sorghum was harvested and transported to Mossman for crushing the following day. The sweet sorghum crop was ratooned and the first ratoon harvested on February, 23. The crop was harvested with a forage harvester and made into silage, with samples collected for laboratory juice extraction and analysis. Harvesting and transport The sweet sorghum was planted on beds with 1.6 m centres to allow harvesting with a conventional sugarcane harvester. Haulouts delivered harvested sweet sorghum billets from the harvester to conventional Mossman Mill cane bins, which were transported by road to the Mossman Mill rail siding at Cassowary. The bins then travelled by rail to Mossman Central Mill where they were weighed prior to entering the crushing mill. The sweet sorghum was harvested in two separate batches, one batch using the harvester s extractor fans to expel leaf and trash (fans on), and one batch with the extractor fans turned off, with all of the leaf and trash being transported with the sweet sorghum stem for crushing (fans off). The nett weight of sweet sorghum in each bin was recorded at the Mossman Central Mill weighbridge and an average bin weight for both fans on and fans off harvesting determined. These average bin weights are compared with the average bin weights of sugarcane harvested by the same harvester operator during 22. The first ratoon of the sweet sorghum crop reached maturity on February, 23, and was subsequently harvested for silage using a forage harvester. To compare the forage harvester with a cane harvester, the bulk density (kilograms per cubic metre) of the sweet sorghum was estimated, and compared with an estimated bulk density of sugarcane. Bulk density was estimated by measuring the weight of sweet sorghum that packed into a haulout bin. The typical weight that sugarcane packed into the same haulout bin when harvested with a cane harvester was used for comparison. Deterioration analysis A determination of how the quality of sweet sorghum juice is affected by time between harvest and juice extraction was made. The two parameters measured were juice extraction as a percentage of fresh weight and brix of the extracted juice. At harvest on the November 12, 22, nine sweet sorghum samples of approximately 5 kg each were sampled from both the fans on and fans off batches. Each sample was stored in the shade at Mossman Central Mill in a large open plastic bag. Seven similar samples of forage harvested sweet sorghum were collected at harvest of the first ratoon on February, 23. At intervals extending to 78 hours after harvest, the samples were individually processed and juice extraction percentage and brix determined. Juice was extracted by fibrating the entire 5 kg

4 sample in a Jeffco cutter/grinder and placing 5 ±.5 g of fibrated material into a steel cylinder cage and extracting with a Carver Press at a hydraulic pressure of kg for 6 seconds as described by Muchow et al. (1993). Juice extraction is determined by weighing the plug left after carver pressing with the difference from the fresh weight initially placed in the cage equalling extracted juice. Juice extracted is expressed as juice percentage of original carver pressed fresh weight. The juice is collected, allowed to stand for ten minutes and brix determined with a hydrometer and corrected for temperature. The fans on and fans off samples were analysed at hours 3, 8, 11.5, 19, 24.5, 3.5, 43, 52 and 7 after harvest. The forage harvested samples were analysed at hours 2, 19, 29, 42, 54, 68 and 78 after harvest. Milling The sweet sorghum crushing trial took place approximately 13 hours after the last sugarcane had been processed. The mill settings for sweet sorghum crushing were not changed from the settings used for the final sugarcane crushing. No lime, floc, or flashing of the juice took place. Prior to the clarifier, the juice was incubated for 35 minutes at 75 C. The incubators were used for starch reduction and the clarifier acted as a solids settling tank, with no attempt made to clarify the juice prior to concentration in the evaporators. The juice concentration was aborted when juice brix (total soluble solids) reached 6. Hot maceration water was added prior to the final mill. The milling train at Mossman Central Mill consists of six mills. During the sweet sorghum trial crushing, only the first five mills were used, with the sweet sorghum bagasse bypassing the final mill due to maintenance requirements. To examine the performance of the milling trains ability to extract sweet sorghum sugars (measured as brix) the brix extraction for sweet sorghum is compared to the brix extraction of sugarcane. extraction from the first five mills for sugarcane was determined just prior to the sweet sorghum trial with the same mill settings. Mossman Central Mill processed the fans on and fans off batches of sweet sorghum separately. Samples of prepared sweet sorghum (shredded, prior to first mill) and sweet sorghum bagasse (after mill number five) for each batch were analysed for moisture, brix and fibre (BSES, 197). From this a total brix extraction percentage was determined as follows (adapted from BSES, 197): extracted percent brix in sweet sorghum = [1 (F ss x B b )/( F b x B ss )] Eqn 1 Where B ss = percent sweet sorghum B b = percent sweet sorghum bagasse F ss = Fibre percent sweet sorghum F b = Fibre percent sweet sorghum bagasse During the milling trial, samples of first expressed juice and second mill juice were collected during the processing of both fans on and fans off sweet sorghum batches. At Mossman Central Mill, all juice is collected in a buffer tank prior to clarification. Each batch of the trial was not of sufficient quantity to process beyond the milling train in a single run. Consequently, all samples collected beyond the milling train were of a mix of both fans on and fans off batches. Samples of mixed juice (buffer tank), effet supply juice (post clarifier) and concentrated syrup (after

5 evaporation) were sampled and ph, hydrometer brix (temperature corrected) and specific gravity measured. Results and discussion Harvesting and transport The sweet sorghum crop was harvested and transported with a conventional sugarcane harvester and haulout bins, and transported in conventional sugarcane bins by road and rail to Mossman Central Mill. Of the tonnes of sweet sorghum weighed, tonnes were cut as the fans on batch, 7.34 tonnes as the fans off batch and tonnes were cut at varying fan speeds to determine the optimal fan speed for the fans on batch. The average bin weights achieved by the contractor for sweet sorghum and sugarcane harvested during 22 are presented in Table 1. Table 1 Average bin weights of sweet sorghum cut with either fans on (extractor fans operating) or fans off (extractor fans not operating) and average bin weight of sugarcane cut by the same harvesting contractor during 22. Fans on Fans off Sugarcane Total tonnes cut Average bin weight Standard deviation Sweet sorghum stems are generally smaller and individual billets weigh less than sugarcane. Harvesting sweet sorghum with a sugarcane harvester s extractor fans fully on, their usual setting for sugarcane, resulted in many stem billets being extracted with the leaf and trash. The lowest possible setting for the extractor fans was required to prevent loss of sweet sorghum billets. This setting did not remove all of the leaf from the billets, resulting in some leaf remaining in the fans on batch of harvested sweet sorghum. The higher average bin weight of the fans on batch of sweet sorghum compared with fans off illustrates that additional leaf and trash decreases the average bin weight. Table 1 highlights the greatly reduced capacity to bulk pack sweet sorghum compared with sugarcane when harvested with a sugarcane harvester with either the extractor fans operating or not. Lower average bin weight has transport cost implications, with the results presented in Table 1 suggesting fans on and fans off sweet sorghum would cost 159% and 176% of sugarcane to transport respectively. Keating et al. (22) concluded the economic viability of using sweet sorghum as a feedstock for an ethanol plant was highly sensitive to the transport cost of sweet sorghum. At harvesting of the sweet sorghum first ratoon, an estimation of forage harvested bulk density was made. This estimation is compared with the estimated bulk density of sugarcane and sweet sorghum harvested with a cane harvester ( fans on ) in Table 2. Table 2 Estimated bulk density of forage harvested sweet sorghum, sugarcane conventionally harvested and sweet sorghum harvested with a sugarcane harvester. Crop Estimated bulk density (kg/m 3 ) Forage harvested sweet sorghum 4 Cane harvested sugarcane 3 Cane harvested sweet sorghum ( fans on ) 2

6 The bulk density estimations presented in Table 2 are crude estimations. However, they do differ significantly enough to provide an indication that much more forage harvested sweet sorghum could be packed into a single bin than sweet sorghum harvested with a cane harvester. No leaf or trash is removed by a forage harvester. Forage harvested sweet sorghum billets are about 15 2 mm long, compared to a billet length of 2 25 mm from the cane harvester. Forage harvested sweet sorghum would need to be covered during transport to prevent material blowing out due to the very light weight of individual pieces. Harvesting sweet sorghum with a forage harvester is a possible solution to overcoming the transport cost limitations. However, there is a trade off with quality as discussed in the following section. Deterioration analysis Figures 1 3 present the effect of time between harvesting and processing on the quality and quantity of extracted sweet sorghum juice for fans on, fans off and forage harvested sweet sorghum Juice Extraction (%) Juice Extraction Time after harvest (hours) Fig. 1 Juice extraction percentage and brix from time of harvest for fans on harvested sweet sorghum.

7 Juice Extraction (%) Juice Extraction Time after harvest (hours) Fig. 2 Juice extraction percentage and brix from time of harvest for fans off harvested sweet sorghum Juice Extraction (%) Juice Extraction Time after harvest (hours) Fig. 3 Juice extraction percentage and brix from time of harvest for forage harvested sweet sorghum. The initial brix and juice extraction percentage varies in the above three charts, which is inconsequential for the purposes of this study. The reader should consider the relative curves of each chart, reflecting the impact of time on these quality parameters of sweet sorghum. The juice extraction percentage remains mostly constant in each of the three charts, with a slight drop after 24.5 hours in Figures 1 and 2 and after 19 hours in Figure 3. This drop is not

8 substantial and could be an artefact of the method of storage of the samples of sweet sorghum prior to processing. The small volume of material (5 kg) stored has a higher surface area than would be apparent when stored in a cane bin. This surface area is vulnerable to drying out. The relatively constant juice extraction witnessed indicates a cut-to-crush of up to 78 hours would not result in a reduction in sweet sorghum juice extraction. The recorded brix of extracted juice in Figures 1 and 2 does not drop over 7 hours, suggesting no deterioration of the sample over this time allowing a long window for cut-to-crush. Conversely, the brix curve in Figure 3 declines rapidly from the time of initial sampling at 2 hours after harvest and levels off approximately half the initial brix after 68 hours. This swift decline in brix of extracted juice severely limits the cut-to-crush time available for forage harvested sweet sorghum, more than likely negating any transport savings made through being able to achieve higher bin weights. Milling Moisture, brix and fibre of prepared sweet sorghum and sweet sorghum bagasse are calculated for both milled batches of sweet sorghum. Using equation 1 the extraction percentage of brix (brix extracted percent brix in sweet sorghum) is calculated for each batch and presented in Table 3. The results for sweet sorghum are compared with sugarcane crushed under the same milling conditions just prior to the sweet sorghum crushing. Table 3 Fibre, brix, moisture and brix extraction percent of fans on and fans off sweet sorghum milling batches and sugarcane milled under the same crushing conditions. Fans on sweet Fans off sweet sorghum sorghum Sugarcane Prepared No. 5 Mill Prepared No. 5 Mill Prepared No. 5 Mill Moisture Fibre extraction extraction from sweet sorghum is lower than that from sugarcane, with the brix extraction for fans on and fans off sweet sorghum similar. A higher starting fibre negatively affects brix extraction, which is a major factor decreasing the brix extraction percentage of sweet sorghum. Mill settings could be changed in response to the higher fibre of sweet sorghum to increase brix extraction, but were not in this trial. Moisture content of sweet sorghum is consistent with juice extraction percentages reported in Figures 1 and 2. The moisture content of the final bagasse for sweet sorghum was comparable with that for sugarcane when the No. six mill is not used. This moisture content was too high for efficient operation of the boiler, which required the addition of 9.3 tonnes of supplementary oil to maintain adequate pressure during the crushing trial. It is reasonable to suggest the inclusion of the No. six mill would reduce final moisture for sweet sorghum as final moisture for sugarcane with No. six mill operating is in the 49 51% range, sufficient for efficient boiler operation. During the milling trial, samples of juice were analysed at various stages. Samples of first expressed juice and second mill juice were taken from each batch. However, a combination of the

9 fans on and fans off batches was taken of mixed juice, effet supply juice and syrup. Table 4 reports these juice measurements. Table 4 Juice measurements of sweet sorghum first expressed juice (FEJ), second mill juice (SJ), mixed juice (MJ), effet supply juice (ESJ) and syrup. (I = Fans on batch, II = Fans off batch). FEJ I FEJ II SJ I SJ II MJ ESJ Syrup ph Temperature Specific Gravity The brix measurements are a measurement of total soluble solids in extracted juice. Hence, they are higher than the reported values in Table 3 where brix is a calculation of total soluble solids in the whole sweet sorghum supply. Juice from the second mill includes maceration water added as part of the milling process, therefore lowering the brix value through dilution. Mixed juice is a combination of first mill and second mill juice. It was noted during the trial that mixed juice frothed considerably, but this frothing had dissipated by the time juice reached the clarifier. Conclusion This study observed the harvesting, transportation and crushing of sweet sorghum using conventional sugarcane equipment. Harvesting with a cane harvester s extractor fans on fully, their usual setting, resulted in sweet sorghum billets being extracted with the leaf and trash. A fan setting that did not remove sweet sorghum billets also left a proportion of the leaf and trash, resulting in lower bin weights than could have been achieved by billets alone, and considerably lower bin weights than sugarcane. This greatly increases transport costs. These transport cost limitations could be overcome by using a forage harvester. However, there is a trade off in rapid juice deterioration from forage harvested sweet sorghum, reducing the viable cut-to-crush time. extraction from sweet sorghum is lower than that achieved from sugarcane. However, slightly altering the mill settings to account for the higher fibre of sweet sorghum could possibly increase extraction percentage. Moisture content of sweet sorghum bagasse at No 5 mill is comparable with that of sugarcane, but this trial did not use the final mill (No. 6) to determine final bagasse moisture content. Acknowledgements The authors gratefully acknowledge the support of Commonwealth Government funding through the Australian Greenhouse Office s GGAP program for the Douglas Shire Renewable Energy Initiative. The authors are indebted to George Adil, Peter Tibaldi and Oakdare Holdings for growing, harvesting and transporting the sweet sorghum crop respectively. The researchers have received much appreciated support from Brian Hare of Pacific Seeds. The support of Alec Ford of Mossman Agricultural Services and Mossman Central Mill staff, particularly Alan Johnstone, John Quaid and Joe Allan is acknowledged.

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