Petroleum coke pulping and its palpability and reactivity

To solve the problems of low volatile content and poor reaction activity of petroleum coke and realize the efficient and clean utilization of petroleum coke, the paper studied the pulping and pulping properties and reaction activity of petroleum coke from petroleum coke produced by a petrochemical enterprise in Nanjing and Shaanxi Province. The results show that the mass fraction of gasification, fuel, and superfine water coke slurry is 70% ~ 71%, 68% ～ 69%, and 67% ~ 68% respectively, and their viscosity, fluidity, and stability can meet the requirements of the application; The influence of superfine and combustion aids on the reaction activity of water coke slurry was analyzed by thermogravimetry. The results showed that superfine refining can significantly reduce the initial and burning temperature of volatile matter, and the addition of combustion aids can improve the ignition temperature and burning temperature of superfine water coke slurry.

Petroleum coke is a by-product of crude oil that is separated from light and heavy oil by distillation, and then converted into heavy oil by thermal cracking (delayed coking unit). Its physical form is similar to that of coal. It is a hard black or dark gray solid with metallic luster and porous. It is a granular, columnar, or acicular carbon body formed by tiny graphite crystals. According to the different sulfur content, it can be divided into high sulfur petroleum coke (sulfur content is more than 3%) and low sulfur petroleum coke (sulfur content is less than 3%).

Low sulfur petroleum coke is mainly used to make high-power electrodes, which can be used in electrolytic aluminum (anode paste and prebaked anode) and iron and steel industry (graphite electrode). Poor quality petroleum coke with high sulfur is mainly used as fuel and is commonly used in cement kiln, circulating fluidized bed (CFB) boiler, and gasification combined unit (IGCC). However, as the reactivity of petroleum coke is similar to that of lean coal, there are some problems such as the increase of fly ash combustibles and the decrease of boiler efficiency. In recent years, with the deep processing of crude oil and the deterioration and heaviness of crude oil in the world, the output of petroleum coke is increasing (about 29 million tons of Petroleum Coke will be produced in China in 2019). The clean and efficient resource utilization of petroleum coke has been widely valued.

Coal water slurry is low pollution, high-efficiency coal-based fluid fuel, and gasification raw material composed of coal, water, and additives. It has fluidity and stability similar to oil. It can be easily pumped, stored, transported, burned, and gasified. It can significantly reduce the emissions of SO2 and NOx. It can be used in power station boilers, industrial boilers, and industrial kilns to replace oil, coal, and gas, It can also be used as gasification raw material to convert it into syngas through coal water slurry gasification technology, which can be used to produce chemical products, such as synthetic ammonia, methanol, dimethyl ether, olefins, etc., as well as indirect coal liquefaction (oil production), combined cycle power generation (IGCC) and polygeneration based on coal gasification.

Based on the properties of petroleum coke and the characteristics of coal water slurry, some domestic institutions are also researching the preparation of coal water slurry from petroleum coke. East China University of science and technology [4-5] studied the pulping properties of two kinds of petroleum coke. The particle size distribution of petroleum coke for pulping was 74 μ m～420 μ M accounted for 30%, less than 74% μ M accounted for 70%; The results show that the slurry properties of the two kinds of petroleum coke are similar. When the viscosity of water coke slurry is 1 000 MPa · s, the slurry mass fraction is about 69%; The results show that the water coke slurry is mostly expansive plastic fluid with poor stability and hard precipitation within 24 h; When the industrial waste containing polyacrylamide was added as a stabilizer, the water coke slurry was completely transformed into pseudoplastic fluid, the hard precipitation time was more than 72 h, but its viscosity was improved, resulting in the mass fraction of petroleum coke slurry decreased to about 64%.

Maoming Thermal Power Plant of Yuedian group company researched the preparation of water coke slurry from petroleum coke in response to the rising coal price. The results showed that when petroleum coke was used as the sole pulping agent and the mass fraction of additives was 0.3% ～ 0.4%, the mass fraction of water coke slurry was 67% ～ 68%, the viscosity was 600 MPa · s ～ 1 000 MPa · s, the pulping stability was poor, and water separation and precipitation were easy; The mixture of petroleum coke and Shandong coal was prepared with the mass ratio of 2 ∶ 1, 1 ∶ 1 and 1 ∶ 2 respectively. The additive content was 0.4% ～ 0.5%, and the particle size of the mixture was less than 75% μ The results show that the mass fraction of CWS is 66% ～ 68%, the viscosity is 600 MPa · s ～ 900 MPa · s, and the stability of CWS reaches grade B in 24 h. Shandong University of science and technology has also carried out an experimental study on the preparation of coal water slurry from coal and petroleum coke under the condition of adding 0.7% naphthalene additives and stabilizers. However, the viscosity of coal water slurry prepared from coal and petroleum coke at various ratios is more than 1200 MPa · s.

Referring to the use and preparation technology of coal water slurry, the slurryability of petroleum coke for fuel preparation, gasification, and ultra-fine water coke slurry was studied experimentally in this paper. The slurryability effect of petroleum coke was evaluated, and the process conditions such as additive ratio and particle size distribution of petroleum coke for different water coke slurry were determined; At the same time, the problems of low volatile matter and poor reactivity of petroleum coke were solved by a superfine method, and the reactivity of gasification, fuel and ultra-fine water coke slurry were compared and evaluated by thermogravimetric analysis, which provided a reference for the preparation of high reactivity water coke slurry from petroleum coke.

1. Experiment

1.1 Analysis of physical and chemical properties of petroleum coke

Petroleum coke is a compound composed of carbon (80% – 95%), hydrogen (1.5% – 8.0%), sulfur, nitrogen, chlorine and heavy metals. The ash content is generally less than 2%, the volatile content is 6% – 12%, the calorific value is 30 MJ / kg-35 MJ / kg, the total moisture content is generally less than 3%, and the density is generally 800 kg / m3-1 800 kg / m3.

1.2 Experimental methods

1.2.1 Experiment on slurryability of petroleum coke

The slurryability is the standard of the difficulty of coal-water / coke slurry preparation and the performance of the slurry, which can be expressed by the highest slurryability; The highest slurry concentration is usually measured by the mass fraction of coal-water / coke slurry at room temperature (20 ℃), the shear rate of 100 S-1 and apparent viscosity of 1200 MPa · s.

The water coke slurry was prepared by dry pulping. The experimental process was as follows: the petroleum coke sample with particle size less than 3 mm was ground to the maximum particle size of 500 by rod mill μ m. Average particle size ≤ 100 μ And the maximum particle size is 1 000 μ m. Average particle size ≤ 200 μ The coke powder is used as the raw material of fuel water coke slurry and gasification water coke slurry respectively; In the preparation of raw coke powder for ultra-fine water coke slurry, the petroleum coke sample with particle size less than 3 mm was pre-ground by a rod mill, and then ground to the maximum particle size of 20 in a stirred fine mill μ m. Average particle size ≤ 15 μ M. The ground petroleum coke powder, combustion supporting agent for sodium chloride-based coal (0.15% of coke powder dry basis), a certain amount of naphthalene additives and water were added into the pulping container in proportion and stirred for a certain time; Then, the apparent viscosity and concentration of the prepared water coke slurry were tested, and the prepared water coke slurry was well preserved for experimental study on its characteristics.

The fluidity of the water coke slurry is measured by the observation method, in which a – has good fluidity and continuous flow; B – good fluidity, semi-continuous flow; C-has certain fluidity and intermittent flow; D-no liquidity. The stability of the water coke slurry was measured by the method of standing for 24 h.

1.2.2 Experiment on the reactivity of water coke slurry

The reactivity of fuel, gasification, ultra-fine water coke slurry prepared from Nanjing petroleum coke, and ultra-fine water coke slurry added with combustion supporting agent were analyzed by TG / DTG. The experimental conditions were heating rate 40 ℃ / min and working atmosphere air [10].

2. Results and analysis

2.1 Analysis of slurryability of petroleum coke

When the mass fraction of naphthalene additive is 0.5%, the highest slurry mass fraction of fuel water coke slurry can reach 68% – 69%, the highest slurry mass fraction of gasification water coke slurry can reach 70% – 71%, and the highest slurry mass fraction of ultra-fine water coke slurry can reach 67% – 68%. It can be seen that with the decrease of particle size of raw petroleum coke, the maximum slurry mass fraction gradually decreases; Compared with gasification water coke slurry, the highest slurry mass fraction of ultra-fine water coke slurry decreased by about 2%, mainly due to the increase of the specific surface area of petroleum coke after ultra refining and the obvious increase of viscosity of water coke slurry at the same concentration. In addition, with the increase of additive proportion, the maximum slurry concentration of water coke slurry increases, but under the condition of investigated additive mass fraction, the increased range decreases. Therefore, for the two experimental samples, considering the requirements of water coke slurry and the cost of additives, the optimal additive content of gasification water coke slurry is 0.3%, and the optimal additive content of the fuel and ultra-fine water coke slurry is 0.5%.

2.2 Reactivity analysis of water coke slurry

Using petroleum coke as raw material, the fuel, gasification, and ultra-fine water coke slurry prepared to have a higher concentration, lower viscosity, better stability, and fluidity, which meet the requirements of industrial boilers and gasifiers. However, due to the low volatile content and poor reactivity of petroleum coke, the influence of superfine and combustion supporting agents on the reactivity of water coke slurry was studied by thermogravimetric analysis.

3 .Conclusion

3.1 petroleum coke has the remarkable characteristics of high carbon content, high calorific value, low ash, and volatile, which can be used as raw materials for the preparation of water coke slurry. The mass fraction of gasification, fuel and ultra-fine water coke slurry prepared with petroleum coke as raw materials are 70% ～ 71%, 68% ～ 69%, and 67% ～ 68% respectively, and the viscosity, fluidity, and stability of water coke slurry can meet the requirements of industrial boilers and gasifiers.

3.2 through the experimental study of different additive ratios, considering the requirements of water coke slurry and the cost of additives, the best additive content of gasification water coke slurry is 0.3%, and the best additive content of the fuel and ultra-fine water coke slurry is 0.5%.

3.3 compared with the fuel water coke slurry, the initial temperature of volatile matter of ultra-fine water coke slurry decreased by 20 ℃, the burnout temperature decreased by 13 ℃, and the ignition temperature decreased only by 5 ℃, indicating that the initial temperature of volatile matter and burnout temperature of petroleum coke slurry can be significantly reduced after ultra refining, but the influence on ignition temperature is small.

3.4 after the addition of combustion supporting agent, the initial temperature of volatiles decreased from 450 ℃ to 447 ℃, decreased by 3 ℃, the ignition temperature decreased from 575 ℃ to 566 ℃, decreased by 9 ℃, and the burnout temperature decreased from 778 ℃ to 761 ℃, decreased by 17 ℃, indicating that the addition of combustion supporting agent can significantly reduce the ignition temperature and burnout temperature of water coke slurry.