Supercritical　water　(SOW)　fluidized　bed　is　a　new　reactor　concept　for　hydrogen　production　from　biomass　or　coal　gasification.　In　this　paper,　a　comparative　study　on　flow　structure　and　bubble　dynamics　in　a　supercritical　water　fluidized　bed　and　a　gas　fluidized　bed　was　carried　out　using　the　discrete　element　method　(DEM).　The　results　show　that　supercritical　water　condition　reduces　the　incipient　fluidization　velocity,　changes　regime　transitions,　i.e.　a　homogeneous　fluidization　was　observed　when　the　superficial　velocity　is　in　the　range　of　the　minimum　fluidization　velocity　and　minimum　bubbling　velocity　even　the　solids　behave　as　Geldart　B　powders　in　the　gas　fluidized　bed.　Bubbling　fluidization　in　the　supercritical　water　fluidized　bed　was　formed　after　superficial　velocity　exceeds　the　minimum　bubbling　velocity,　as　in　the　gas　fluidized　bed.　Bubble　is　one　of　the　most　important　features　in　fluidized　bed,　which　is　also　the　emphasis　in　this　paper.　Bubble　growth　was　effectively　suppressed　in　the　supercritical　water　fluidized　bed,　which　resulted　in　a　more　uniform　flow　structure.　By　analyzing　a　large　number　of　bubbles,　bubble　dynamic　characteristics　such　as　diameter　distribution,　frequency,　rising　path　and　so　on,　were　obtained.　It　is　found　that　bubble　dynamic　characteristics　in　the　supercritical　water　fluidized　bed　differ　a　lot　from　that　in　the　gas　fluidized　bed,　and　there　is　a　better　fluidization　quality　induced　by　the　bubble　dynamics　in　the　supercritical　water　fluidized　bed.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.