Alternative Straw Processing Systems

CONCLUSIONS

1. Straw (dry and green leaves) separation efficiencies in DCS were 17–49 wt% (kg of straw separated by 100 kg fed straw, wb).

2. The efficiencies of separation of MI in DCS ranged from 18 wt% to 76 wt% (kg of separated MI by 100 kg of fed MI, wb).

3. The straw and MI separation efficiencies were related. Therefore, since the cleaning system increased straw removal, it is very likely that the MI removal efficiency will also increase.

4. The efficiencies obtained in the tests are lower than those reported by the manufacturers and previous studies. However, deficiencies involving DCS design, operation, and maintenance parameters were identified during the trials.

5. DCS processing capacity is important because of its relationship with the cane layer height at the system inlet, and a higher layer height can prove detrimental to the separation process.

6. The separation efficiency of DCS plays a key role in the operational costs. In Mill 2, considering the operation under ideal conditions, the operating cost of energy would be USD 0.80 per ton of sugarcane straw processed.

Figure 53: DCS separated straw washing and transporting in a water channel.

Figure 54: Washed straw drainage in the cush-cush screen.

Figure 55: Straw feeding into the last mill of the milling tande. Figure 56: Independent mill for straw shredding.

The assessment of these alternative systems was carried out in collaboration with three partner mills located in the Brazilian state of São Paulo (Mills 5, 6, and 7). During the evaluation trials, straw and bagasse samples were collected throughout the conditioning process and the following physicochemical analyses were conducted: moisture, ash, elemental analysis (C, H, N, S, and Cl), ash chemical composition by X-ray fluorescence, higher heating value (HHV), and particle size distribution.

Additionally, to better understand the straw washing and leaching processes, laboratory- and bench-scale studies were carried out in shakers and extractors under varying process conditions, such as water temperature (20–60 °C), washing time (5–55 minutes), straw-to- water ratio (1:20–1:75 w/w), and agitation (250–1500 rpm).

RESULTS AND DISCUSSION

I. STRAW WASHING AND LEACHING PROCESSES

Although the systems examined were not optimized, the straw washing process proved to be rather promising, given that the average efficiency for ash content reduction for the tested plants was 39%–46%, as shown in Figure 57.

0 2 4 6 8 10 12 14 16 18 20

Total Ash Content (wt%, d.b)

Washing Efficiency (Mill 5)

39.3%

Washing Efficiency (Mill 6)

46.1% Washing Efficiency (Mill 7) 46.4%

B A B A B A

Figure 57: Total ash contents before (B) and after (A) straw washing process.

In addition, the washing process has been shown to promote the leaching of chemical elements that are critical for burning in boilers; these include silicon (Si), potassium (K), sulfur (S), and chlorine (Cl). Lastly, it was noted that leaching efficiencies can reach greater values in optimized washing processes.

This hypothesis was confirmed by the results of the laboratory- and bench-scale experiments. The best efficiencies were obtained for the leaching process in the extractor (Figure 58) under the following process conditions: dry and shredded straw (20 g); distilled water (1.5 L); 20 °C; 1,500 rpm; and 3 minutes of operation; this was done twice more (a total of 3 steps of washing) using 1.5 L of clean fresh water.

100 20 30 50 50 60 70 80 90 100

Leaching Efficiency (%)

Potassium (as K₂O) Ash

74

Chlorine 93

82

Silicon (as SiO₂)

62

Sulfur 16 Figure 58: Water

washing and leaching efficiencies obtained during trials carried out in an extractor.

From laboratory - and bench-scale studies, parameters were identified to be optimized in commercial systems, such as agitation efficiency, contact time, temperature, and washing water quality.

II. USE OF CONVENTIONAL MILL FOR STRAW SHREDDING

The assessment of the straw shredding processes using conventional sugarcane mills to reduce the particle size distribution presented very interesting results. After shredding using an independent mill, it was observed that 90 wt% (on average) of the straw refers to particles smaller than 12.5 mm (Figure 59); this shows a strong similarity with the bagasse particle size distribution. In addition, the mill presented higher operational regularity than the hammer and knife shredders. The mill produced straw samples with higher homogeneity and smaller particle size variations.

Feeding straw in the last mill of the milling tandem produced more homogeneous bagasse-straw mixtures, with a particle size distribution very similar to that of bagasse (90 wt% were smaller than 12.5 mm). By contrast, adding straw to bagasse on belt conveyors produces more heterogeneous mixtures (Figure 60). In short, mills have proven to be more efficient than shredders because they are better at achieving the desired straw particle size distribution. Preliminary studies carried out by SUCRE and reported in the project reports indicate that CAPEX should stay between 50% for Alternative 1 and up to 70% for Alternative 3, in relation to a conventional sugarcane straw processing system.

These alternatives are detailed in the next section (3.3.5). However, only after completing the detailed investment studies can a definitive assessment of the amounts involved be made.

0 20 40 60 80 100 120

0 10 20 30 40 50 60 70 80 90 100

Cumulative undersize mass (wt%, db)

Sieve opening size (mm)

Shredded Straw: Independent Mill

SS1 SS2

SS3 SS4

Figure 59: Cumulative undersize mass (wt%, db) as a function of the sieve opening size of shredded straw (inde- pendent mill) samples (SS1, SS2, SS3 and SS4) collected at Mill 7.

Figure 60: Typical appearance of straw processed by conventional hammer shredder (A), straw shredded in an independent mill (B) and bagasse-straw mixture produced through the straw feeding in the last mill of the milling tandem (C).

A B C