Study On The Combustion And Pollutant Emission Characterictics Of Biomass And Blending With Coal

Energy is an important substantial foundation of human's survival and development. It is the motive power of human being engaging in various economic activities. With the development of the social economy, the energy sources consumption is increasing quickly, but the energy shortage in all over the world is facing great challenge at present time. At the same time, the gas pollution caused by coal utilization is endangering the safety of resource, development of the social economy and human survival seriously. Facing this critical situation, new kinds of energy must be exploited and used to reduce or replace the application of fossil fuel. As a kind of renewable energy, biomass energy has great practical value. The co-combustion with coal is one of easy, simple and economical methods in using biomass, but its utilization is facing many difficulties and challenges.In this paper, the combustion characteristics of biomass, coal and their blending were studied by thermal gravimetric analysis (TGA). The effects of biomass species, heating rates and mixed proportions were studied and the dynamic parameters of combustion were obtained. The SO₂ and NO pollutants emission characteristics in combustion of biomass, coal and their blending respectively were investigated through a tube heating furnace and the influences of bed temperature and biomass proportions were discussed.First of all, a thermal analyzer modeled TGA/SDTA851 produced by Mettler-Toledo was used to study the effects of experiment parameters on the combustion characteristics. A kind of bituminous coal collected from Shandong Liyan power plant was used as coal sample, and the maize straw, rice straw, wheat straw, peanut shell, cotton straw and poplar chip were selected as biomass samples. In comparison, some studies on lignin, paper sullage and cellulose were performed also.The results show that, the ignition and burnout temperature of coal are 429.0℃and 626℃respectively, while the ignition temperature of biomass are 266.0℃, 271.7℃, 267.3℃, 286.8℃, 274.0℃, 306.5℃, 372.6℃, 317.2℃and 338.9℃respectively, and the burnout temperature are 480℃, 492.5℃, 443℃, 443℃, 384℃, 420℃, 1014℃, 487℃and 447℃respectively. Comparing with coal, the ignition and burnout temperature of biomass (excepting for lignin) are lower, the combustion characteristic index are larger, the occurring time of exothermic peak was earlier, and the burnout time was shorter. The blending of biomass and coal has lower ignition temperature than coal, and the volatile matter releasing and burning are uniform, then the burning of coke is going ahead of schedule. With the increase of heating rates (from 30 to 100℃/min), the maximum combustion rate of volatile and coke, burnout temperature, combustion characteristic index increase, but the peak value and its corresponding temperature increase at first and then decrease. With the increasing of biomass proportion to coal (from 5% to 30%), the ignition and burnout temperature, combustion rate of coke, the exothermic peak value and its corresponding temperature decrease. The combustion characteristic index decrease or increase at the period of coke or volatile combustion respectively. During the stage of volatile combustion, the values of activation energy and exponential factor of biomass are greater and the combustion rates are higher, and that of blending increase with the augment of biomass proportion but decrease with the heating rate. During the coke combustion stage, the values of activation energy and exponential factor of coal are small, but that of blending are smaller indeed, which decrease with the increase of heating rates and biomass proportion.Next, the effects of bed temperature and biomass proportions on the pollutant emission characteristics of SO₂ and NO were researched through a tube heating furnace. The results indicated that biomass have a little sulfur, some have no sulfur. Compared with coal, the peak value of SO₂ release of biomass is smaller and its release time is earlier. The conversion of SO₂ and NO of biomass are lower than that of coal. The conversion ratio of SO₂ is 76.19% with coal combustion while they are 22.9% and 36.63% for wheat straw and cotton straw combustion respectively, and no SO₂ releasing in maize straw and poplar chip combustion. Adding biomass in the coal, the conversion of SO₂ is reduced to some extent but with the increase of biomass proportion, the conversion ratio of SO₂ has no good regularity, while the conversion of NO decrease. When the biomass proportion are from 5% to 30%, the conversion ratio of NO decrease from 5.09% to 17.25%, 3.77% to 14.30%, 4.34% to 11.89%, 3.47% to 15.24% for the blending of coal with wheat straw, maize straw, cotton straw and poplar chip respectively. As the furnace temperature rise (from 850, 900 to 950℃), NO conversion ratio increases; the SO₂ releases moving up, the peak weakens and the conversion ratio of SO₂ increases, which reach 65.13%, 65.83%, 74.66% for maize straw, 68.76%, 69.58%, 72.99% for wheat straw, 63.87%, 67.78%, 75.58% for cotton straw and 67.55%, 73.07%, 81.11% for poplar chip respectively.At last, a BP artificial neural network was applied to forecast the emission characteristics of SO₂ and NO of different blending. By analyzing and calculating, two prediction models were created. For predicting SO₂, the trainbfg model was selected as training function, which node numbers of implication layer was 7. While for predicting NO, the trainlm model was more suitable as training function with 4 node numbers. 28 sample data were used to train and 4 samples data were used to check up the validity of the network. The result showed that the trained artificial neural network models were accurate enough to forecast the testing samples and met the need of error range. That means that the BP artificial neural networks created in this paper can be used to forecast the conversion ratio of SO₂ and NO.