Background:
The use of energy crops and agricultural residues is expected to increase to fulfill the legislative demands of bio-based components in transport fuels. Ensiling methods, adapted from the feed sector, are suitable storage methods to preserve fresh crops throughout the year for e.g. biogas production. Various preservation methods, namely ensiling with and without acid addition for whole crop maize, fiber hemp and faba bean were investigated. For the drier fiber hemp, alkaline urea treatment was studied as well. These treatments were also explored as mild pre-treatment methods to improve the disassembly and hydrolysis of these lignocellulosic substrates.
Results:
The investigated storage treatments increased the availability of the substrates for biogas production from hemp and in most cases from whole maize but not from faba bean. Ensiling of hemp, without or with addition of formic acid, increased methane production by more than 50% compared to fresh hemp. Ensiling resulted in substantially increased methane yields also from maize, and the use of formic acid in ensiling of maize further enhanced methane yields by 16%, as compared with fresh maize. Ensiled faba bean, in contrast, yielded somewhat less methane than the fresh material.Acidic additives preserved and even increased the amount of the valuable water-soluble carbohydrates during storage, which affected most significantly the enzymatic hydrolysis yield of maize. On the other hand, preservation without additives decreased the enzymatic hydrolysis yield especially in maize, due to its high content of soluble sugars that were already converted to acids during storage.Urea-based preservation significantly increased the enzymatic hydrolysability of hemp. Hemp, preserved with urea, produced the highest carbohydrate increase of 46% in enzymatic hydrolysis as compared to the fresh material. Alkaline pretreatment conditions of hemp improved also the methane yields.
Conclusions:
The results showed that ensiling and alkaline preservation of fresh crop materials are useful pre-treatment methods for methane production. Improvements in enzymatic hydrolysis were also promising. While all three crops still require a more powerful pre-treatment to release the maximum amount of carbohydrates, anaerobic preservation is clearly a suitable storage and pre-treatment method prior to production of platform sugars from fresh crops.