Combustion of lignin-oil-water mixtures in a rotary kiln

• Main Author: Thammachote, Nualpun
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/1484

Lignin recovery from black liquor has been proposed to de-bottleneck recovery boiler limited Kraft mills. The precipitated lignin would be used as a fuel in the lime kiln, replacing the external fuel, such as natural gas or fuel oil, presently used. In this work lignin-oil-water mixtures were investigated as a fuel. The rheology of the lignin-oil-water mixtures was studied; and a pilot scale preparation facility and firing system were devised. Tests were then made in the 0.4 m inside diameter, 5.5 m long UBC pilot scale lime kiln. The lignin was purchased in dry powder form from Westvaco Co., USA; the oil used in this experiment was No. 2 fuel oil; and a small amount of surfactant, Tergitol NP-9, from Sigma Chemical Co., was added to lignin-oil-water mixtures. The rheology of lignin-oil-water mixtures was found to be complex. The lignin-oil-water mixture viscosity was measured using the Haake (Model VT 500) viscometer. The viscosity results show time-dependent, both thixotropic and rheopectic, behavior depending on the solid content in the mixture. The lignin-oil-water mixture viscosity at steady state was found to be a function of shear rate. The higher the shear rate, the lower the viscosity of lignin-oil-water mixtures in the range of the shear rate studied (50-250 s⁻¹). At 25°C, the steady state viscosity of 37-47% lignin, 10-20% oil, 43-47% water mixtures was in the range of 0.3-0.7 Pa‧s at shear rate 100 s⁻¹. In the combustion experiments, lignin-oil-water mixture was prepared in a 43 litre tank with a mixer from McMaster Carr Supply Co.. A Moyno pump was used to circulate the mixture in the tank. A Masterflex pump system, which uses peristaltic action to propel fluid through the tubing, was used to control the volumetric flow rate of lignin-oil-water to the kiln by a variable speed drive. A double pipe heat exchanger was installed to keep the mixture temperature at about 30°C at steady state. Lignin-oil-water mixture was fed to the kiln via a nozzle inserted concentrically through a modified North America, Model NA 223G-3, natural gas burner. The nozzle had a separate water cooling jacket, and was connected to a twin fluid type, round spray pattern, stainless steel atomizer from Spray Systems Co. The conditions for each combustion experiment were set at a limestone flow rate 40 kg/h, kiln rotational speed 1.5 rpm and kiln inclination angle 1 degree. Natural gas fired tests were used as controls. The oxygen content in the flue gas was controlled between 2-3% for both natural gas and lignin-oil-water mixture firing. A gas chromatograph, an oxygen analyzer and a Fourier transform infrared spectrometer were used to measure the oxygen, carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen oxides, and methane concentrations in the flue gas. The results from the combustion experiments in the pilot lime kiln show that lignin-oil-water mixtures burned satisfactorily with a long luminous flame. The mixtures contained about 37-41% lignin, 12-20% oil, 43-47% water, and 1000 ppm surfactant. The percent calcination of the lime product from lignin-oil-water mixture firing was 99%.The reactivity of lime product from lignin-oil-water firing was comparable to that from natural gas firing. The flue gas during lignin-oil-water mixture firing contained on average 16 ppm CO, 352 ppm total NOx and 345 ppm SO₂. Compared to natural gas firing, the higher NOx level, and higher gas flue gas flowrates which could enhance dusting are potential disadvantages of LOW firing. As well, in the present work the sodium and sulphur balances were not closed. Further work is needed to explore these issues before mill scale trials are undertaken.