9.3.1 Straw Quality and Availability

The first and foremost investigating issue is the residual amount on standing crops as well as the amount left on the ground after harvesting of green cane. A wide variation is observed in the documented data and the available information does not allow complete comparison due to lack of knowledge on a number of essential issues like methodology followed, varieties of sugarcane studied, moisture content of yield and straw, etc.

Hassuani et al. (2005) summarized the results of a seven-year project covering many aspects of availability of sugarcane straw, its quality, routes of recovery and related cost, agronomic impact, impact on the environment, and use for power gen­eration in advanced systems (biomass integrated gasification/gas turbine). Hassuani et al. reported three varieties of sugarcane in three (3) different ages, i. e., first har­vest, third harvest, and fifth harvest. The straw amount varied from 10 to 18 mg ha-1; dry basis and the ration of straw (db) to stalk (wet basis) lied in the range of 11-17 %. The literature showed that the yield of straw ranged from 7.4 to 24.3 mg ha-1 (db), whereas the straw to stalk ratio was between 9.7 and 29.5 %. The average was

14.1 mg ha-1 and 18.2 %, respectively (Hassuani et al. 2005). In potential assess­ments, it is normally accepted that the straw to stalk ratio lies between 14 and 18 %, which clearly reveals that the total amount of straw available is highly dependent on the sugarcane yield.

With regard to the characteristics of straw, the main focus is on two of the most promising uses of straw, i. e., fuel for generation of power and provision of feedstock for the biofuel of second generation. As far as fuel for power generation is concerned, it is mandatory to gather all the information about the proximate and ultimate analyses including the ultimate analysis of the mineral content as the comparison with bagasse is of extreme importance since bagasse can be burned in the same type of equipment, either separated or mixed. The studies further reveal that analyses for three main straw components were carried out: green leaves, dry leaves, and tops. This separation was generated because it had the tendency to display characteristics with marked differences and their role in the final composition of the recovered straw, which depends on the methods used for its collection and the procedures used in harvesting, for instance, whether or not the tops should be removed by the har­vester. Several field experiments were conducted in which the average participation was 31, 62, and 7 % for green leaves, dry leaves, and tops, respectively (dry basis) (Leal et al. 2013).

Similar results were presented by straw and bagasse except for the content of moisture, i. e., in terms of combustion chamber design; the bagasse-fired boiler could be used for straw burning. Other than that, the higher content of chlorine, especially in the tops, could cause corrosion in the boilers. The highlighting differ­ences between the components of bagasse and straw are as follows:

1. Increased content of potassium in straw (mainly in the tops) that can be the rea­son of the deposits found on hot surfaces, ash slags, and corrosion.

2. Levels of magnesium and calcium in the straw. Even though they are in higher concentration when compared with bagasse, still it is expected that they do not cause any problem in the boilers (Leal et al. 2013).