ISO/TS 28924 — Solid Biofuels — Fuel Specifications for Graded Non-Wood Fuels

ISO/TS 28924 addresses a critical gap in the bioenergy standards landscape by providing fuel specifications for graded non-wood solid biofuels. While wood fuels have long dominated the biomass heating market, the growing demand for sustainable energy has driven interest in a much broader range of biomass feedstocks, including herbaceous biomass (straw, grasses, Miscanthus), agricultural residues (husks, shells, stalks), fruit biomass (olive pits, nutshells), and aquatic biomass (algae, water hyacinth). Each of these feedstocks has distinct physical and chemical properties that significantly influence combustion behavior, emissions profiles, and ash management strategies. This specification establishes a classification framework that enables fuel suppliers, appliance manufacturers, and end-users to confidently trade and utilize these alternative biofuels with predictable quality.

Classification Framework for Non-Wood Biofuels

ISO/TS 28924 organizes non-wood solid biofuels into four main categories based on botanical origin: herbaceous biomass (including cereal straws, grasses, and Miscanthus), agricultural residues (including husks, shells, stalks, and fruit stones), fruit biomass (including olive residues, palm kernel shells, and nut shells), and aquatic biomass (including macroalgae and microalgae biomass). Each category is further subdivided into quality classes that reflect the processing level and permissible impurity content.

A distinctive feature of ISO/TS 28924 compared to ISO/TS 28923 (wood fuels) is the broader range of acceptable ash content values. While premium wood pellets require ≤0.7% ash, herbaceous biomass pellets may have ash content ranging from 2% to 10% depending on the feedstock and quality class. This reflects the fundamental compositional difference: herbaceous plants accumulate silicon, potassium, and calcium in their structural tissues, resulting in inherently higher ash yields. The specification sets different ash limits for each feedstock category rather than imposing a single universal limit, recognizing that ash content alone is not a reliable predictor of combustion quality across all non-wood biomass types.

Engineering Challenges in Non-Wood Biomass Combustion

The most significant engineering challenge addressed by ISO/TS 28924 is ash-related operational problems. Non-wood biomass ash is characterized by high concentrations of alkali metals (particularly potassium), silicon, and chlorine, which form low-melting-temperature eutectic compounds. The ash deformation temperature (DT) for many herbaceous biomass fuels falls below 900°C — compared to above 1200°C for clean wood — meaning that slag formation in the combustion chamber is almost inevitable unless the combustion system is specifically designed for these fuels. The specification requires that the ash melting behavior (DT, ST, HT, FT) be declared for all non-wood fuel classes, and provides guidance on critical temperature thresholds for different combustion system types.

A second major challenge is chlorine-induced corrosion. The chlorine content of herbaceous biomass can be 10-50 times higher than that of clean wood. During combustion, chlorine is released primarily as HCl and KCl(g). Alkali chlorides condense on heat exchanger surfaces, forming sticky deposits that accelerate tube corrosion through a mechanism known as active oxidation (chlorine-induced corrosion). ISO/TS 28924 sets maximum chlorine limits for each quality class and recommends chlorine-sensitive corrosion protection measures (including the use of high-alloy steels, ceramic coatings, and additives such as ammonium sulfate) for boilers firing high-chlorine non-wood fuels.

The nitrogen oxide emissions challenge is particularly acute for non-wood fuels. While wood fuels typically contain 0.1-0.3% nitrogen (dry basis), herbaceous biomass and agricultural residues can contain 0.5-3.0% nitrogen. This fuel-bound nitrogen is the primary source of NOx emissions in biomass combustion (thermal NOx being relatively low due to moderate flame temperatures). ISO/TS 28924 provides a classification of NOx emission potential based on fuel nitrogen content and recommends appropriate NOx control strategies — including air staging, flue gas recirculation (FGR), and selective non-catalytic reduction (SNCR) — for different fuel classes.

Fuel Preparation, Blending, and Logistics

ISO/TS 28924 provides extensive guidance on fuel preparation and blending strategies to upgrade lower-quality non-wood feedstocks. The specification recognizes that many agricultural residues have physical properties that are suboptimal for automated combustion systems — low bulk density, poor flowability, and high fines content. Mechanical processing recommendations include drying (to reduce moisture below 15% for pelletization), grinding (to achieve a particle size distribution suitable for densification), and pelletization or briquetting (to produce a uniform, high-density fuel with consistent handling properties). The specification provides target values for pellet dimensions, density, and mechanical durability specific to each feedstock category.

Fuel blending is presented as a practical strategy to mitigate the problematic properties of individual feedstocks. For example, blending high-chlorine straw pellets with low-chlorine wood pellets in a 30:70 ratio can reduce the chlorine content of the blend below the corrosion threshold for standard boiler materials. Similarly, blending high-ash rice husks with low-ash Miscanthus can produce a fuel with acceptable ash content while maintaining the high silica content beneficial for fluidized bed combustion systems. ISO/TS 28924 provides a blending calculation methodology and examples for several common blending scenarios.