Alkaline wet oxidation of biorefinery lignin from wheat straw

Abstract

Climate change and depletion of fossil resources have generated a need for energy production based on renewable resources such as lignocellulosic biomass. Agricultural residues, like wheat straw, is lignocellulosic biomass that could contribute to the energy supply if it was used for biogas production. One issue with biogas production from lignocellulosic materials is, however, the high content of recalcitrant lignin. The inhibiting lignin fraction can, nevertheless, become more easily degraded if it undergoes pretreatment. A well-known industrial process to degrade lignin is alkaline wet oxidation, which is examined in this study with biorefinery lignin from wheat straw as raw material. The effect of temperature, dry matter (DM) content, reaction time and NaOH amount is examined, with the objective to contribute to the development of lignin pretreatment methods for subsequent biogas production.

The optimal reaction conditions using 10 bar O2 were found at 180˚C, 6.5% DM content, 23 minutes reaction time and 9.8 wt% NaOH. The total yield of the identified phenolic compounds, including vanillin, vanillic acid and guaiacol, was 0.197%. The approximate yield of all organic acids was 35.5%, including formic acid, acetic acid and seven other unidentified compounds. pH stability is a critical factor in the outcome of the oxidation and is very influenced by the relation of DM content and NaOH loading.

In this study, it has been shown that alkaline wet oxidation can be applied to degrade wheat straw lignin, although many improvements should be made in the experimental procedure. In future studies, alternative wet oxidation processes with more benefits for subsequent biogas production could, however, be examined.

Abstract (Swedish)

Klimatförändringar och överkonsumtion av fossila naturresurser har skapat ett behov av en energiproduktion som baseras på förnybara energikällor, däribland biomassa innehållandes lignocellulosa. Restavfall från jordbruket, som vetehalm, är material innehållandes lignocellulosa som kan bidra till energitillförseln om de används till biogasproduktion. Ett problem med biogasproduktion från sådana material är emellertid den höga halten av svårnedbrytbart lignin. Den inhiberande ligninfraktionen kan dock göras mer nedbrytbar med lämplig förbehandling. En välbeprövad industriell process för att bryta ned lignin är alkalisk våtoxidation, vilket undersöks i denna studie med vetehalmslignin utvunnet från bioraffinaderiprocesser som råmaterial. Effekten av temperatur, torrhalt, reaktionstid samt mängd NaOH undersöks, med målet att bidra till utvecklingen av metoder för förbehandling av lignin för efterföljande biogasproduktion.

De optimala reaktionsförhållandena med 10 bar O2 hittades vid 180˚C, 6.5% torrhalt, 23 minuter reaktionstid och 9.8% NaOH. Det totala utbytet av de identifierade fenoliska ämnena, innefattandes vanillin, vanillinsyra samt guajakol, var 0.197%. Det approximativa utbytet av alla organiska syror var 35.5%, där inräknat myrsyra, ättiksyra samt sju andra ämnen som inte kunde identifieras. Stabiliteten av pH är en kritisk faktor för utfallet av oxideringen och påverkas starkt av förhållandet mellan torrhalt och mängd NaOH.

I denna studie har det visats att alkalisk våtoxidation kan appliceras för att bryta ned vetehalmslignin, även om många förbättringar behöver implementeras i den laborativa proceduren. För framtida studier kan alternativa våtoxidationsmetoder, med fler fördelar för efterföljande biogasproduktion, undersökas.

Popular Abstract

When agricultural residues are used for biogas production, some non-degraded leftovers always remain. The leftover fraction remains because it is too complex for the biogas-producing microorganisms to degrade. Fortunately, there are ways to help these microbes out! By using a certain chemical pretreatment, where high temperature, high pressure of oxygen and a strong base are used, some degradation products arise from the leftover fraction. These degradation products can be more easily used by the microbes, making it possible to produce even more biogas. Not such a bad deal to simultaneously enable the use of residual material and enhance the production of energy, right?

The most favorable conditions for this chemical pretreatment were examined in this study, where a leftover fraction of wheat straw was used. Wheat straw is an agricultural residue that is produced in enormous amounts every year. The highest output of degradation products was achieved when the lowest amounts of the leftover fraction was used in the pretreatment. Some degradation products that were formed included vanillin, a well-known flavouring agent, acetic acid and formic acid. Which degradation products that were formed was, however, dependent on the pH stability during the treatment.

Studies like this are very relevant for the transition towards an energy production based on renewable resources, which is crucial due to climate change and depletion of fossil resources. One important part of the future energy production is biomass, as it is both renewable and seen as the fourth largest energy source in the world, after oil, coal and natural gas. Agricultural residues are an example of lignocellulosic biomass, which is of particular interest since it is inedible for humans and, therefore, does not compete with food production for arable land. A very efficient way to transform biomass to a useful type of energy is through biogas production, as it is naturally produced in multiple steps by microorganisms. Biogas is a mixture of methane and carbon dioxide, but where methane is the desired compound since it can be used as a fuel in vehicles or used for generation of electricity or heat.

One issue with producing biogas from lignocellulosic biomass, such as agricultural residues, is the high content of lignin. The lignin is, in other words, the leftover fraction that remains after biogas production from lignocellulosic materials. Lignin has a very complex structure, consisting of aromatic units connected in chains, which is why it is so hard for the biogas-producing microorganisms to degrade. With the studied chemical pretreatment, a well-known process called alkaline wet oxidation, the connections between the aromatic units in lignin are broken down and degradation products, such as aromatic compounds or organic acids, arise.

The findings of the study are a first step to optimize the use of the leftover fraction in agricultural residues. If the investigated chemical pretreatment is further developed, so more parts of agricultural residues can be made available for biogas production, the easier could a transition to renewable energy production become. So, let’s continue to investigate how to help the biogas-producing microorganisms and, by doing so, develop a pretreatment that can contribute to a sustainable energy production!

• author: Lindblad, Maja
• supervisor: Ola Wallberg; Borbala Erdei
• organization: Chemical Engineering (M.Sc.Eng.)
• alternative title: Alkalisk våtoxidation av lignin från bioraffinaderiprocessad vetehalm
• course: KETM05 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: alkaline wet oxidation, lignin oxidation, wheat straw, chemical pretreatment, biorefinery lignin, anaerobic digestion, response surface-modelling, chemical engineering
• language: English
• id: 9066729
• date added to LUP: 2021-11-08 11:53:00
• date last changed: 2021-11-08 11:53:00