Pyrolysis And Combustion Experiments Mechanism Research Of Seaweed Biomass

As a limited energy source, fossil fuel could hardly afford long-lasting consumption. And environmental contamination becomes an apparent and serious issue. All facts hasten a growing concern to the exploitation and application of biomass. Many countries pay attention to the importance of biomass that is an abundant renewable energy resource. Such as Denmark, the Netherlands, Germany, France, Canada, Finland and other countries has been conducting their own research and formed a distinctive biomass energy R&D system for many years. But the energy plants around the world is mainly focus on wood and crops, in fact the U.S. Science magazine said that in some cases, the use of food crops such as bio-fuel, not only fail to mitigate climate change, it actually are likely to increase greenhouse gas emissions.The development of biomass energy properly handles the relationship between energy and food, so the development of new biomass sources becomes particularly important. Seaweed biomass live in the sea, not occupy area land. Its resources development has great potential. In particular, China are surrounded by wide coastal areas and territorial seas. If those rich seaweeds sources are explored efficiently and put into clean and proper use, they may contribute a lot to the future of our country in the competition of the worldwide energy usage, both theoretically and industrially.The research on this seaweed biomass is still relatively small around world, which is a novel topic. In this paper, the thermochemical conversion processes (combustion and pyrolysis) of the typical large seaweed were studied using a systematic experimental and theoretical method.Researching fuel characteristics is the foundation of studying seaweed biomass. In chapter 2, the seaweed biomass is studied from proximate and ultimate analysis. It is found that the heating values of the seaweed samples are low and the moisture and ash contents of seaweed are high. In relation to the elementary analysis, seaweed has a lower oxygen content than terrestrial biomass. The low ash fusion temperatures of seaweed make the ashing temperatures recommended by both GB and ASTM norms to exceed the limit of temperature to prepare seaweed ash. At a high ashing temperature, Gracilaria cacalia and Sargassum natans will generate some high-melting matters which influence the identification of the ash fusion points. So it is more exact and referenced that the seaweed biomasses were ashed at a lower temperature (such as 530oC). The ash fusing characteristics on three sorts of seaweed (a kind of marine biomass) were studied by using thermal microscope, X-ray diffractometer, ash composition analysis, and simultaneous thermogravimetry/differential thermal analysis. It was presented that there were lots of alkali metals especially K and Na in all seaweed ash samples. XRD analysis shows that the crystalline phase intensities of alkali chlorides reduce with increasing the ashing temperature, due to the evaporization of alkali chlorides. Therefore the evaporization of alkali chlorides in seaweed biomass should be considered during the thermal conversion.At the same time, the specific heat capacities of three sorts of seaweed (a kind of marine biomass) during 40 oC -550 oC were measured by using the NETZSCH DSC404 differential scanning calorimeter, which were modified with the mass loss on the base of traditional solution method. The results show that the heating process are composed of three main intervals, dehydration, large devolatilization seni-coke state, during which the specific heat capacities are great different. The cause is that the residues in every interval are changed. Comparing the specific heat capacities of three sorts of seaweed, Gracilaria cacalia is the largest, Enteromorpha clathrata the second and Sargassum natans the lowest. In this paper, the mathematic relations for specific heat capacities and temperatures are presented during 40 oC -550 oC. The results can be a reference for the thermal chemical conversion energy utilization and numerical simulation of seaweed biomass.Thermal analysis was a important method used to analyze the pyrolysis and combustion process. In chapter 3, pyrolysis and combustion experiments of Enteromorpha clathrata (ENT) (a species of seaweed) have been conducted using a DTG-60H Thermal Analyzer. The results indicated that the non-isothermal mass loss process of samples is composed of dehydration, rapid mass loss, slow mass loss and solid residue decomposition. The devolatilization stage of ENT started earlier than that of woody biomass because the basic components in seaweed are preferable for pyrolysis compared to lignocellulosic materials. The FTIR analysis was employed to investigate the changes in the main components of sample, while the TG-MS analysis was used for the gaseous products analysis during the pyrolysis. The characteristic parameters of pyrolysis at different heating rates showed that the maximum rate of pyrolysis mass loss, the peak temperature, the initial and final temperature for devolatilization, and the heat release would increase with increasing heating rate. The kinetic parameters were calculated by using the Coats–Redfern method, which indicated that the second order mechanism function was suitable for the pyrolysis of Gracilaria cacalia and Laminaria japonica. The Zhuralev,Lesokin and Tempelman mechanism function was suitable for the pyrolysis of Enteromorpha clathrata and Sargassum natans at a low temperature; the second order mechanism function was also suitable for their pyrolysis at a high temperature. The ignition mode of seaweed was homogeneous and the ignition temperature was low. The combustion process was composed of dehydration, the pyrolysis and combustion of volatile, transition stage, the combustion of char and the reaction at high temperature. And the combustion characteristic parameters were obtained such as ignition temperature, maximum rate of combustion, burnout temperature etc. The combustion models of these seaweeds were also analyzed. The combustion characteristics and model differences between the seaweed and woody biomass were caused by the differences of volatile components. TG-MS analysis was used for the gaseous products analysis during the combustion. During the combustion of seaweeds, NO2 emissions were similar with CO2 emission. They were both accord the mass loss peaks in DTG curve and exothermic peaks in DTA curve.The obvious SO2 in the combustion stage were likely caused from the decomposition and oxidization of sulfated polysaccharide. However the SO2 emission at the high temperature was about the decomposition of S-containing ash. Several NO emission peaks in the low temperature region were obvious. The cause was mainly associated with the protein in seaweed. At last, activation energies were determined using Arrhrnius model that was solved by binary linear regression method.In the chapter 4, the fluidized bed combustion of seaweed particles (Enteromorpha clathrata and Sargassum natans) was studied in a bench scale combustor. The devolatilization times of seaweed in fluidized were short, are about 1 minute. When Enteromorpha clathrata particles were put into the fluidized bed, dehydration and the release and combustion of volatile happened, followed by burning char that accorded with the shrinking core model. The carbon burnt layer by layer from outside to inside the nuclear, while the ash layer was almost constant radius. Sargassum natans particles fractured immediately soon after they were fed into the fluidized bed, which was due to the release of a large number of volatile. SEM analysis was used to study the cross-sections of Enteromorpha clathrat particles at different fluidization times. With the combustion of EN particle, the pores inside the particle increase. The micro-pore surface appeared rough, and even partial happened to melt. During the continuous combustion experiments in fluidized bed, if the feeding Enteromorpha clathrat matched the air, they can burn stability at 750 oC. Slagging and big temperature fluctuations were not been found. Particle size distribution of bottom ash mainly concentrated around the value of larger and smaller values. Sargassum natans can burn at low temperatures, but there would be a serious slagging with the combustion process, which mainly due to its low ash melting point. Therefore the fluidized bed combustion was suitable for Enteromorpha clathrata, while Sargassum natan was opposite. At last, Enteromorpha clathrata and its bottom ash were collected for pore structure analysis. The number of porosity, pore volume and specific surface area increased after combustion. The expansion internal pore can be seen in ash. Fractal analysis showed that original seaweed has smooth surface. After combustion, the surface was irregular and rough.In order to know more about the combustion characteristics of single seaweed particle in fluidized bed, the fluidized combustion experiment of several seaweed particles (Enteromorpha clathrata, while Sargassum natan) were studied. When Enteromorpha clathrata particles were burning, the relative concentration of NOx increases, while the relative concentration of CO decreased. The SO2 and NOx released from Sargassum natan were more than that from Enteromorpha clathrata. The CO concentration also decreased with increasing the bed temperature. The devolatilization of two kinds of seaweed in advance, and the burnout times are both shorten, because the rate of heat transfer increases due to the raise of bed temperature. However the velocity of air flow had no regular influence as bed temperature on the gas concentration of Enteromorpha clathrata and Sargassum natan. Increasing the velocity of air flow made two kinds of seaweed burn easily. With the change in bed height, two kinds of seaweed were not found in the gas precipitation significant to change regularly. The increasing in bed height made the heat carrier increase, which helped to burn. The combustion time had also been shortened.By analyzing the influence of three factors (bed height, bed temperature, fluidized air velocity) on the fixed carbon residue of Enteromorpha clathrata particles after combustion, the result showed that the bed height is the minimum factor on the fluidized combustion of seaweed. The influential extent of operating parameters on the burnout of fixed carbon was sequenced by the gray relation analysis: bed temperature (℃)> fluidized air velocity(m/s) > bed height (mm). The result showed that the bed temperature was the main influence factor on the combustion in the range of working conditions.At last, the mathematical model for the fluidized bed combustion of Enteromorpha clathrata single particle was established. The simulation resulted generally consistent with the experimental comparison.In chapter 6, seaweed biomass fast pyrolysis process was studied. Analysis of working parameters of various types of seaweed biomass pyrolysis characteristics were investigated. This chapter analyzed the effects of main factors of seaweed biomass pyrolysis work condition on pyolysis characteristics.Pyrolysis reaction temperature had a certain influence on the bio-oil yield of seaweed. The increase in reaction temperature was benefit to generate pyrolysis and no good for generation of carbon. At about 500℃, there was the maximum liquid yield. Retention time (carrier gas flow rate) had little effect on the seaweed bio-oil yield. The bio-oil yield increased slightly with reducing the residence time. The liquid yield from protein pyrolysis was high. The fast pyrolysis of protein produced little char. The result indicated that the protein was easier to pyrolysis. The volatile proportion in char pyrolyized decreased with the increasing the pyrolytic reaction temperature, while the fixed carbon proportion increased, which indicated that the higher temperatures had more volatile released. The testing results showed that seaweed bio-oil has high hydrocarbon content and the advantage of low oxygen content compared with the general land biomass. At last, the chapter discussed that the char and incondensable gas obtained from pyrolysis of seaweed can provide the heat that is needed in pyrolysis process. Therefore the energy self-balance, seaweed fast pyrolysis for bio-oil production and combustion of semi-coke and incondensable gas, can be achieved.To further understand the characteristics of seaweed bio-oil, Chapter 7 analyzed two kinds of seaweed bio-oil (Enteromorpha clathrata and Sargassum natans) obtained under different conditions by using GC-MS analysis. The main components of seaweed bio-oil were obtained. A number of major components of seaweed bio-oil were hydrocarbon, ketones, aldehydes, alcohols and phenolic compounds, as well as large molecular weight carboxylic acids and their derivatives, and includes a small amount of heterocyclic compounds (derivatives of furan, pyran, pyridine, etc.).There were lots of hydrocarbons, carboxylic acids and their derivatives in Enteromorpha clathrata oil. While Sargassum natans oil contains many steroids and alcohols compounds. The oleic acid, palmitate, and peanut acid were also detected in Sargassum natans oil. The bio-oils pyrolyzed under different work conditions were very similar in composition, but the relative contents of compositions were different. Pyrolysis temperature played an important role on the distribution of seaweed oil compositions, while the influence function of the carrier gas flow rate was not obvious.The chapter also studied the mechanism of fast pyrolysis of seaweed biomass. The majority of nitrogen-containing compounds in seaweed oil are related to the thermal decomposition of the protein. The hydrocarbon materials are mainly consistent with carbohydrate and lipid substances. Pyrolysis of proteins includes the thermal decomposition of peptides and amino acids in general. The main mechanism of pyrolysis of amino acids is draw off CO2 molecules reaction, as well as the Strecker reaction of poly solution, and then continue to pyrolysis reaction resulting the formation of carbonyl-containing and two carbonyl compounds. Little lipid in seaweed can also generate hydrocarbons and esters from the decarboxylation and substitution reaction during pyrolysis process.The dissertation ends with summary and research prospects.