We consider the self heating process in a two dimensional spatially dependent model of a compost pile which incorporates terms that account for self heating due to both biological and oxidation mechanisms. As moisture is a crucial factor in both the degradation process and spontaneous ignition within a compost pile, this model consists of four mass-balance equations, namely, energy, oxygen, vapour and liquid water concentrations. Analyses are undertaken for different initial water contents within the compost pile. We show that when the water content is too low, the reaction is almost negligible; whereas when it is too high, the reaction commences only when the water content evaporates and the water ratio drops to within an appropriate range. However, for an intermediate water content range, the biological reaction is at its optimum and there is a possibility of spontaneous ignition within the compost pile. References F. Akgun and R. H. Essenhigh, Self-ignition characteristics of coal stockpiles: theoretical prediction from a two-dimensional unsteady-state model, FUEL, 80, 409--415, 2001, doi:10.1016/S0016-2361(00)00097-1 P. C. Bowes, Self-heating: Evaluating and Controlling the Hazards, Elsevier Press, Amsterdam, The Netherlands, 1984. W. F. Brinton, E. Jr. Evans, M. L. Droffner and R. B. Brinton, Standardized test for evaluation of compost self-heating, BioCycle, 36, 60--65, 1995, http://www.jgpress.com X. D. Chen, On the fundamentals of diffusive self-heating in water containing combustible materials, Chemical Engineering and Processing, 37, 367--378, 1998. FlexPDE$^{tm}$, PDE Solutions Inc, http://www.pdesolutions.com B. F. Gray, M. J. Sexton, B. Halliburton and C. Macaskill, Wetting-induced ignition in cellulosic materials, Fire Safety Journal, 37, 456--479, 2002, doi:10.1016/S0379-7112(02)00002-4 R. T. Haug, The Practical Handbook of Compost Engineering, Lewis Publishers, USA, 1993. W. Hogland, T. Bramryd and I. Persson, Physical, biological and chemical effects of unsorted fractions of industrial solid waste fuel storage, Waste Management and Research, 14, 197--210, 1996, doi:10.1006/wmre.1996.0019 P. F. Hudak, Spontaneous combustion of shale spoils at sanitary landfill, Waste Management and Research, 22, 687--688, 2001, doi:10.1016/S0956-053X(01)00077-0 M. Khanahmadi, R. Roostaazad, A. Safekordi, R. Bozorgmehri and D. A. Mitchell, Investigating the use of cooling surfaces in solid-state fermentation tray bioreactors: modelling and experimentation, Journal of Chemical Technology and Biotechnology, 79, 1228--1242, 2004, doi: 10.1002/jctb.1117 M. Krajciova, L'. Jelemensky, M. Kisa and J. Markos, Model prediction on self-heating and prevention of stockpiled coals, Journal of Loss Prevention in the Process Industries, 17, 205--216, 2004, doi:10.1016/j.jlp.2004.02.002 F. Kuwahara, Y. Sano, A. Nakayama, K. Nakasaki and T. Fukazawa , Numerical modelling of a composting process with aeration, Journal of Porous Media, 10(12), 927--938, 2009. T. Luangwilai, H. S. Sidhu, M. I. Nelson and X. D. Chen, Modelling the effects of moisture content in compost piles, Proceedings of the 39th Australian Chemical Engineering Conference, Chemeca-2011, Engineers Australia, 2011, http://www.conference.net.au/chemeca2011/papers/411.pdf T. Luangwilai and H. S. Sidhu, Determining critical conditions for two-dimensional compost piles with air flow via numerical simulations, ANZIAM J.(E), 52, pp. C463--C481, 2011, http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/3753 N. O. Moraga, F. Corvalan, M. Escudey, A. Arias and C.E. Zambra, Unsteady 2D coupled heat and mass transfer in porous media with biological and chemical heat generations, International Journal of Heat and Mass Transfer, 52, 5841--5848, 2009, doi:10.1016/j.ijheatmasstransfer.2009.07.027 D. Nield and A. Bejan, Convection in porous media, Springer-Verlag, New York, 1992. R. Rynk, Fires at composting facilities, BioCycle Magazine, 41(1), 54--62, 2000, http://www.jgpress.com M. J. Sexton, C. Macaskill and B. F. Gray, Self-heating and drying in two-dimensional bagasse piles, Combustion Theory and Modelling, 5, 517--536, 2001, doi:10.1088/1364-7830/5/4/302 R. A. Sisson, A. Swift, G. C. Wake and B. F. Gray, The self-heating of damp cellulosic materials: I. High thermal conductivity and diffusivity, IMA Journal of Applied Mathematics, 49, 273--291, 1992, http://ftp.math.utah.edu/pub/tex/bib/toc/imajapplmath.html H. S. Sidhu, M. I. Nelson and X. D. Chen, A simple spatial model for self-heating compost piles, ANZIAM J., 48(CTAC2006), C135--C150, 2007, http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/86