Moisture meters for biomass, hay, straw, wood chips and pellets are instruments dedicated to the bioenergy sector for measuring the moisture content of solid fuels of plant origin. Moisture content directly affects the usable calorific value of the fuel (high moisture = reduced calorific value, because the water has to evaporate before combustion), storage stability (biomass that is too damp ferments and produces toxic gases, as well as losing substance) and the efficiency of biomass-fired boilers and heating plant. They are used in the trade in firewood, pellets and wood chips, in biomass heating plants, on farms and in cattle sheds for hay and straw, and in waste-to-energy plants.
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Measuring technologies
The technologies used are: capacitive plate meters for wood chips and loose biomass (a measuring cell filled with the material, measuring the dielectric constant); long-pin meters for bales of hay and straw (with lances 50-100 cm long inserted into the bale to measure moisture at the core); microwave meters for non-destructive measurements on material flowing past; and thermogravimetric balances for the reference gravimetric method. The typical moisture ranges are very wide: 5-15% for quality pellets, 15-60% for green wood chips, 10-25% for hay and straw, and up to 80% for freshly cut wood.
Moisture meters for pellets
Wood pellets for domestic use (stoves, boilers) are governed by the ENplus quality standard (ENplus A1, A2, B): moisture must be ≤ 10% for class A1 (premium quality for stoves), ≤ 10% for A2 (residential) and ≤ 12% for B (industrial). Moisture meters dedicated to pellets have a measuring cell of specific geometry, with a calibration database for pellets of various origins (beech, fir, pine, mixed conifers, agricultural sources). Typical accuracies are ±0.5-1% on moisture content. They are used in pellet production (quality control of the finished product), by distributors (acceptance testing of deliveries) and by end users (checking the quality of what they have bought).
Wood chips for heating plants
Wood chips are the main fuel of many medium and large biomass heating plants, rural district heating schemes and cogeneration installations. Wood chips are generally traded on a dry weight equivalent basis: moisture significantly affects the price. For storage, wood chips must not exceed 30-35% moisture for extended periods (beyond that they ferment, self-ignite and lose dry matter). Portable and benchtop meters are used for acceptance testing (with sampling to standard), for periodic checks on stocks, and for optimizing natural drying (in open storage, moisture falls over time through natural evaporation).
Hay and straw for livestock
For hay and straw intended as livestock feed or bedding, moisture control is critical to keeping quality: hay with moisture > 17-20% at the core of the bale risks unwanted (anaerobic) fermentation, nutritional losses, the formation of molds dangerous to livestock and, in extreme cases, self-ignition of bales stored in the barn. Long-pin meters make it possible to measure at the core of the bale without having to break it open. They are used by farmers, agricultural inspectors and health inspectors visiting cattle sheds. EN ISO 18133 governs the measurement of moisture in solid biomass.
Calorific value and moisture
The net calorific value (NCV) of biomass is closely correlated with moisture: wood at 0% moisture has an NCV of about 18 MJ/kg; at 20% moisture the NCV falls to about 14 MJ/kg; at 50% moisture the NCV is only 7-8 MJ/kg, because combustion consumes heat to evaporate the water. For biomass cogeneration plants, fuel moisture is a fundamental technical specification: variations in moisture alter the efficiency of the process and call for adjustments to the system (excess air, combustion profile). Advanced systems incorporate continuous moisture sensors on the plant’s fuel feed.
Reference standards
The main standards are: EN ISO 18134 (moisture in solid biofuels – the reference gravimetric method, parts 1, 2 and 3 for different applications); EN ISO 17225 (technical specifications for solid biofuels); EN ISO 14780 (sample preparation); ENplus (the European pellet certification, managed by AIEL in Italy). Commercial instruments are subject to calibration criteria in an ISO 17025 accredited laboratory. Periodic verification against the gravimetric method is recommended.
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