VOCs Waste Gas Treatment in Coal Chemical Industry

What is coal gasification

Classification of Coal Gasification Processes:

• Raw materials: pulverized coal, crushed coal, coal water slurry;
• Gasification bed: fluidized bed, fluidized bed, fixed bed;
• Fixed bed is made of lump coal (15-50mm) as raw material;
• Fluidized bed is made of crushed coal (less than 10mm) as raw material;
• Airflow bed is made from pulverized coal (less than 0.1mm) as raw material; 

Survey Table of Crude Gas Components Produced by Different Gasification Processes

What is Low Temperature Methanol Wash

Low temperature methanol washing process: using cold methanol as the absorption solvent, utilizing the high solubility of methanol in acidic gases at low temperatures, to remove acidic gases from the feed gas, mainly CO2 and H2S.
Low temperature methanol washing is a method jointly developed by Linde and Lurgi in the early 1950s to remove acidic gases from raw material gases. In 1954, it was first used for gas purification in the coal pressurized gasification industry in South Africa.

• Low temperature methanol washing process package providers include Linde, Lurgi, Dalian University of Technology, etc;
• There are various gasification processes, which can be divided into three categories: fixed bed, fluidized bed, and fluidized bed;
• The determination of whether the low emission gas has residual heat recovery value mainly considers the CH4 content in the exhaust gas;
• The concentration of CH4 depends on the gasification process, and fixed bed gasification processes include Lurgi furnace, BGL furnace, etc;

Waste gas properties

Determination of oxygen supplementation air volume

• LM is the lower explosive limit of the mixed gas,%
• Li is the lower explosive limit of component i,%
• Vi is the volume fraction of a certain combustible component to the combustible component,%

Usually designed without considering the influence of inert gases, the lower explosive limit of exhaust gas is calculated, and the air dilution ratio is determined based on the relationship between exhaust gas concentration and 25% LEL. This calculation can ensure intrinsic safety, but the exhaust gas volume is relatively large.

Due to the presence of a large amount of inert gas CO2 in the low-temperature methanol washing exhaust gas N2， A small amount of combustible components,
According to the calculation method for a mixture containing n flammable gases and p inert gases, it can be determined that the low-grade mixed exhaust gas of flammable and inert gases is non flammable and non explosive.

Therefore, the exhaust gas from low-temperature methanol washing has no upper or lower explosive limits.
The amount of air replenishment for low-temperature methanol washing waste gas can be determined based on the oxygen content of the flue gas after complete oxidation being greater than 3%.

Calculation process of combustibility of mixed exhaust gas in air 

① Material Balance

 ② Heat balance

Typical engineering cases

Situation Analysis

Xinye Energy Chemical’s 525,000 tons/year methanol unit uses crushed coal pressurized gasification technology. In addition to the main components CO2 and N2, the low-temperature methanol washing exhaust gas also contains methane, non-methane total hydrocarbons, CO, methanol, etc. This exhaust gas is currently discharged through the boiler chimney. According to environmental protection requirements, VOCs removal treatment is required. In addition, the polyoxymethylene unit also has three exhaust gases that need to be treated.

Process route

Based on the characteristics of combustible components in exhaust gas, our engineers have decided to adopt the treatment technology route of “RTO purification+medium pressure steam waste heat boiler for heat recovery”; According to our company’s unique “Le Chater&Inert Gas Correction Theory Safety Air Distribution Algorithm”, we have decided to select a 270000 air volume rotary valve RTO, with an oxygen content of 5% in the exhaust gas after incineration; Simultaneously select a 5.1MPa/46T steam boiler with a 120 meter end chimney design to reduce the impact of exhaust emissions on the factory environment;

The main device adopts a single 270,000 air volume rotary valve RTO, square layout, equipped with 3 rotary air distribution valves and 36 heat storage chambers

• Comprehensive treatment:
  Equipped with a 270,000 air volume rotary RTO, polyoxymethylene tail gas mixed with air for oxygen supplement
  Waste heat utilization: 46t/h, 5.1MPa saturated steam
  
• Environmental protection standards:
  non-methane total hydrocarbon emissions <50mg/m³, annual carbon emission reduction of about 860,000 tons;
  
• Payback period: 3 years

Safety Analysis

1. Complex Control/Chain Briefing
2. HAZOP analysis
3. SIL rating

Innovation 1— Breakthrough in the lower explosion limit under inert conditions

Consider the effect of inert gas on the lower explosion limit

Considering only the oxygen supply, the air supply is 100,000, and the total air volume is 220,000

1. Air background, the lower explosion limit at 900℃ is 25%LEL;
2. Inert background, non-flammable and non-explosive at room temperature, but at high temperature?

Innovation 2— Design and application of square RTO structure with large air volume

√ Main economic indicators