Rationale for dry deacidification of baking soda instead of slaked lime

Slaked lime is no longer suitable for dry deacidification

   1 Low deacidification efficiency of slaked lime

Dry deacidification of slaked lime can not meet the more and more stringent environmental discharge standards.

Hainan local standard:

The dry deacidification efficiency of slaked lime is generally about 30% or even lower. For household waste incineration device, even if the concentration of SO2 in the flue gas is usually relatively low, the lime milk semi-dry method + slaking lime dry method can not guarantee the stable emission of flue gas below 20mg/m3. For most hazardous waste incinerators, dry deacidification does not account for more than half of the total deacidification load, even with large quantities of slaked lime.

2 Low utilization efficiency of slaked lime

The reaction of slaking lime Ca(OH)2 with HCl and SO2 actually depends on a layer of water film on the surface of the particles to proceed relatively quickly, so the semi-dry deacidification process requires as low as possible dew point temperature difference and as large as possible residence time.In dry deacidification process, there is no high humidity, low temperature and long residence time and other conditions, coupled with the quality of lime in different places, so with low deacidification efficiency at the same time, the utilization efficiency of slaked lime is far less than 50%.Another reason is that the molar volume of the reaction products of slaked lime is highly expansive, and the products such as CaSO3 soon clog the surface and pore of the reactants, resulting in a very low utilization rate inside the particles.

Baking soda not only has finer particle size, but also the thermal decomposition reaction occurs first in the reaction process. Due to the rapid release of CO2 and H2O, a large number of micropores similar to activated carbon are formed, which improves the specific surface area and avoids surface clog. This is also the reason why baking soda NaHCO3 is used instead of soda Na2CO3 directly.

3 Slaked lime produces more by-products

More byproducts are produced after the slaked lime reaction, with an average of more than 1.5kg of ash per kg of slaked lime:

According to domestic and foreign literature reports and statistical analysis of fly ash sample analysis data, a large number of soluble salts exist in fly ash of waste incineration, mainly in the form of KCl, NaCl, MgCl2, ZnCl2 and CaCl2. The mass fraction of soluble salts is generally 15%-30%, and most of them are concentrated above 20%.

CaCl2 is highly soluble in water, and CaSO3 is unstable. Therefore, the simple view that the byproducts of lime deacidification are insoluble is not accurate. According to the new Hazardous Waste Landfill Pollution Control standard (GB 18598-2019 replaces GB 18598-2001), which will be implemented on June 1, 2020, soluble salt < 10%, landfill disposal must go into a rigid landfill. In the utilization and development of fly ash by-salt resources, a large amount of Ca salt will inevitably cause scaling problems in membrane separation and evaporation equipment. Therefore, dry deacidification of slaked lime has no advantage in the disposal of by-products.

4 Slaked lime is not cheap

With the strict implementation of environmental protection, lime mine closure, lime kiln closure, transportation treatment and other measures, the cement price in some areas is close to 800 yuan /t, the price of slaked lime is more than 1400 yuan /t, the raw material cost of slaked lime has been comparable to caustic soda, and considering the by-product disposal cost, its cost has far exceeded baking soda dry deacidification. Dry deacidification of slaked lime has become the most expensive method of deacidification.