the possibility of more efficient de-sox and De-NOx in CFB boiler furnace

Transcription

1 the possibility of more efficient de-sox and De-NOx in CFB boiler furnace Yue Guangxi Thermal Engineering Department Tsinghua University Beijing China

2 The development Low bed inventory CFB KPa bed inventory needs higher pressure head of draft fan. The average auxiliary power of CFB is 2-3% higher than PC combustion. 2 Coarse particle splashing causes serious erosion at inter-section between bottom refractory line and vertical water wall. This erosion has big impact on the availability of boiler operation. Ash transportation.217% Transformer 1.293% Compressor.867% SA fan.96% PA fan 2.22% Limestone crusher.19% Pump 1.373% ID fan 1.529% Auxiliary power total 8.33% Coal crusher.77% Approach: decreasing the overall bed inventory.

3 Possibility to decrease the bed inventory Bed particles can be distinguished as large un-entrained and small entrained particles. The former one composing dense bed at bottom and later one composing fast bed at upper part of furnace. The margin of bed inventory: The inventory of fine particle should be more than a limitation that creates circulating rate larger than Rmin. The bed inventory of coarse particles should guarantee enough resident time for large coal particle burn out. Chemical engineer and thermal engineer have different philosophy to deal with bed inventory. 炉膛高度, m 二次风 总悬浮浓度,kg /m 3 粗颗粒浓度,kg /m 3 As h cir cu lat in g ra te, G s, kg /( m 2 s) = Commend A ed B C D E F G H I + So Haft rd co coal al 细颗粒浓度,kg /m 3 5 Limits for erosion protection 8 1 Fluidized velocity, u f, m/s

4 G in (i) Solution for lower bed inventory The conceptual model of material balance in CFB boiler E(i)*X(i) X(i) F(i) 1 8 Effi cie ncy, 6 η I, % 4 2 Drain efficiency Bed material size distribution Particle size, d i, µm fractional accumulation efficiency of CFB η = η οi + η i -1 Cyclone efficiency Accumulation efficiency Frequency distribution P i %/µm G out (i)

5 First demonstration a CFB boiler with capacity 75T/h showed the bed inventory could be safe operating within KPa without big variation of temperature. Accumulative mass fraction,% t/h, 322Pa 75t/h, 383Pa 75t/h, 568Pa 75t/h, 733Pa 5t/h, 354Pa size of ash formation Coal size,µm 距布风板高度,m t/h, 322Pa 75t/h, 383Pa 75t/h, 568Pa 75t/h, 733Pa 5t/h, 354Pa 炉膛温度, o C

6 Commercialization of low bed inventory CFB technology Fan power saving 3% Erosion sharply decreasing Boiler efficiency increasing 1% Availability >6h/year Hundreds of units in operation instead of traditional CFB. 22T/h 75T/h 13T/h 44T/h

7 New challenge on the emission for CFB boiler New emission standard- GB SO 2 1 mg/nm 3 NO x 1 mg/nm 3 Particulate 3 mg/nm 3

8 General understanding on the De-NOx and De-SOx in CFB furnace Furnace NOx /mg.nm 3 NOx Fuel-N Volatile Temperature O 2 concentration CFB boiler PC boiler New standard 2mg/Nm Volatile Dry ash free basis, Vdaf /% 脱硫效率 /% SOx Limestone size Ca/S ratio Temperature O 2 concentration 温度 / o C SO 2 (ppmv) Fuel Limes tone CaSO 4 Air CaO 8 o C 9 o C CaS CO (ppmv)

9 The impact of bed quality on the De-NOx and De-SOx in CFB As bed quality improved: 1.Combustion in dense bed becomes more reducing- NOx SOx 2. Combustion in freeboard tends more reducing- NOx SOx 3.The utilization of limestone particle improved- SOx The overall effect of bed quality improvement looks like positive for emission control. Second ary air Lean Oxygen Core Second ary air

10 Case study for comprehensive De-NOx and De-Sox in CFB A demonstration of 26T/h CFB with improved circulating system was operating this year. The coal for this boiler: 编号 moistur e Ash volatil e High Fix carbon Sulfur Heat value Low heat value Mt Aad Vad FCad St,ad Qgr,ar Qnet,AR % % % % % J/g cal/g

11 The average pressure drop of freeboard is over 6Pa/m. D 5 of fly ash 1µm. D 5 of circulating ash 1µm Furnace T 87 C NOx CO [mg/nm 3 ] before cyclone After cyclone 139 After SH 36 <2 Ca/S =1.5 SOx <1 Observation: NO continuously reducted by CO or C in cyclone and SH. The utilization of limestone fines was improved dramatically. It is possible to meet the new emission standard for CFB without SCR, FGD.