Biomass: how to measure sugarcane bagasse moisture?

The measurement of the moisture content of the sugarcane bagasse is made by means of electronic meters applied directly to the bagasse outlet spout, just after the last milling suit (also used in the lines with diffuser). These meters provide continuous readings at a rate of 5 Hz and are applied to the central part of the ‘spout’ or at three points (see illustrations below). This last option shows the operator the moisture profile in the transverse direction of the flow, which is important for adjusting the pressure in the drying suit rollers.

Why measure the humidity of sugarcane bagasse?

Knowing the moisture content of sugarcane bagasse when leaving the crushing process is fundamental in at least two points of view: The first refers to extraction, that is, the crushing process that removes sugars from the cane. If the bagasse has a high moisture content, it means (among other factors) that part of the juice still remains in the bagasse, resulting in a reduction in the amount of sugar removed from the cane. If the bagasse is low in moisture, it is possible that part of the sugar still remains in the bagasse because there was not enough water to remove it. With this information, the operator will be able to update the soaking valves or the pressure of the rollers to reach the optimum point. The second takes into account the subsequent process, usually the burning of bagasse for the production of thermal energy in large boilers. From this point of view, the boiler control system must know the moisture content of the fuel it will receive to adjust the control parameters in order to guarantee its performance. Fuel with a high moisture content for too long ‘overturns’ the boiler (decreases its ability to produce steam), and can also cause explosions inside the furnace.

Accuracy of in-line humidity measurements

The technology applied to the line’s meters was developed about 15 years ago and the applications in sugarcane bagasse started in 2009. Until then, measurements were made by electropneumatic samplers that take samples of bagasse and are sent to the laboratory for analysis (some few samples per shift and the accuracy of the results is compromised by several factors (non-uniform sample and delay in the collection of accumulated material, among others). Electronic meters have very accurate results (see example below) and the sample volume is very significant. Suppose a line with a crushing capacity of 1 million tons of sugarcane per harvest. During this period, about 300,000 tons of bagasse will be produced. Considering a medium standard installation (spout size, flow speed, etc.), it appears that about 22 tons of bagasse have their moisture content analyzed by a single UmiChip moisture meter every DAY (2% of the bagasse produced in that period). If we use three sensors in the outlet spout this amount will be 65 tons per day, 6% of the total! To compare: in the same period, with hourly samples, the volume of bagasse analyzed would be 7.5 kg (0.0075 ton or 0.0007%). In addition to the number of samples analyzed being very high, the accuracy of the measurements is not affected by the traditional collection and analysis procedures.