Factors Leading to Spontaneous Combustion in Yard Wastes

 

Richard Buggeln (1)  and  Robert Rynk (2)

 

(1) Center for Industrial Services, The University of Tennessee, Knoxville, TN, (2) Senior Technical Editor, The JG Press, Emmaus, PA, USA

 

Spontaneous combustion (SC) is defined as combustion of material in the absence of  "forced ignition", i.e., an externally applied spark or flame.  This paper describes how SC occurs in piles of yard wastes, starting with chemical and physical events that initiate heat-producing reactions via biotic and abiotic processes involving oxygen and a little water. The accumulation of heat generated within a pile depends on the balance between rate of internal heat production and rate of heat loss to the external environment.  When the former rate is higher than the latter, a "critical" internal temperature may be reached at which time run-away ignition, i.e., SC, is unavoidable.  Water plays the interesting role of "governor" of temperature change and heat exchange within a pile of yard wastes as well as heat loss from a pile. While heat, in the form of water vapor, is being lost from a self-heating pile, the pile temperature does not rise above 70 -90C until all free water has been removedas in the example of potatoes in a pot of boiling water that only start to burn, i.e., rise above the boiling temperature of water, after the water has boiled off.  Important experiments with Eucalyptus leaves, sawdust, and other types of plant material demonstrate the inverse relationship between mass of material and ambient temperatures than can lead to SC. That is, the bigger the pile the lower the ambient temperature at which SC may occur. SC has been studied most intensively in coal, and the roles of particle size, pile compaction, and other potentially catalytic phenomena may have application to the propensity of SC to occur in piles of yard wastes.  Finally, several incendiary experiences at composting/mulching operations will be described together with guidelines on the prevention of costly fires that are very hard to extinguish.

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Effects of Kinetic Parameters On the Management of Composting Systems with Air Recirculation

 

K EKINCI (1), H M Keener (1), F C Michel, Jr (1), D L Elwell (1)

 

(1) Food, Agricultural, and Biological Engineering, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio USA, USA

 

A problem in composting reactors is heat generation exceeding heat removal so that the composting process becomes temperature and oxygen limited.  This is especially true for composting systems with air recirculation when the airflow leaving the system is reduced.  Because kinetics and heat generation of the process are dependent upon the feedstock(s), the total airflow and recirculation ratios will be dependent on feedstock.  However, information is limited on management of the composting system with air recirculation. This paper investigated the effects of compost feed stocks on the thermodynamics, kinetics, and energy usage for composting systems with air recirculation for paper mill sludge with broiler litter, biosolids, yard waste, and manure.  Results were developed by simulating system performance using a two-dimensional finite difference numerical model of the composting reactor and a two component first order kinetic model of decomposition. The numerical model was developed based on heat and mass balance equations. It predicts the system variables, temperature, mass, moisture content, ash content, maturity ratio, organic matter reduction, and O2 consumption as a function of time. Validation studies of the numerical model (Ekinci, Ph.D. Dissertation, 2001) showed it could accurately predict compost variables. Using published data on yard waste and biosolids composting (decomposition rate and compost equilibrium values), the numerical model was used to investigate the effects of decomposition rate on management of the composting system. The study especially focused on the energy usage as a function of fan on-off time, airflow, compost bed depth, and air recirculation ratio. Preliminary results on composting of paper mill sludge with broiler litter indicated that lower airflows with low air recirculation rates lead to longer composting time to reach a certain reduction in organic matter. Results enable estimating variables and fixed cost of composting system as a function of aeration rate and air recycling rates.

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Composting of Municipal Biowaste: the Influence of Temperature, O₂-And CO₂-Content and the Respiratory Quotient

 

Bidlingmaier,Werner(1),MONNIG,KARLA(1),Kuhne,Michael(2)

 

(1)Bauhaus- Universitat Weimar, (2)Materialforschungs-und Prfanstalt Weimar, Germany

 

The active phase of composting in reactors can be influenced by using controlled ventilated systems.Actuating variables and the reaction of the process are measured. The complex interaction in the system of material/ microbes/ reactor can not be categorized.An essential element was the creation of a system,which allowed sufficiently precise control and measurement:Three well-insulated 0.17 m3 reactors, adjustable aeration for controlling the temperature or/and the O2-content, on-line analyses of O2 and CO2 in the off-gas. New sensors for measuring the water-content in the material and the O2-content of the gas were also checked by comparing with conventional technique. The trials revealed tendencies which then were tested in larger 2.3 m3 reactors aerated with recirculation (Herhof- Company).The results support the way to reduce the stream of fresh air because a higher content of CO2 doesn't disturb the biodegradation, and also holding the temperature at 50 oC has no negative effect on the O2- consumption. The respiratory quotient seems to be no indicator during the hot phase of composting.