(1) Ammonia-soda process: Using table salt (sodium chloride), limestone (calcined to produce quicklime and carbon dioxide), and ammonia gas as raw materials to produce soda ash, first introducing ammonia gas into saturated salt water, and then introducing carbon dioxide gas. Therefore, the answers are: NH3; CO2;

(2) The combined alkali production method involves reacting carbon dioxide gas with a saturated NaCl solution of ammonia to produce sodium bicarbonate. By decomposing the sodium bicarbonate, sodium carbonate can be prepared. The relevant equations for the reaction are: NH3 + CO2 + H2O + NaCl (saturated) = NaHCO3↓ + NH4Cl, 2NaHCO3

Na2CO3+CO2↑+H2O, so the answer is: NH3+CO2+H2O+NaCl (saturated) = NaHCO3↓+NH4Cl, 2NaHCO3

Na2CO3+CO2↑+H2O；

(3) CO2 is an important raw material in the alkali production industry. In the combined alkali production process, CO2 is derived from the calcination of limestone. In the ammonia-soda process, CO2 is sourced from the waste gas of the synthetic ammonia industry,

Therefore, the answer is: synthetic ammonia plant; calcination of limestone;

(4) Based on the analysis, the substances recycled in the "combined alkali production process" are NaCl and CO2. Therefore, the answer is: NaCl, CO2