The need for sustainability, energy security and reduction of global warming has brought many alternative energy sources into the foreground. Already there are
well established technologies that can produce renewable energy but when it comes to the production of renewable liquid fuels and chemicals, biomass is the primary
feedstock. Biomass is a renewable source of energy that can provide heat, electricity and transport fuels. However, utilisation of biomass posses some limitations such as
land availability and competition of energy crops with food crops. In order to overcome these problems "third generation" biofuels from alternative feedstock such
as macro-algae have recently come into the foreground. Oceans and seas cover over 70% of the earth's surface, most of which is under exploited, resulting in additional
potential for biomass production.
This thesis concentrates on the potential for production of bio-energy and chemicals from macro-algae through thermochemical processes such as pyrolysis, combustion and hydrothermal liquefaction. Utilisation of aquatic biomass for production of bioenergy is a very recent concept and there is a lack of information on their thermochemical behaviour. This investigation contributes to a wider study
and forms part of the Supergen II bionergy programme investigating the potential for utilisation of macrolage in the UK. This investigation includes a detailed characterisation of the fuel properties and thermal behaviour of a range of wild seaweeds around the UK provided by the Scottish Association of Marine Sciences.
In addition, a range of model biochemical components have been investigated, in particular, the model carbohydrates present in macro-algae.
Alginic acid, mannitol, laminarin, fucoidan and cellulose are the main carbohydrates present in brown macro-algae. The rest of the plant material comprises of protein and ash. Freshly harvested macro-algae contain 80-90wt%
moisture. Their ash content is high, reflecting their high inorganic content. Potassium is the most abundant metal present in macro-algae although other metals are also present including sodium, calcium, magnesium, Their carbohydrate, protein and ash content undergo a seasonal variation during their growth cycle. This variation was found to affect their properties as fuel.
Carbon content reaches its maximum during summer - early autumn. During the same period, the inorganic (and thus ash) content is at its minimum suggesting summer - early autumn as the optimum period for harvesting macro-algae for bioenergy. The high carbon and low inorganic content during this period is reflected in its higher heating value but it is still relatively low (13-14 MJ/kg) when compared
with terrestrial biomass. The nature of the inhabitant location was found to significantly influence macro-algae fuel properties with samples grown in the open
ocean having better fuel properties (higher HHV and lower inorganic content) than samples growing in canals and estuaries.
Investigation of the pyrolysis behaviour was performed using thermal analysis such as TGA and Py-GC/MS. The volatile matter evolved during pyrolysis was higher for samples collected during summer and early spring due to their higher carbon content. The main volatiles evolved during decomposition were found to originate either from their carbohydrates or from their protein content. Specific
marker compounds were identified for the carbohydrates such as dianhydromannitol, 1-(2-furanyl)-ethanone, 2-hydroxy-3-methyl-2-cyclopenten-1-one and furfural for manitol, laminarin and alginic acid respectively. Proteins are found to produce a range of indoles and pyrroles. Some of the compounds identified may have industrial applications indicating the possibility of producing chemicals through
pyrolysis of macro-algae. The high moisture content of seaweed necessitates that significant amounts of water must be removed before this feedstock can be converted by pyrolysis.
The high moisture content is similary an issue for combustion which has been assessed by a combination of TGA and characterisation of the biomass. Macro-algae
have a low HHV, high halogen content and high ash content and are predicted to have high slagging and fouling behaviour in conventional combustion chambers.
This fouling behaviour is predicted through empirical indexes such as the alkali index and is shown to be higher than terrestrial biomass even during summer - early
autumn when their inorganic content is at minimum. Typical ash contents vary from 18 to 45wt% and contain mainly oxides of K20, Na2O, CaO and MgO.
Pre-treatment prior to combustion can significantly reduce the ash content leading to improved combustion properties, but this also leads to removal of some
biochemical components. Using an acid pre-treatment, some of the seaweed's biopolymers, such as mannitol or fucoidan, can be removed presenting the possibility for acquiring valuable chemicals from seaweed before combustion of the
residue.
An alternative processing route, capable of processing wet feedstocks called hydrothermal liquefaction (HTL) involves the processing of the macro-algae in subcritical water. HTL converts the starting material into four product streams
including a bio-crude, a char, an aqueous stream consisting primarily of process water and a gaseous stream. A parametric study of HTL has been investigated using
high pressure batch reactors with or without the presence of catalysts. The bio-crude produced from the liquefaction of macrolgae was found to have a high heating value
and resembling chemical composition to crude-oil. It can be used directly as a fuel however it still contains significantly high nitrogen levels and will required suitable upgrading (e. g. denitrogenation). The bio-char was found to also have a high heating value. Both bio-crude and bio-char produced from HTL are virtually free of alkali
metals suggesting they are suitable for combustion. Reaction conditions such as temperature and the ratio of biomass to water have the greatest influence on product
yields and properties. Typical bio-crude yields were in the range of 10 to 19wt% on a daf basis with their HHVs ranging from 32 to 38 MJ/kg. The yields of bio-chars were in similar range (IOwt% to 19wt% on a db) with HHVs between 10 and 26 MJ/kg. An energy balance was calculated in order to investigate the energy required to heat the mixture of macro-algae and water. The energy recovery in the bio-crude
and bio-char was relatively low, between 50 and 65%, indicating that a significant portion of the energy content of macro-algae is passing in to the other product
streams.
The aqueous phase (process water) was found to be rich in metals, especially alkali metals, and sugars, and its composition suggests it maybe possible to utilize it
as a fertilizer. A fraction of the sugars present in macro algae (mannitol and laminarin) pass in to the aqueous stream, suggesting there is also potential for fermentetion to bioethanol. The gaseous stream is composed mainly of CO2, N, CO and lower concentrationso H and CH4.
The most suitable thermochemical processing route for macro-algae is proposed to be hydrothermal liquefaction and has potential for utilization of all the product streams
producing fuels and chemicals using a bio-refinery concept.