1.2.1.1 Different biomass for co-combustion

Biomass includes forest wastes, agricultural wastes, animal wastes and anthropomorphic wastes. Considering co-combustion with straw and coal could achieve large-scale and efficient utilization, it is attracting more and more attention and research. Most methods for research are concentrated in the laboratory using thermal gravimetric analysis, or measuring the combustion characteristics of mixtures of pollutants (including toxic gases and heavy metals, etc.) emission characteristics and ash melting characteristics through combustion or pyrolysis of different coals and biomass. The conclusions gained through these methods are an important reference for the design calculations and material choices of biomass-fired boilers, but the site condition is quite different from the experimental condition.

(1) Co-combustion ofcoal and agricultural wastes

In the Northeast Institute of Electric Power Engineering, experimental research on co-combustion of coal and corn stalk were carried out (Lu et al., 2005). The results showed that the co-combustion of coal and corn stalk was helpful for coal burnout. With the increase of co-combustion rate from 20% to 80% (mass biomass to coal ratio), the burnout efficiency was increased, the burnout time was shortened and the burnout temperature was decreased. In Shandong Univer­sity, Zhang et al. (2006) researched on the characteristics of straw co-combustion with coal by the thermal gravimetric analysis method. Cotton stalk, cornstalk, wheat-straw were chosen for co-combustion with coal at different heating rate (30, 50, 75 and 100K/min) and different co-combustion rate (1:20,1:10, 3:20,1:5,1:4, 3:10). The results showed that the co-combustion of coal and straw was helpful for coal burnout. With the rise of heating rate, the ignition temperature of straw mixed coal was decreased and the rate of combustion was increased.

In order to making clear the effect of the alkali metal K on nitrogen conversion in co-combustion of coal and straw, a series of experiments were carried by Yang et al. (2009). The results indicate that it is effective to inhibit the release of NO to add a certain proportion of straw. When the content of K increases in the de-ashed coal samples mixed with a low proportion of straw and KOH, it has a stronger catalytic effect on the reduction reaction of NO, and when the content of K reaches a certain value, the catalytic effect does not increase. The lower the O2 content in the combustion atmosphere, the better the reduction of NO. Dong etal. (2010) have taken some experimental tests. The experiments were carried out at a 400 t/h power station boiler to test its economy and emission characteristics. Considering each operation controllable factor, the best running condition were optimized, which could keep better economy and emission performance. The optimized condition consisted of oxygen content 3.6%, combustion temperature 1278K, pulverized coal fineness R90 = 20%, straw particle size 15 mm, primary air with average coordination, secondary air with waist type, and co-combustion ratio 20% (a heat ratio value). [3]

is 0.3876 million tonnes, the proportion of treated garbage (%) is 77.9%, all which are shown at Table 4.2, Table 4.3 and Table 4.4.

As shown in Figure 4.2, garbage disposed by burning isn’t the most important way in China, which takes 22% and less than other disposing ways e. g. sanitary landfill. Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials (Andrew, 2005). Incineration can reduce the solid mass of the original waste by 80-85% and the volume (already compressed somewhat in garbage trucks) by 95-96%, depending on composition and degree of recovery of materials such as metals from the ash for recycling (Ramboll, 2006).

In China, researchers have focused on co-incineration performance tests and experiments of coal and different types of MSW Gu et al. (2003) focused on the co-combustion research of municipal sewage sludge and coal. With a thermogravimetric method, the research results showed that co-combustion could enhance activation energy with a lowering of the ignition temperature.

The fuels have basically attained devolatilization characteristics in the co-combustion process. Liu’s (2006) experimental research showed that the reactivity of the blend with 20 wt. % of sludge is similar to that of coal. When the blend is with 50 wt. %, there are two temperature zones with obviously different reactivity trends. In the lower temperature zone (less than 430°C), the reactivity of the blend is similar to that of the sludge, and in the higher temperature zone (greater than 430°C), the reactivity of the blend is close to that of the coal. Zhao etal. (2005) researched on co-combustion of sludge/residue in a paper mill with high moisture content and low heating value coal at the hot circulating fluidized bed test facility. His research showed that when the secondary air rate increases, temperature in the dense bed decreased slightly and temperature in the dilute phase region declined, while the combustion efficiency was increased. When the excess air coefficient was increased, temperature in the dense bed increased, temperature in the dilute phase region increased at first and then declined forming an optimum value corresponding to the highest combustion efficiency. When the ratio of paper mill waste to coal was increased, the decline in temperatures in both dense bed and dilute phase region was decreased, and the combustion efficiency was decreased. Lu et al. (2004a) indicated that co-combustion of sewage

sludge with coal on a circulating fluidized bed was stable at water contents of 30-60% in sewage sludge and co-combustion rates of 25-100%.

Co-combustion of coal and refuse derived fuel (RDF) were carried out in a bubbling fluidized bed combustor by Sun et al. (2006). The feasibility of solidification and co-combustion of waste

products in oily wastewater with coal was analyzed by Liu et al. (2005). The combustion process, ignition and burnout characteristics of waste tire and coal with a tire-coal ratio of 10%, 30% and 50% were investigated by means of thermogravimetric analysis (TGA), which were carried out by Li etal. (2007), whose research showed that co-combustion with waste tires could improve the burnout characteristics. Co-combustion of waste plastic and coal in fluidized bed were researched by Jin et al. (2001) and co-combustion of Medical Solid Waste and coal in a CFBC by Pu Ge et al. (2003).