Page 182 - New Trends In Coal Conversion

Technologies for control of sulfur and nitrogen compounds and particulates 145

6.2.1.3.2 In-combustion control techniques
Low NO x burners (LNBs). NO x control in LNBs is based on maintaining a lower flame
temperature and staged mixing of the fuel and air in the flame. The challenge is to bal-
ance temperature, O 2 profiles, and mixing time to obtain both combustion efficiency
and low NO x operation. In the primary combustion zone, about 65% of the fuel is
injected plus all of the air. The rest of the fuel is injected downstream of the primary
combustion zone. Moreover, staged fuel can reduce NO x formation by up to 60% for
gas-fired boilers and heaters.
Remarkable novel technologies in this field are as follows:

• Flame anchor low NO x burner. In this case, an LNB is configured in a special way of oper-
ation that hinders the formation of NO x . Combustion reactions are supported at a selected
fuel mixture when anchoring the flame by exposing it to an electrical charge, which can
be an electrostatic charge provided through a corona discharge. The combination of burner
geometry and electrostatic anchoring can support a flame with a relatively low temperature
and a lower concentration of oxygen. This leads to lower NO x production (Patent US 2015/
0140498 A1, 2015).
• Oxygen-rich low NO x burner. The objective of this technique is to adapt the low NO x tech-
nology to an oxygen-enriched combustion to achieve higher energy efficiency while
ensuring low NO x formation (Patent CN103791495A, 2016).
Over fire air (OFA). This technique is focused on reducing NO x formation by
creating a fuel-rich primary combustion zone through the removal of a portion of
the air flow from the burners instead of leading it to the primary combustion zone
(about 10%e20% of the primary air). The air supplied by the over fire air ports facil-
itates the full combustion of the CO produced in the primary combustion zone (CECO,
2017).
Close-coupled over fire air (COFA) and separated over fire air (SOFA) are two
kinds of different systems to reduce NO x emissions. The first one can achieve NO x re-
ductions of between 30 and 50. If higher NO x reductions are required, they could be
accomplished using SOFA, which can achieve reductions from 40% to 60% (Bell and
Buckingham, 2010).
Flue gas recirculation (FGR). The aim of FGR is to reduce the oxygen content of
the combustion air by mixing it with recycled flue gas, typically at a rate of 9:1. This
technique is also used in gas-fired and oil-fired furnaces (Bell and Buckingham, 2010).
The equipment necessary for retrofitting a coal-fired unit with this technique are duct-
works, fans, and a fly ash collecting device. Some operational problems in the flame
and increased steam temperatures could be produced by excessive recirculation
(IEA, 2017). Flue gas recirculation alone in coal-fired boilers achieves a low NO x
reduction efficiency (<20%). This is because the ratio of thermal NO x to total NO x
emissions is relatively low in coal-fired plants. The technique is being used in coal-
fired units in combination with other primary measures for NO x control (IEA, 2017).
Burners out of service (BOOS). Fuel-rich and fuel lean zones are created by
disabling one or more burners, obtaining a control of about 10% of NO x emissions.
This technique is difficult to carry out in some types of pulverized coal-fired plants
(IEA, 2017). BOOS has been applied successfully in many industrial and utility