Research On Dynamic Characteristics Of Coal And Biomass Blends And Desulfuration Characteristics Of Its Briquettes

Energy scourses and environmental problems have become two focuses throughout the world. Along with the drying up of fossil energy and environmental depravation, developing clean coal technology, achieving undefiled use of coal, empoldering and utilizing biomass energy have become a global task . Coal-Biomass briquette technology is a partial of clean coal technology with the characteristics of using biomass energy efficiently and reducing atmosphere pollution. In this paper, the pyrolysis , combustion and emission characteristics of Coal-Biomass briquette have been discussed, and much valuable information were obtained. The main contents of this research is as follows:1. By using TGA technology, the pyrolysis characteristics under different conditions of coal and biomass blends has been discussed.The pyrolysis process of pure coal and biomass have three stages, they are dehydration stage, main pyrolysis stage and remnant char producing stage. The temperature range of three stages of coal and biomass is different. The pyrolysis curve of blends shows the characteristics with two peaks. All the curves of blends are similar with the pure samples , which is related with the component of the blends. Results also show that there is partial intersection of the two components during pyrolysis process. With the increase of biomass component, the value of the first peak is increased , well the second peak is decreased, the intersection range becomes longer, the temperature of the max pyrolysis velocity becomes lower gradually.Heating rate has slight impact on the initial pyrolysis temperature of biomass. There are two peaks under low heating rate. With the increase of heating rate, the max pyrolysis velocity increases prominently, the second pyrolysis peak becomes less prominent, the whole pyrolysis time is longer. The increase of heating rate accelerates the pyrolysis process in forepart.Heating rate has slight impact on initial pyrolysis temperature of hard coal. There is only one peak on pyrolysis curve, with the characteristics of lower peak value decreases and moves towards lower temperature and longer pyrolysis time when heating rate increases. When heating rate increases to 50℃. min^-1, the coal shows incomplete pyrolysis in given temperature area.An exponent of pyrolysis characteristics of coal and biomass blends is defined which can reflect the characteristics of blends comprehensively. It offers theoretical basis for evaluating the pyrolysis characteristics of blends.Two phases parallel reaction model is founded to describe the pyrolysis process of blends. The dynamic parameters are worked out. The results show that the active energy is bigger well frequency factor is smaller in first phase.2. By using TGA technology, the combustion characteristics under different conditions of coal and biomass blends has been discussed.The combustion process of biomass has four stages, they are dehydration stage, volatile separating and combustion stage, char combustion stage and burn out stage. The combustion process of hard coal also has four stages, they are apparent weight increasing stage, volatile separating stage, char combustion stage and burn out stage.The combustion curve of blends shows the characteristics with two peaks. The combustion rate of blends is bigger evidently in forepart than in backpart. It shows strong heating release capability with blends than pure coal. The combustion rate of backpart is decreadsed with the increase of biomass component, with the result of lower heating release. By combustion of blends, the ignition performance is improved in different degree, the combustion process trends to lower temperature, heating release capability is boosted up, and the heating release distribution is improved .With the increase of heating rate, the initial volatile separating temperature increases slightly, the combustion rate increases, the max combustion rate trends to moveing to the higher temperature. The two phases of biomass combustion becomes unconspicuous, well still distinctive in blends combustion. With the increase of heating rate, the combustion range becomes longer, the combustion capability becomes stronger, the burn out temperature becomes higher, the max heating release peak trends to moving to higher temperature.An exponent of combustion characteristics of coal and biomass blends is defined which can reflect the characteristics of blends comprehensively. It offers theoretical basis for evaluating the combustion characteristics of blends. Two phases reaction with middle competetion model is founded to describe the combustion process of blends. The dynamic parameters are worked out. The results show that the active energy is bigger in first phase than in second phase. The impact to active energy of biomass component is slight in first phase well distinctive in second phase with quick dereasing in value. Different heating rate has slight impact to active energy in first phase, well distinctive in second phase with an irregular change maybe caused by experimental error.3. The impact on coal-biomass briquette combustion characteristics of different factors is checked by orthogonal experiment, with volatile rate, burn out rate and combustion time as targets.Results show that the impact of biomass component to volatile rate of the four facters in turn is biomass component ratio, hearth temperature, type of biomass and shaping pressure. To burn out rate in turn is hearth temperature, biomass component ratio, type of biomass and shaping pressure. To combustion time in turn is biomass component, hearth temperature, type of biomass and shaping pressure. Component ratio of biomass and hearth temperature have significant impact, well type of biomass and shaping pressure have slight impact to all targets.Ash melting characteristics of hard coal has higher target value with 250～300℃higher temperature compared with biomass. Co-firing with biomass changes the ash melting characteristics of coal and result in ash fusibility trendency.4. The desulphrization theory of calcic reagent during coal-biomass briquette combustion is analyzed, and the impact of different combustion conditions to combustion is also discussed. The results show that the sulfur retention is CaSO₄ if the briquette has completely combusted or it will be CaSO₄ and CaS. No other calic sulfide is found.The desulphrization rate of different briquettes is checked. The rate from high to low is coal biomass briquette with desulphrization reagent, general briquette with desulphrization reagent and general briquette with no desulphrization reagent. The sulfer retention in radial distribution is CaSO₄ in exterior , in interior are CaSO₄ and CaS when burned completely. On radial distribution, the sulfer retention content from high to low is coal biomass briquette with desulphrization reagent, general briquette with desulphrization reagent and general briquette with no desulphrization reagent.Combution temperature has distinct impact on desulphrization rate of coal -biomass briquette. Desulphrization rate increases with the increase of temperature. The desulphrization rate decreases when hearth temperature exceed 1000℃because of partial decomposition of CaSO₄ and agglomeration of CaO.Type of biomass and shaping pressure has slight impact on desulphrization rate well component ratio of biomass has distinct impact. The desulphrization rate increases when Ca/S ratio increases but has no distinct increase when the ratio value exceed 2. The Iron oxide and MgO as additive improved desulphrization rate of calcic reagent in a certain degree.Sulfer release of briquette is quick during forepart of combustion. The release amount from high to low is general briquette with no desulphrization reagent, general briquette with desulphrization reagent and coal biomass briquette with desulphrization reagent. The desulphrization reaction determines desulphrization rate in great degree during forepart of combustion.5. Thermal characteristics and emission characteristics of coa-biomass briquette are monitored in a small industrial boiler.Experiment results show that coal biomass briquette has stronger heating release capability than general briquette during forepart and weak heating release capability during backpart. The boiler starts up quickly by burning coal-biomass briquette. The thermal efficiency of boiler burning coal biomass is higher than burning general coal.Four kinds of briquette's soot letting concentration is between 0～20mg. Nm^-3, which is much lower than state standards.All briquettes shows two peaks characteristics in SO₂ release concentration during combustion. Thers is a higher concentration in initial stages for a short time, well a lower concentration during char combustion time for a longer time. SO₂ release concentration of general briquette is higher than coal-biomass briquette. Accession of biomass in briquette decreases SO₂ release amount in a certain degree. SO₂ release concentration of coal-biomass briquette with desulphrization reagent is lower than briquette with no desulphrization reagent during whole combustion period.