Preparation Of Briquettes Or Formed Cokes From Low Rank Coal And Bonding Mechanism Of Its Process

Preparation of briquettes or cokes?used for gasification? from low rank fine coals is a scientific and reasonable technological approach for low rank coal conversion.. At present, the low volatile carbonaceous materials?such as anthracite coal, coke, semi coke powder? as the main raw material for preparing briquettes or coke has been studied extensively, but the investigation of briquettes/coke preparation by cold briquettingand carbonizing the lignite and low metamorphic bituminous was limited. According to present research status, the meaning problems are as follows:1.The effect factors of briquette/coke strength have interactions, and the bonding mechanism is influenced by the type and properties of coal. All these issues have not been formed an unified understanding.2. The strength of the briquette/coke is impacted bythe briquetting and carbonizing process, which are not investigated systematically. Especially, the type, source and strength of the bonding forces and the corresponding impacted factors are not clear.3. The kind of binder is abundant, but the bonding mechanism of the binder and coals in carbonization process needs to be further clarified.In this study, low metamorphic bituminous SM coal was applied as the main raw material, the influence of briquetting conditions and carbonization conditions on the strength of briquettes or coke in molding process, and the bonding mechanism in cold and hot briquetting processwas investigated. The type, source and strentgh of bonding force in the briquetting process were analyzed in detail.and simulated by using MATLAB software. The pyrolysis characteristics and its kinetics of the SM coal, MC binder and the briquette were investigated as well. In additions, the influence of MC binder on the strength of different metamorphic grade coal were studied to find out the adaptability of MC binder to various coals.. The main results and conclusions of the study are as follows.1?The strength of the no binder briquettes was greatly influenced by the briquetting conditions as the rank of coal, briquetting load?40 150 KN?, particle size distribution, moisture content?8 24%? and among the parameters existed interaction. The compressive strength increased with decreasing particle size, increasing briquetting load and moisture content, while the drop strength first increased and then decreased slightly. The optimalproportion of pulverized coal?less than 0.125 mm? increased from 60% to 80% with increasing water addition. The influence sequence on Briquettes crack completely drop times was particle size distribution > moisture content > briquetting load. The optimal binderless briquetting conditions were: briquetting load 100 KN, proportion of pulverized coal?less than 0.125 mm? 60%, moisture content 15 20%. Under these conditions, the compressive strength of briquettes was 3.16 4.40 MPa, the drop strengthwas greater than 90%, and the crack completely drop times were 4.2?In binderless briquetting, Oxygen-containing functional group, potential characteristics and pore structure characteristics had significant effect on the bonding process. The more oxygen-containing functional group content and micropore volume in resulting of high wettability were benefit to binderless briquetting. The hydrogen bonding?O-H···O? formed between the oxygen-containing functional groups and liquid molecules on the surface of coal was the main bonding force in the binderless briquetting stage.3?The type of binder and its proportion were the main factors affecting the strength of briquettes. Both Organic binder?PVA, PR, AS, HPMC? and inorganic binder?Na2Si O3, bentonite? could increase the drop strength of briquettes significantly, but the change tendency varies with the added binder content. The wet and dry drop strength of briquettes were greater than 95% with the proportion of PVA, PR, AS, HPMC followed by 0.6%, 5%, 10%, 5%. Thedry drop strength were 94.6% and 96.6% as adding 20% of the Na2 Si O3 and bentonite, respectively, but the wet strength were as low as 86.1% and 84.1%.4?The briquettes conditions, carbonization conditions, binder type and its adding amount had different influences on the carbonization yield, Cr and the strength of briquettes or coke. The carbonization yield and Cr had changed with the content of adding water and the carbonization temperature obviously. With the increase of briquetting load, the pulverized coal?less than 0.125 mm? content, the moisture content and the MC binder content, the compressive strength of briquettes after carbonizing increased at different degrees, and the sequence of the influence factoris MC binder content>moisture content>briquetting load>particle size distribution. The drop strength of briquettes after carbonizing showed a trend of increase first and then decrease. With the increase of carbonization temperature from 350 oC to 850 oC, the compressive strength and drop strength of carbonization briquettes decreased first and then increased, and the maximum compressive strength was up to 9.10 MPa. When carbonization temperature was 450 oC, the compressive strength and drop strength reached a minimum value as 2.05 MPa and 69.2%.5?With the increase of carbonization temperature from 350 oC to 850 oC, the total pore volume and average pore diameter first decreased and then increased, and the BET areas had no significant variation; Aromatic layer spaceing d002 decreased from 3.58? to 3.52 ? gradually, aromatic nucleus effective packing height Lc and aromatic layer diameter La increased from 14.31 ? and 41.42 ? to 26.75 ? and 68.32 ?, respectively. The aromaticity fa was 0.30 0.63, which indicated that the degree of condensation of the aromatic rings increased gradually and the carbon network structure was more stable; Aliphatic C-H, aromatic C=C and C=O characteristic peaks became weak gradually, which illustrated that the process of carbonizing removed a large number of aliphatic side chain and the aliphatic bonds and some oxygen-containing functional groups; The adsorption and softening? 450 oC?, infiltration and adhesion?450 650 oC?, and the curing shrinkage?650 850 oC? of MC binder took in turns at the surface of coal.6?The bonding forces of briquetting stage were mainly: van der Waals force vF, liquid bridge lF, adhesion force aF, solid bridge sF and mechanical force mF. According to the numerical simulation and analysis, the factors such as characteristics of coal, radius of coal particles R, distance between coal particles s, surface tension s and viscosity m of liquid had different effects on bonding force. When briquetting with binder, the order of bonding force was: mF >sF >aF >lF >vF, the mechanical force and solid bridge played an important role in adhesiving process. When briquetting without binder, sF and l2 F were approximately zero, and the order of bonding force was: mF >aF >l1F >vF, the mechanical force and hydrogen bonding force were significant in the adhesiving process.7?Three samples mainly released H2, CH4 and CO2. The initial pyrolysis of MC started earlier than SM and the generated H2 promoted the pyrolysis of briquettes. The pyrolysis kinetics of SM coal, MC binder and briquette was described by a first-order reaction, and fitted using the Coats-Redfern integral method., The results shown that the fitting of SM and MC were good at every temperature stage, the fitting of briquettes was better at the temperature lower than 500 oC.. With the increase of carbonization temperature from 350 oC to 850 oC, the activation energy E of three samples increased first and then decreased. The activation energy E of SM, MC and briquette reached maximum value at 350 500 oC, which were 59.66 KJ·mol^-1?52.59 KJ·mol^-1?44.84 KJ·mol^-1, respectively.8?The Carbonization yield of BRXL lignite briquette was 48.83% byusing the MC binder, and carbonized at 850 oC after cold briquetting. of.The carbonization yield of SM, HS, KG, LA bituminous coal briquette was 60 65%, and the JC anthracite briquette was 77.66%. BRXL carbonization briquette had a density of 0.81 g/cm³ and compressive strength of 3.99 MPa. SM, HS, KG, LA carbonization briquettes had a density of 0.96 1.13 g/cm³ and compressive strength of 5.45 9.42 MPa. JC carbonization briquette had a density of 1.18 g/cm³ and compressive strength of 17.05 MPa.