Two-Stage Conversion of Land and Marine Biomass for Biogas and Biohydrogen Production

Abstract

The replacement of fossil fuels by renewable fuels such as biogas and biohydrogen will require efficient and economically competitive process technologies together with new kinds of biomass. A two-stage system for biogas production has several advantages over the widely used one-stage continuous stirred tank reactor (CSTR). However, it has not yet been widely implemented on a large scale. Biohydrogen can be produced in the anaerobic two-stage system. It is considered to be a useful fuel for the future due to its high energy density and clean combustion with the emission of only water vapour. Anaerobic digestion can be used to treat wastewater and for energy production, leading to a reduction in eutrophication and greenhouse gases. The material remaining after treatment can also be used as a fertiliser as long as certain standards are met. The production of biogas and biohydrogen from a range of land and marine biomasses was studied in this work. The reduction of the heavy metal content of seaweed was also studied in order to improve fertiliser quality.



Two-stage, dry anaerobic digestion of mussels, reeds, seaweed, solid cow manure, and a mixture of seaweed and manure was studied. The system consisted of a leach bed reactor for hydrolysis and an upflow anaerobic sludge blanket (UASB) reactor for methane production. The results showed that mussels with shells, seaweed, and the seaweed and manure mixture were efficiently digested in the two-stage system; 68 to 83% of the methane being produced in the UASB reactor. The manure by itself, and reeds, which are slowly degradable, were efficiently digested in the one-stage dry leach bed process, in which most of the biogas was produced. Seaweed and manure can also be co-digested in the one-stage dry digestion process, since methano¬genic conditions prevailed in the leach bed reactor, thus reducing the cost of operating two biogas reactors. Technically, both the new feedstocks and the one- and two-stage dry anaerobic systems have great potential for biogas production. However, economic evaluations are needed to validate practical applicability.



The removal of heavy metals from seaweed hydrolysate was studied in the two-stage system. The heavy metals Cd, Cu, Ni and Zn were adsorbed using iminodiacetic acid Cryogel® carriers. However, removal of the heavy metals resulted in low methane yields, possibly due to the removal of micro¬nutrients needed for anaerobic digestion. It is therefore suggested that the metals be removed after methane production in a UASB reactor. Alkaline and autoclave post-treatment of the seaweed digestate resulted in 86% organic matter solubilisation and the leachate may be treated in a UASB reactor, providing a means of handling digestate with high heavy metal content. Co-digestion of leachates from the leach bed reactor and the post-treatment resulted in a high methane yield, 0.34 l/gVSadded in a batch test. Subsequent treatment of the leachate from the leach bed reactor resulted in a high methane productivity at a loading rate of 20.6 g COD/l.day in a UASB reactor. Treatment of the seaweed leachate in the UASB reactor resulted in a stable process without the need for additional nutrients or buffer. As the seaweed leachate was rich in nutrients and buffer capacity, its co-digestion with wheat straw hydrolysate in the UASB reactor resulted in a stable process.



Biohydrogen and biogas were co-produced from wheat straw hydrolysate in a two-stage system consisting of a CSTR and a UASB reactor, employing the thermophile, Caldicellulosiruptor saccharolyticus in the first H2 reactor. Straw hydrolysate was efficiently produced by acid-catalysed steam and enzyme pretreatment, giving a 95% sugar yield of the theoretical yield. High biofuel production rates of 1.8 to 3.5 l H2/l.day and 2.6 to 4.0 l CH4/l.day were obtained under stable operational conditions and treatment efficiencies. However, the cost of nutrient supplementation was high, and cheaper nutrient sources will be required to make the production cost economically competitive.



This research has demonstrated the versatility of a two-stage system that allowed the digestion of new kinds of biomass such as seaweed with sand, mussels with shells, reeds, manure and wheat straw. It has also been shown to be possible to remove heavy metal from seaweed to improve fertiliser quality. High hydrogen and methane production rates were also demonstrated, and the two-stage anaerobic system is thus, technically, a promising reactor configuration for the production of biofuels.

Abstract (Swedish)

Popular Abstract in English

Peter is a part-time cocoa farmer who produces cocoa beans that are used for chocolate production. However, he has always wondered about the efficient utilisation of the residual cocoa pods, which seem to contain sugar as they attract bees. He discussed this with a student studying a biological process whereby organic material can be converted to biogas and a residue that can be used as a fertiliser.



This student was studying interesting ways of producing renewable sources of energy such as biogas and biohydrogen from organic materials on land and in the sea, and also a new method of improving fertiliser quality. Abundant marine organic materials such as mussels, reeds and seaweed, which do not compete with food grown on agricultural land, were used to produce biogas. Manure, which is an abundant waste resulting from intensive animal farming, was also used for biogas production. The biogas production system consisted of an anaerobic two-tank system. The reason for using a two-tank process is to separate the fast-growing microbes that break down complex organic material from the slow-growing microbes that produce biogas. In this process, both groups of microbes can grow better than if they were mixed in the same tank. The results demonstrated that biogas production in a two-tank process was efficient for mussels including the shells, seaweed, and a mixture of seaweed and manure, as most of the biogas was produced in the second reactor. In the case of the mussels, the shells remained in the first tank and were then easily removed. In addition, the digestion of a mixture of seaweed and manure reduced the effects of toxic substances such as sulphate and ammonia present at high concentrations in each of these materials. On the other hand, biogas production from the reeds or the manure alone was not efficient in the two-tank process since they degrade slowly. Hence, a one-tank process, which is a simple system to operate (even on farm-scale) could be cost effective for reed digestion.



Biogas production from seaweed and the reduction of contaminating heavy metals were also studied. As the seaweed contains high levels of heavy metals, the digested residue can not be used as a fertiliser. Biogas production and removal of the heavy metals were performed in the two-tank biogas process. During the breakdown of organic matter, the liquid produced ferments or sours, due to acid production. This process favours the release of metals, which can easily be removed. Removal of the metal was performed with a sponge-like material called Cryogel®, which is highly porous and has special metal-binding sites. The resulting liquid, with low heavy metal content, was used for biogas production. It was found that biogas can be produced from seaweed, and that the seaweed liquid, which was rich in nutrients, can replace the nutrients and buffer that are usually added to biogas processes. The heavy metals could be reduced using the two-tank system, but more research is needed before the residue is used as a fertiliser.



Finally, biogas and biohydrogen were produced from wheat straw, which is an abundant agricultural residue that does not compete with food cultivation. Since straw degrades slowly, and has a structure similar to that of reeds, the material was first treated to release the sugars into a liquid. The liquid was then used for biohydrogen production, and the resultant waste from this process was in turn used for biogas production, thus using most of the sugars contained in the liquid. Biohydrogen is produced in a similar, but incomplete process like biogas. The processes were very efficient, resulting in high production rates of biohydrogen and biogas. The only emission from the combustion of hydrogen is water vapour, and the addition of a small amount of hydrogen during the combustion of methane significantly enhances combustion. Hence, the production of such fuels from cheap renewable resources will be very beneficial for the environment and reduce climate change.



In conclusion, exploring land and marine organic materials and the pretreatment of slowly degrading materials can increase biogas production. In addition, the two-tank biogas process was versatile in handling a wide range of organic materials, and can be optimised for the combined production of biohydrogen and biogas. This system also offers the possibility of heavy metal removal to improve fertiliser quality.



The student’s advice to Peter was, thus, to use the cocoa pods for biogas production; providing renewable energy to dry his product especially during the rainy season, avoiding the use of firewood. The residue from the biogas process can also be used to improve vegetable production in Theresia’s farm hence, providing enough vegetables for the family.

• author: Nkemka, Valentine
• supervisor: Marika Murto
• opponent: Dr. Nordberg, Åke, Swedish University of Agricultural Sciences, Uppsala, Sweden
• organization: Biotechnology; Environmental and Energy Systems Studies-lup-obsolete (research group)
• publishing date: 2012
• type: Thesis
• publication status: published
• subject: Industrial Biotechnology
• keywords: steam pretreatment, dry digestion, biogas, biohydrogen, Caldicellulosiruptor saccharolyticus, cadmium, anaerobic digestion
• pages: 156 pages
• defense location: Lecture Hall B, Sölvegatan 39, Center for Chemistry and Chemical Engineering, Faculty of Engineering, Lund University
• defense date: 2012-11-29 13:15:00
• ISBN: 978-91-89627-87-1
• language: English
• LU publication?: yes
• id: 40713139-5b09-4ef0-b690-5057dd2db40d (old id 3163813)
• date added to LUP: 2016-04-04 10:04:04
• date last changed: 2023-06-01 13:11:47