Overview of the Cane Crushing and Paper and Pulp Industries and the use of residues

2.1 SUGAR CANE CRUSHING INDUSTRY

Sugarcane Crushing Industries (SCCI) are represented by Autonomous Ethanol Distilleries and Raw Sugar Industries, and a conceptual process flow diagram of a typical sugar mill in the Raw Sugar Industry (RSI), which is largely representative of the SSCI in South Africa, is shown in Figure 3. There are currently 14 mills in South Africa and the average size of the mills is a processing capacity of 300 tons of wet cane per hour. The cane is washed to remove the impurities before it undergoes size reduction with cutting knives, which are typically driven by steam in the South African context[60,61]. The sugar is then extracted from the cane particles, either by a crushing system, or by a diffuser. In the South African sugar mills, diffusers are mostly used, since they are more energy efficient than the milling system[62,63]. The juice then undergoes clarification with lime addition, to remove all solid impurities and then concentrated from a concentration of 15% solids to produce a concentrate of about 65% in a series of multi-effect evaporators, which in South Africa, operate with 5 or 4 effects[64]. The concentrate then enters the crystallisation units to generate raw sugar crystals in the vacuum pans, which are heated using the vapour that is bled from the multi-effect evaporators[64,65]. The crude product from the crystallisation unit is separated into molasses and crystals and the crystals are dried using exhaust steam[60,63].

Older sugarcane mills that are less energy efficient will typically demand 0.55 tons of steam for every wet ton of cane (WTC) processed[60]. Modifications and modernisation of energy utilisation in sugar mills, which require significant capital investments, could lead to more efficient energy usage, with specific steam demands as low as 0.28 ton/WTC in the long term[60,66], while a mid-term target for

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the steam consumption is estimated at 0.40 ton per WTC[64]. The mid-term goal of 0.40 ton per WTC is the maximum permitted steam usage if the mill is to be considered as a platform for exporting renewable energy[64]. Modifying the sugar mill to reduce the steam demand from 0.55 to 0.40 ton per WTC, for the purpose of making surplus bagasse available export to energy-producing facilities, includes the following technical measures:

 Optimising imbibitions rate to reduce the amount of evaporation needed[64].

 Convert from the batch pans to continuous pans, and reduce the pan movement water[61,64,65].

 Use a 5 effect evaporator where vapour is bled to the vacuum pans at a lower effect[65].  Optimise the flashing of condensates for steam recovery[64].

 Electrify the turbine drivers[61,67].

These modifications will entail an investment cost that is estimated at US$ 17.32 million[61,64] (all currency quoted in $2012) for the average size mill in South Africa, which crushes 300 ton WTC per hour[64].

A typical representative sugarcane mill generates about 0.30 tons of bagasse per ton of sugar cane processed[68]. The post-harvesting residues of sugarcane (trash), can typically be generated at a yield of 1.16kg for every kilogram of bagasse generated[69]. Currently, bagasse is inefficiently converted to heat and electricity by means of a low pressure boiler ranging from 15-22 bar[60,65], with some electricity available for export, depending on the efficiency of the mill, while the trash is left on the field. This steam is then expanded with turbines to 4 bar[70] to generate electricity that is sufficient for the mill and a minor amount for export.

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Figure 3: Process Flow Diagram of a Sugar Mill

2.2 PULP AND PAPER MILLS

Paper and pulp industries (P&PI) includes the Sulphite and Kraft Processes, and these generate onsite residues that include the bark that is generated when the logs are debarked and the liquid residues generated by the digestion of the wood chips[71]. In sulphite pulping processes the liquid residue is referred to as Spent Sulphite Liquor (SSL) [34], whereas the liquid residue from Kraft and alkaline pulping of lignocellulose is known as Black Liquor. In the South African P&PI, the Sulphite pulping process is primarily applied for the production of chemical cellulose, which is therefore the focus of the present study. As an industrial context for this study, the Saiccor facility at Umkomaas, KZN would be considered, which currently processes 8000 tons per day of Eucalyptus Globulus hardwood.

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Figure 4: Process Flow Diagram of a Spent Sulphite Process in South Africa

An overview of the Sulphite Process is shown in Figure 4, where wood is firstly debarked and then chipped. The wood chips are then mixed with water and enter the digester to remove the hemicellulose and lignin through solubilisation[72]. The solubilisation occurs at a temperature of approx. 160o_{C with sulphur dioxide in combination with either magnesium oxide (Mg-O) or calcium}

oxide (Ca-O). Currently, the Saiccor facility has three digestion lines, two of which use Mg-O as the solubilisation agent while the third utilises Ca-O. The digester product slurry is then washed and separated into the pulp, which is bleached and rolled into sheets, and diluted SSL, which contains lignosulphonates (water-soluble lignins), sugars and organic acids and phenolic compounds[73]. The SSL from the Mg-O lines is processed for the recovery of energy (steam and power) and pulping chemicals, by concentrating the SSL in a multi-effect evaporator prior to combustion of the resulting syrup in a recovery boiler, which recovers magnesium oxide and sulphur dioxide for digestion process [74]. The energy generated from the combustion of SSL is often not sufficient to satisfy the energy demands of the mill itself, and an additional fuel source such as bark, field biomass residues

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(collected and processed into hog fuel) or coal is needed [71,74]. The SSL from the Ca-O digestion at Saiccor is split into two streams, one of which is sent to a neighbouring lignosulphonates recovery plant, and the second is discharged as effluent.

2.3 ADVANCED ELECTRICITY GENERATION FROM BIOMASS RESIDUES

The efficient and cost effective conversion of biomass to electricity is paramount to promoting the generation of heat and power from renewable resources. Biofuel production from lignocellulose is also frequently associated with co-generation of surplus electricity, for export to the grid. In this section, competing technologies for efficient conversion of biomass residues to electricity are discussed. Such technologies apply to both fresh biomass and solid residues from biomass processing, available from biofuel production, such as pre-treatment residue, the solid components in fermentation residue or supplementary bio-wastes.

Newer systems for generating electricity from biomass utilise boilers that operate at medium-to- high pressures (45-105 bar), which result in greater generation of electricity [60,67,70,75] when compared to the low-pressure systems (15-22 bar) typically installed in the SCCI in the past. Export electricity generation from sugar-mills is dependent on the steam pressure of the boiler, as demonstrated by Venkatesh and Roy[67], whom reported that the installation of a 66 bar boiler allowed for an electricity export of 0.067 MW/WTC, while Moor 2008 reported an electrical export of 0.04MW/WTC when a 45 bar boiler was installed. Nsaful et al [70] reported that increasing the biomass boiler pressure from 30 bar boiler to 40, 63 or 82 bar boilers had increased the potential export electricity by 17.3, 35.9 and 40.1%, respectively. Thus, integrating biofuel production in existing biomass processing facilities should consider high pressure boilers, to maximise the co- generation of surplus electricity production from available bio-residues.

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The Biomass Integrated Gasification and Combined Cycle (BIGCC) is a more advanced and efficient system for electricity production than the high pressure boiler combined with a steam turbine. Bridgewater et al[76] compared a BIGCC and a high pressure boiler system that operated at about 45bar for the generation of electricity from woody feedstock. That study showed that the boiler- steam-turbine system only attained an efficiency of 24% while the BIGCC system had an efficiency of about 40%, which is the typical upper limit for BIGCC efficiency[77]. That study[76] also showed however, that the boiler-steam-turbine system was more economically viable than the BIGCC due to the capital costs of the latter. Similarly, it was found [78] that the BIGCC system was less economically viable compared to a high pressure boiler-steam-turbine system for generating electricity, and also when integrated into sugar mills[79]. Moon et al[80] reported the contrary conclusion with regards to the economic comparison between gasification systems and combustor systems. The most likely explanation for this apparent discrepancy is the status of technological costs on the learning curve. Bridgewater et al[76] and others[78,79] based costs of the BIGCC system on emerging technological cost estimates, while the ordinary combustor system was based on mature costs. In Moon et al[80], the cost basis for the BIGCC was already maturing and was thus subject to some degree of cost reduction.