Review pubs.acs.org/EF
Cite This: Energy Fuels 2018, 32, 4069−4095
Review of Pulverized Combustion of Non-Woody Residues Miriam Rabaçal,† Sandrina Pereira, and Mário Costa*
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IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa 1049-001, Portugal ABSTRACT: The intense use of wood-derived fuels in (co)firing processes results in an enormous pressure on the forest. In order to alleviate this pressure and to proceed with the CO2 emissions reduction process, it is necessary to increase the use of non-woody residues, in particular herbaceous materials and agricultural residues. (Co)firing using such residues can cause a number of problems due to the presence of alkali metals, chlorine, and other ash-related impacts as well as corrosion of metallic surfaces and particulate matter emissions. This may limit the variety of biomass residues that can actually be used in (co)firing processes. This review aims to summarize recent developments in the combustion of pulverized non-woody residues and includes experimental and numerical studies of single particle combustion and combustion in small- and large-scale furnaces. The review provides an overview of the properties of non-woody residues, describes the existing research facilities to study the subject, and summarizes the experimental studies on the combustion of non-woody residues, including studies on single particle combustion, drop tube furnaces and entrained flow reactors, and large-scale furnaces. The review also concentrates on numerical modeling, namely on the formulation of combustion models and their application in computational fluid dynamics. Finally, the main conclusions are summarized and the research needs listed.
1. INTRODUCTION Biomass can have a vegetal, human, or animal origin. Wood from forest, waste from agricultural and forestry practices, and waste from human, animal, and industrial activities represent different forms of biomass. This variety of resources added to the existing conversion processes for energy production turns biomass into an attractive solution to increase the integration of endogenous and renewable energy sources in the world energy mix.1 Direct combustion is widely used to convert biomass into heat and/or electricity, namely in cofiring processes with coal. Cocombustion of biomass with coal contributes to the reduction of power plant greenhouse gas (GHG) emissions. Demirbas,1 Kuo and Wu,2 and Zhang et al.3 also report that the combustion of biomass instead of coal can reduce the emissions of SOx and NOx compared with pure coal firing because of factors such as the lower content of sulfur and nitrogen present in the biomass, retention of the sulfur by the alkali/alkalineearth compounds present in the biomass, or, in case of NOx emissions, the lowering of the flame temperature as a consequence of the relatively high moisture content present in the biomass. At the economic level, Karampinis et al.4 report that the investment costs needed to implement cofiring in a power plant are lower than the investment costs in a new hydro power plant or a new onshore wind power plant. Nevertheless, the cofiring of coal with biomass, namely in pulverized fuel (PF) power plants, poses some difficulties. First, ash that results from biomass combustion, especially for biomass residues with higher levels of alkalis and chlorine than coal, may increase phenomena such as slagging, fouling, and corrosion. Second, in comparison with coal, biomass presents lower heating values due to its high moisture and oxygen contents.3,5 Despite these drawbacks, a number of biomass residues are currently used in cofiring and, more recently, in pure biomass firing processes, including forestry and sawmill residues, short-rotation coppice wood and other wood materials; solid waste materials from olive, palm, sunflower and rape seed oil and other crops; dried © 2017 American Chemical Society
sewage sludge; and cereal straws and other baled agricultural residue materials.6 Woody residues have low ash contents and can be fired at relatively high cofiring ratios with coal or in pure firing processes. In contrast, non-woody residues, such as those processed from agriculture and related industries, present generally high ash and alkali metal contents, which make their use in cofiring processes with coal, even at relatively low cofiring ratios, or in pure pulverized firing processes rather difficult. For example, grasses, reeds, and straws have high ash content and are rich in silica, lime, potash, and phosphate, which together with their low ash fusion temperatures potentiate both slagging and fouling formation in conventional pulverized combustion systems. There is a need to improve the current understanding of the combustion of pulverized non-woody residues in order to increase their penetration in cofiring and pure firing processes. Recent important related reviews include those by Niu et al.,7 Chen et al.,8 Toftegaard et al.,9 Williams et al.,10 and Scheffknecht et al.,11 which focus on various aspects of the combustion of solid fuels based on both experimental7−9 and modeling10,11 studies. Niu et al.7 reviewed the research on ashrelated issues during biomass combustion, with emphasis on the formation mechanisms, needs, and potential countermeasures. Chen et al.8 discussed the impacts of wildfire and anthropogenic emissions from biomass burning in China on public health and climate and identified research priorities and insights on biomass burning in the country. Toftegaard et al.9 revised the current knowledge on oxy-fuel combustion, focusing on flame temperature, heat transfer, ignition, burnout, Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 23, 2017 Revised: December 7, 2017 Published: December 12, 2017 4069
DOI: 10.1021/acs.energyfuels.7b03258 Energy Fuels 2018, 32, 4069−4095
Review
Energy & Fuels Table 1. Non-Woody Biomass Composition and HHVsa alfalfa stems
a
wheat straw
fixed carbon volatile matter ash
15.8 78.9 5.3
17.7 75.3 7.0
carbon hydrogen oxygen nitrogen sulfur
47.2 6.0 38.2 2.7 0.2
44.9 5.5 41.8 0.4 0.2
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 P2O5 others
5.8 0.07 0.3 18.3 10.4 1.1 28.1 1.9 7.6 26.4
55.3 1.9 0.7 6.1 1.1 1.7 25.6 4.4 1.3 1.9
18.7 pistachio shells
17.9 olive stones
fixed carbon volatile matter ash
17.0 81.6 1.4
16.3 82.0 1.7
carbon hydrogen oxygen nitrogen sulfur
50.2 6.3 41.2 0.7 0.2
52.8 6.7 38.3 0.3 0.05
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 P2O5 others
8.2 2.2 35.4 10.0 3.3 4.5 18.2 3.8 11.8 2.7
30.4 10.6 9.9 9.9 7.2 2.9 18.7 0.7 7.4 2.3
18.2
21.6
rice husk
coffee husk
switch grass
sugar cane bagasse
Proximate Analysis (wt %, dry) 16.3 1.4 14.3 12.0 73.2 91.7 76.7 85.6 10.5 6.8 9.0 2.4 Ultimate Analysis (wt %, daf) 40.7 38.2 46.7 48.6 6.0 5.1 5.8 5.9 32.9 36.8 37.4 42.8 0.5 2.4 0.8 0.02
