Because the design and operation of primary coolers in China have not been standardized, primary coolers are generally prone to blockage, which requires frequent cleaning, affects pressure control in the gas collecting main, causes coke oven smoking, and increases blower power consumption. To address this, the Process Optimization Working Group of the China Coking Industry Association carried out detailed theoretical calculations, built a primary-cooler spraying test platform, and implemented pilot retrofits and operation. These efforts resolved the blockage problem in primary coolers, ensured long-cycle stable operation, and effectively and significantly improved the removal rate of naphthalene in coke oven gas. In July 2025, the group standard “Process and Technical Specification for Coke Oven Gas Primary Coolers” (T/CCIAA 34-2025), edited by Shandong Metallurgical Design Institute Co., Ltd., was released. To better help technical management personnel in coking enterprises implement this group standard, the Process Optimization Working Group will provide an interpretation focusing on the causes of primary-cooler blockage, theoretical calculations for spraying, and key operating points.

I. Analysis of the causes of primary-cooler blockage
During the stepwise cooling of coke oven gas in the primary cooler, tar in the gas phase gradually condenses into tar droplets, and gaseous naphthalene gradually desublimates into solid naphthalene. Because solid naphthalene has a large specific surface area, it readily adheres to the pipe walls, with tar droplets and dust attaching as well. Continuous accumulation in this manner leads to blockage of the primary cooler. As temperature continues to decrease, tar fluidity diminishes, which aggravates the degree of blockage. The purpose of spraying an ammonia-liquor/tar mixture (hereinafter referred to as the spraying liquor) into the middle and lower sections of the primary cooler is to dissolve gaseous naphthalene, solid naphthalene, and agglomerated deposits in the coke oven gas into the ammonia-liquor/tar mixture and route them to the ammonia-liquor/tar separation unit, thereby preventing primary-cooler blockage. Among the influencing factors, the composition of the spraying liquor is critical; spray uniformity and spray density are also important.

The spraying liquor is in fact a mixture of ammonia liquor and coal tar; its composition is expressed by the mass fraction of coal tar in the spraying liquor and the mass fraction of naphthalene in the coal tar. Some domestic practitioners refer to the spraying liquor as “light tar,” but in fact light tar should refer to coal tar collected from the primary cooler and the electrostatic tar precipitator, which alone may be called light tar.

In the early 1990s, Beijing Coking & Chemical Plant introduced the German AS process, carrying out a systematic overhaul of the residual ammonia-liquor deoiling, primary-cooler spraying, electrostatic tar precipitator, and circulating-water systems. The AS process has operated well, with the primary-cooler pressure drop remaining stable at below 800 Pa for many years. The former Shougang Coking Plant also used the AS process with stable primary-cooler resistance. The key point is to continuously supplement product tar into the lower-section spraying liquor of the primary cooler and maintain the tar proportion in the lower-section spraying liquor.

II. Theoretical calculations

The theoretical calculation of the spraying liquor first determines the naphthalene generation (desublimation), coal-tar generation (condensation), and condensate-water generation within the primary cooler; then it calculates the saturated solubility (by mass) of naphthalene in coal tar at different temperatures; and finally it calculates the required amount of product coal tar to dissolve the generated naphthalene.

Assumed conditions: primary-cooler inlet temperature 78 °C, middle-section outlet temperature 40 °C, lower-section outlet temperature 21 °C, average pressure −1000 Pa.

1. Calculation of naphthalene generation
   Naphthalene sublimates at ambient temperature and pressure and has a melting point of 80.2 °C, so its formation in the primary cooler can be regarded as desublimation, i.e., only solid naphthalene is formed. According to the “Physicochemical Constants for Coking Production” (p.73), which lists saturated naphthalene vapor contents in coke oven gas at different temperatures and pressures (see Appendix Table 1), the saturated naphthalene vapor contents at the middle-section and lower-section outlets of the primary cooler can be calculated as 3511.1 mg/Nm³ and 479.4 mg/Nm³, respectively. Based on literature review and actual test results from Yizhou Technology, the naphthalene content at the primary-cooler inlet is estimated to be about 7500 mg/Nm³. Therefore, the naphthalene generation in the middle section is 3988.9 mg/Nm³, and that in the lower section is 3031.7 mg/Nm³.

2. Calculation of coal-tar generation
   Basis (literature data and association statistics): the tar recovery rate by ammonia-liquor spraying in the riser is 60%, with the remaining 40% unrecovered; the coal-tar content at the electrostatic tar precipitator (ETP) inlet is 5 g/Nm³; tar yield is 3% (on dry coal); the coke oven gas yield (dry gas) is 320 Nm³ per ton of dry coal (charged coal Vd 25.5%). From these, the coal-tar content in coke oven gas at the furnace top is 93.75 g/Nm³, and at the primary-cooler inlet is 37.50 g/Nm³; the naphthalene content in tar is 10%.

Naphthalene distribution (per Nm³): naphthalene contained in coal tar is 9.375 g/Nm³, of which 7.021 g/Nm³ is generated in the primary cooler, and 0.5 g/Nm³ in the ETP (with the generated tar containing 10% naphthalene). Thus, the naphthalene content in coal tar recovered by the riser is estimated as
$(9.375 − 7.021 − 0.5) / (93.75 × 60$.
This indicates that naphthalene in coal tar mainly originates from the primary-cooling process.

The coal-tar generation in the primary cooler is 25.48 g/Nm³ (after deducting the naphthalene generated in the primary cooler). The coal-tar generation rate in the primary cooler is 0.447 g/(Nm³·°C) (assumed linear and proportional). Therefore, the coal-tar generation in the middle section is 16.99 g/Nm³, and in the lower section is 8.49 g/Nm³.

3. Calculation of condensate-water generation
   Basis: according to the “Physicochemical Constants for Coking Production” (see Appendix Table 2) on the gas volume and water-vapor content of coke oven gas at different temperatures, the dew point of the gas at the primary-cooler inlet is 76 °C (i.e., 2 °C lower than the inlet temperature). The condensate-water generation in the middle section is 469.43 g/Nm³, and that in the lower section is 42.97 g/Nm³.

4. Calculation of primary-cooler condensate
   Assuming all solid naphthalene dissolves into coal tar, the total condensate generated (condensate water + coal tar + naphthalene), in g/Nm³, is:

• Middle section: $469.43 + (78 − 40) × 0.447 + 3988.9/1000 = 489.90$

• Lower section: $42.97 + (40 − 21) × 0.447 + 3031.7/1000 = 54.49$

Mass fraction of naphthalene in the tar phase of the condensate:

• Middle section: $3.989 / (16.99 + 3.989) = 19.02$

• Lower section: $3.031 / (8.49 + 3.031) = 26.30$

Based on literature estimates, the saturated solubility of naphthalene in coal tar is about 35% at 40 °C (by mass) and about 20% at 21 °C; the measured value by Yizhou Technology is 19.54%. Therefore, the tar generated in the middle section can dissolve the naphthalene generated there, whereas the lower section must be supplemented with product tar to dissolve its generated naphthalene. Otherwise, solid naphthalene will block parts of the gas pipeline upstream of the exhauster (blower); the remainder, after being pressurized by the blower, will resublime into gaseous naphthalene, then desublimate again in the desulfurization precooler and benzol-washing final cooler, blocking equipment once more, and may even affect gas utilization and coke-oven heating all the way downstream.