Properties of pastes and mortars containing ammonia contaminated fly ash

Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) are widely used for NOx emission control in coal-fired power plants. In the SCR and SNCR processes, liquid ammonia (NH3(aq)) or urea is injected into the system or flue gas stream and reacts with NOx in the flue gas to yield diatomic nitrogen (N2) and water (H2O). Unreacted ammonia in the flue gas after the NOx reduction process is defined as slip and it contaminated the fly ash, yielding ammonia-contaminated fly ash. Ammonium sulfate (NH4)2SO4 and ammonium carbonate (NH4)2CO3 are the forms mostly found in ammonia-contaminated fly ash. This study is aimed to investigate the effects of ammonia that contaminate high CaO and low CaO fly ash on the properties of pastes and mortars. (NH4)2SO4 or (NH4)2CO3 solutions are set in a concentration range of 0 to 1000 ppm (mg of NH3 per kg of fly ash). The results confirmed that both (NH4)2SO4 and (NH4)2CO3 reduced the normal consistency and led to less water requirement of fresh pastes. (NH4)2SO4 decreased the compressive strength of the mortars. Oppositely, (NH4)2CO3 increased the compressive strength of the mortars. The addition of (NH4)2SO4 or (NH4)2CO3 increased the autoclave expansion of pastes. Sulfate expansion test results showed that both of (NH4)2SO4 and (NH4)2CO3 increased the sulfate expansion of the mortars, but the expansion was still lower than the OPC-only mortar. The formation of gypsum and ettringite was considered to cause the expansion of the paste and mortar specimens. The results of autogenous shrinkage indicated that (NH4)2SO4 and (NH4)CO3 decreased autogenous shrinkage of the mortars.

选择性催化还原（Selective Catalytic Reduction, SCR）与选择性非催化还原（Selective Non-Catalytic Reduction, SNCR）广泛应用于燃煤电厂的氮氧化物（NOₓ）排放控制领域。在SCR与SNCR工艺中，通常向系统或烟气气流中注入液态氨（NH₃(aq)）或尿素，使其与烟气中的NOₓ反应，生成双原子氮（N₂）与水（H₂O）。氮氧化物还原工序后，烟气中残留的未反应氨被定义为氨逃逸（slip），此类未反应氨会污染粉煤灰，进而形成受氨污染的粉煤灰。受氨污染的粉煤灰中，主要检出硫酸铵[(NH₄)₂SO₄]与碳酸铵[(NH₄)₂CO₃]两种赋存形态。本研究旨在探究污染高氧化钙与低氧化钙粉煤灰的氨，对水泥净浆与砂浆性能的影响。试验设置硫酸铵或碳酸铵溶液的浓度范围为0~1000 ppm（以每千克粉煤灰中氨的毫克数计）。试验结果证实，硫酸铵与碳酸铵均可降低水泥净浆的标准稠度，同时减少新拌净浆的需水量。硫酸铵会降低砂浆的抗压强度；与之相反，碳酸铵可提升砂浆的抗压强度。硫酸铵或碳酸铵的掺入会增大净浆的蒸压膨胀率。硫酸盐膨胀试验结果表明，硫酸铵与碳酸铵均可提升砂浆的硫酸盐膨胀量，但该膨胀量仍低于纯普通硅酸盐水泥（Ordinary Portland Cement, OPC）砂浆。研究认为，石膏（gypsum）与钙矾石（ettringite）的生成是导致净浆与砂浆试块膨胀的主要原因。自收缩试验结果显示，硫酸铵与碳酸铵均可降低砂浆的自收缩值。