Dear Tijjani,

The following publications cover the answer to your question. The first publication is a review article describes the pretreatment process of lignocellulosic wastes to improve ethanol and biogas production. The second publication is a paper that describes the pretreatment process of orange peel by leaching of limonene for improving biogas production.

1-Int J Mol Sci. 2008 Sep; 9(9): 1621–1651.

Published online 2008 Sep 1. doi:  10.3390/ijms9091621

PMCID: PMC2635757

Pretreatment of Lignocellulosic Wastes to Improve Ethanol and Biogas Production: A Review

Mohammad J. Taherzadeh1,* and Keikhosro Karimi1,2

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This article has been cited by other articles in PMC.

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Abstract

Lignocelluloses are often a major or sometimes the sole components of different waste streams from various industries, forestry, agriculture and municipalities. Hydrolysis of these materials is the first step for either digestion to biogas (methane) or fermentation to ethanol. However, enzymatic hydrolysis of lignocelluloses with no pretreatment is usually not so effective because of high stability of the materials to enzymatic or bacterial attacks. The present work is dedicated to reviewing the methods that have been studied for pretreatment of lignocellulosic wastes for conversion to ethanol or biogas. Effective parameters in pretreatment of lignocelluloses, such as crystallinity, accessible surface area, and protection by lignin and hemicellulose are described first. Then, several pretreatment methods are discussed and their effects on improvement in ethanol and/or biogas production are described. They include milling, irradiation, microwave, steam explosion, ammonia fiber explosion (AFEX), supercritical CO2 and its explosion, alkaline hydrolysis, liquid hot-water pretreatment, organosolv processes, wet oxidation, ozonolysis, dilute-and concentrated-acid hydrolyses, and biological pretreatments.

For more details, see attached file.

2-Improvement of Biogas Production from Orange Peel

Waste by Leaching of Limonene

Rachma Wikandari,1 Huong Nguyen,2 Ria Millati,3

Claes Niklasson,2 and Mohammad J. Taherzadeh1

Limonene is present in orange peel wastes and is known as an antimicrobial agent, which impedes biogas production when digesting the peels. In this work, pretreatment of the peels to remove limonene under mild condition was proposed by leaching of limonene using hexane as solvent. The pretreatments were carried out with homogenized or chopped orange peel at 20–40∘ C with orange

peel waste and hexane ratio (w/v) ranging from 1 : 2 to 1 : 12 for 10 to 300 min. The pretreated peels were then digested in batch reactors for 33 days. The highest biogas production was achieved by treating chopped orange peel waste and hexane ratio of 12 : 1 at 20∘ C for 10 min corresponding to more than threefold increase of biogas production from 0.061 to 0.217 m3 methane/kg VS. The

solvent recovery was 90% using vacuum filtration and needs further separation using evaporation. The hexane residue in the peel had a negative impact on biogas production as shown by 28.6% reduction of methane and lower methane production of pretreated orange peel waste in semicontinuous digestion system compared to that of untreated peel.

For your question you may need to read the first publication (review), however, I attached the second publication that might be helpful for you as well.

Hoping this will be helpful,

Rafik

Rafik's answer is good, but I really want to stress that it is very difficult to make biogas from citrus waste because of the limonene. Limonene is a very powerful inhibitor. Microbes have great difficulties growing and metabolizing when it is present. Usually this manifests itself as a ery long lag time in batch experiments. It takes a very long time before the bacteria adapts to the environment and starts growing. This has a huge impact on the economic feasibility of a commercial bioreactor. Any citrus waste will make the biogas production inefficient and hence uneconomic.

Professor Taherzadeh in Boras, Sweden, has done alot of work on the subject. Check out papers by Taherzadeh, Ylitervo, Sarvari Horvath, Lennartsson, Niklasson, Millati, Wikandari

All of these are at the University of Boras, Sweden. It is one of the best groups in this subject.

Dear Jonas,

The two attached publications to my answer is studies for the Swedish group you just mentioned.

Rafik

Ooh thanks alot! Drs Nyman and Karaman for your contributions.