Characterization of almond processing residues from the Central Valley of California for thermal conversion

•By-products from almond processing constitute large volumes residue material in California estimated as high as 2.7 million tons annually•Almond residue ash has a very high K content, varying between 18–36% K, suggesting a potential value-added processing strategy•Trace element concentrations are mostly well below local area soil suggesting few regulatory utilization challenges for disposal and utilization of ash•High ash content together with the very high K content may cause adverse bed behavior, corrosion, and fouling in thermal boilers

Characterization of biomass relevant to thermochemical conversion processes and other applications is critical to the design and proper operation of energy conversion, biorefining, and other facilities, especially in regard to estimating critical problems related to fouling and slagging from ash constituents. Residue feedstock from almond production was obtained from seven huller and sheller facilities located throughout the Central Valley of California. Results of proximate (moisture, ash, volatile and fixed carbon content), ultimate (C, H, N, S, O composition), heating value, major and trace elements, and melting behavior analyses (all reported on a wt.% dry basis) reveal many similarities and also differences that potentially affect their utilization. The moisture content of air-dried feedstock is an average of 9.7% with only the separated hull material having a higher value (12.2%) and the fine component (< 2 mm) a lower value (8.2%) on an as received basis. The volatile matter is relatively constant (72–76%). The ash content reflects a variable soil component in most fractions with a low average in shell of 3.5% and increasing to 22% in the fine fraction. The elemental C/O ratio is constant at about 1.15 and only appears slightly higher in the woody fraction (1.21). Nitrogen (0.4–0.8%) and sulfur (0.2–0.3%) are elevated compared to many other types of biomass, with the large variation in N probably related to irrigation water source and fertilization practices. Chlorine is generally low (< 0.05) and varies without KCl control in both the crude feedstock and the ash. The ash of the almond biomass is very high in K, varying between 18–36% and only S, Ca, and P reaching substantial amounts. The trace element concentrations are mostly well below local soil compositions with only Ga, Sr, and Cu well above and thus suggest few, if any, regulatory utilization challenges. The elevated feedstock concentrations of S and N may be sufficient to cause some environmental concern for certain types of thermal conversion processes, mostly in relation to NOx and SOx emissions. The high ash content together with the very high K content can cause adverse bed behavior, corrosion, and fouling in boilers, despite the relatively high ash melting temperatures (> 1100 °C) suggested by pellet fusibility test.