Study On Pyrolysis And Combustion Characteristics Of Microalgae,Coal And Their Blends

There is an abundant of biomass resources on the earth. Exploiting and using biomassresources cannot only ease the energy crisis arising as a result of economic development, butalso mitigate environmental problems. The first generation biofuels, which are generated fromfood and oil crops, have some shortcoming, such as small biological yield, the occupation ofcultivated land. Besides, the weather is seriously than before and the rapid economic growthdemand more energy. All above mentioned promotes more experts and scholars to focus onsecond generation biofuels—microalgae.Compared with traditional biofuels, microalgae have high fat content, fast growth andshort growth cycle, a strong environmental adaptability, no occupation of cultivated land andreduction of carbon dioxide emission, etc. Thus, microalgae are recognized as the mostpotential alternative to fossil fuels. Many countries in the world are dedicated to study thetechnology of microalgae biomass.This paper studied the characteristics of a green freshwater microalgae—Chlorellavulgaris by an improved drying oven, thermal gravimetric analysis and microwave heatingequipment, etc. The two types of coal-fired power generation system with CO₂capturetechnology are analysis and evaluation by a life cycle assessment method, and using capturedCO₂captured to cultivate microalgae was explored. Many useful conclusions can be obtainedfrom this paper.Drying characteristics of Chlorella vulgaris under different moisture content, drythickness and the drying temperature were studied. The fitting results of ten common thin-layerdrying model showed that "Two term model" is the best one. The effective moisture diffusioncoefficient of Chlorella vulgaris is3.36×10^-9-2.23×10^-8m²/s. With the increase of temperature,the effective moisture diffusion coefficient is increased. The values of activation energy andpre-exponential factor of Chlorella vulgaris are E_a=34.12kJ/mol and D0=5.30×10^-4m²/s,respectively.The impact of microwave power, reaction atmosphere, microwave absorber, activatedcarbon addition, the solid residue addition and the amount of raw materials on the microwavepyrolysis of Chlorella vulgaris are studied. The results showed that, in N2atmosphere, themaximum oil production and weight loss per unit microwave power were obtained under thepower of1500W; The maximum gas production and residues production were obtained underthe power of2250W and750W, respectively; Without microwave absorbent, the bio-oil production was maximum, after adding microwave absorber, the best catalytic effect wasactivated carbon, followed by calcium oxide; The catalytic effect of the silicon carbide wasclose to the effect of solid residues; Oil production was maximum by adding3%activatedcarbon, residue production is maximum by adding10%activated carbon, the maximum gasproduction and weight loss is obtained by adding5%activated carbon; The maximum averagetemperature rising rate and the bio-gas production is obtained by adding5%solid residues, thelargest bio-oil production is obtained from pure samples, the maximum weight loss is obtainedby adding20%solid residue; The maximum gas production, the smallest residues production,the largest residue production and the maximum weight loss were obtained when using dosagesof Chlorella vulgaris sample were25g,35g,20g and35g, respectively; When the dosages ofChlorella vulgaris sample was15g, the unit solid, liquid and gas production was maximum. InCO₂atmosphere, maximum yield of solid, liquid and gas was at power of750W. Themaximum oil and residues productions were obtained by adding5%silicon carbide, themaximum gas production was obtained with no microwave absorber.Co-pyrolysis characteristics of Chlorella vulgaris and coal were studied by thermalgravimetric analysis. The results showed that the main pyrolysis zone of blends was172-600°C,which close to that of pure Chlorella vulgaris, but different from coal; Chlorella vulgaris andcoal did not exist synergy in the solid-solid phase during their co-pyrolysis. When thetemperature was higher than550°C, Chlorella vulgaris can hinder pyrolysis of coal; Theaverage reaction rate of the main pyrolysis stage and the final residue yield were approximatelystraight line relationship with the content of Chlorella vulgaris in the blend; The activationenergy of blends calculated by KAS method was in the range of320.77-416.01kJ/mol, and byFWO method,312.89-421.19kJ/mol. The activation energy of50%Chlorella vulgaris/50%coal is minimum.The combustion characteristics of Chlorella vulgaris under different oxygenconcentrations were studied by thermal gravimetric analyzer. The results showed that thesecond stage was the main de-volatilization and combustion stage; The DTG curves ofChlorella vulgaris had three obvious peaks, and the weight loss rate was maximum at the firstpeak; The main elemental constituents of Chlorella vulgaris minerals were sulfur, potassium,phosphorus, chlorine, magnesium and calcium, the influence of mineral matter content on theweight loss of Chlorella vulgaris is small; As the oxygen concentration increased, the ignitiontemperature, the final combustion temperature and the residue mass of Chlorella vulgaristended to decrease, while the maximum reaction rates increased; With the increase of theheating rate, the ignition temperature, the weight loss rate of the peaks and the residual mass of Chlorella vulgaris combustion tended to increase; When the oxygen concentrations rangingfrom20vol.%to80vol.%, the activation energy were134.28-241.69kJ mol-1; The optimaloxygen concentration for oxygen-enriched combustion of Chlorella vulgaris was25-35vol.%.Two CO₂capture technologies, oxy-fuel combustion technology and post combustioncapture technology used in the300MW coal-fired power generation units were studied by a lifecycle assessment method. The energy consumption and pollutant emissions of the powergeneration system with oxy-fuel capture technology in a life cycle were carried out, and theresults compared with post combustion capture technology. The CO₂captured from the powergeneration system with oxy-fuel capture technology used to culture microalgae was discussed.The results showed that the oxy-fuel combustion system to generate1kWh of electricity in alife cycle consumed more energy (0.065MJ) than post combustion capture system, and theefficiency of the system was less0.17%than post combustion capture system; Environmentalimpact load of oxy-fuel combustion system to generate1kWh of electricity was9.06382mPET2000, which was less4.61%than post combustion capture system; When CO₂capturedfrom300MW oxy-fuel combustion power generation system was used to cultivate microalgae,the production of microalgae was1728.99t/d.