Article pubs.acs.org/EF
Co-combustion of Oil Shale Retorting Solid Waste with Cornstalk Particles in a Circulating Fluidized Bed Hong-peng Liu,*,† Wen-xue Liang,† Ming-hao Wu,‡ and Qing Wang† †
Engineering Research Centre of Oil Shale Comprehensive Utilization, Ministry of Education, Northeast Dianli University, Jilin, Jilin 132012, People’s Republic of China ‡ Beijing Jingneng Gaoantun Gas Power Company, Limited, Beijing 100024, People’s Republic of China ABSTRACT: The combustion of mixtures of oil shale retorting solid waste and cornstalk particles in a circulating fluidized bed under different conditions was analyzed while investigating the effects of combustion chamber temperature distribution on slag and fly ash. The concentrations of O2, CO, and NOx in the flue gas at different locations within the combustion unit were also obtained. The results show that oil shale retorting solid waste mixed with cornstalk particles in the appropriate ratio can undergo completely stable and efficient combustion. With increasing cornstalk particle fractions, the combustion characteristics are improved, such that the furnace exit temperature is elevated, while the concentrations of O2 and CO are decreased and the combustible fraction of the bottom ash gradually drops below 2%. Although the N content of the cornstalks is greater than that of the retorting solid waste, increases in the proportion of cornstalk material in the fuel is not associated with increased emissions of NOx. Thus, a high cornstalk/coke ratio in the fuel both maintains the furnace temperature within an optimal range and also keeps the concentration of NOx at a low level. In particular, a 30% proportion of cornstalk particles in the fuel, together with a primary air rate of 66.67%, results in the maximum combustion efficiency.
1. INTRODUCTION Oil shale, being a sedimentary rock, consists of a combination of mineral and organic matter,1 with the organic material primarily composed of kerogen. Oil shale retorting solid waste (RW), formed in the thermal processing of oil shale, is a lowgrade fuel. This material typically has a low volatile content and a minimal calorific value along with a high proportion of ash and is also difficult to ignite and burn out. Discarded retorting solid waste tends to use up valuable landfill space and also contains toxic chemicals, such as polycyclic aromatic hydrocarbons, phenolic compounds, and sulfuret, all of which are potentially harmful to the environment.2−4 For the above reasons, there is currently significant research interest in developing effective methods of treating retorting solid waste. Arro et al.5 investigated the co-combustion of oil shale semi-coke with oil shale in a circulating fluidized bed (CFB) boiler and found that the resulting SO2 could be sufficiently fixed. Wang et al.6,7 studied the co-combustion of oil shale semi-coke with oil shale, adding increasing proportions of cornstalk particles (CS), and found a decrease in the ignition temperature. These results suggest that the use of cocombustion during the treatment of semi-coke in a CFB is feasible.8 Wang et al.9 investigated the co-combustion of oil shale semi-coke with oil shale, including the effects of different proportions and particle sizes on the heating rate. In addition, distributed activation energy model (DAEM) reaction models show that the combustion of oil shale semi-coke with oil shale improves the co-combustion characteristics. Increasing the relative proportion of CS also increases the volatile content of the combustion mixture and improves the combustion characteristics of the semi-coke.10 Biomass is a potential green energy source and has many advantages, including easy storage, high burning efficiency, © 2015 American Chemical Society
lower pollution potential, low dust emissions, and high heat values.11 However, biomass is often incompletely combusted, which is not only a waste of energy but also increases emissions of pollutant gases and dust particles. RW and biomass, thus, have similar challenges with regard to their practical use as fuels, because it is difficult for cornstalk biomass to be used independently as fuels, as they may result in the formation of boiler slag as well as corrosion.12−14 The present study attempted to mitigate this problem by experimenting with the combustion of mixtures of Huadian RW with cornstalk biomass in a bench-scale CFB test apparatus.
2. MATERIALS AND METHODS 2.1. Samples. RW was sourced from the oil shale refinery in Huadian, China. The retorting equipment consisted of Fushun-type retorts employing wet slag extraction. Therefore, the RW should balance weight in the air. CS grains were obtained by the on-site hot extrusion of raw CS material. The particle size of CS is 8 mm. Table 1 provides the proximate and elemental analysis data for each fuel specimen employed in the present work. Table 2 provides the particle size distributions of RW. As seen from Table 1, the RW used in this work was a low-grade fuel with a low volatile content and a high proportion of ash. In comparison to the RW, the CS material had high fixed carbon and volatile values but low ash content, all of which are advantageous with regard to combustion. The CS also had higher proportions of C, H, and O relative to the RW. 2.2. Working Conditions. The RW was combined with the CS material in various ratios, denoted as R1 (7:3), R2 (8:2), and R3 (9:1). These two fuels were fed by two disc feeders by adjusting the rate of the feeder to change the feeding amount. These two mixed in the fuel Received: August 7, 2015 Revised: September 6, 2015 Published: September 7, 2015 6832
DOI: 10.1021/acs.energyfuels.5b01804 Energy Fuels 2015, 29, 6832−6838
Article
Energy & Fuels Table 1. Fuel Compositions proximate analysis (%)
low calorific value
elemental analysis (%)
sample
Mad
Vad
Aad
FCad
Qnet,ar (kJ/kg)
Cad
Had
Oad
Nad
Sad
CS RW 7:3 RW/CS 8:2 RW/CS 9:1 RW/CS
7.40 0.89 2.85 2.20 2.06
69.86 10.44 28.27 22.32 21.14
6.06 82.62 59.65 67.31 68.84
16.68 6.09 9.27 8.21 8.00
17097.38 3868.29 7837.017 6514.108 6249.52
37.95 11.29 19.29 16.62 16.09
6.47 0.35 2.19 1.57 1.45
40.76 4.21 15.18 11.52 10.79
0.77 0.11 0.31 0.24 0.23
0.59 0.53 0.55 0.54 0.54
Table 2. Particle Size Distributions of RW particle size (mm)
6−5
5−3
3−2
2−1
1−0.5
0.5−0.2
0.2−0.1
