I think I found a bug. The module slots are specified in the code, but does not show up in the game.
so I shuffled areound in the code... and made something functional in the data.lua:
{
type = "furnace",
name = "big-furnace",
corpse = "big-remnants",
icons = {{ icon = "TheBigFurnace-Fork/graphics/big-furnace.png", icon_size = 64 }},
flags = {"placeable-neutral", "placeable-player", "player-creation"},
minable = {mining_time = 1, result = "big-furnace"},
max_health = 350,
dying_explosion = "medium-explosion",
resistances = {{type = "fire", percent = 80}},
collision_box = {{-8.5, -8.5}, {8.5, 8.5}},
selection_box = {{-8.5, -8.5}, {8.5, 8.5}},
drawing_box = {{-8.5, -8.5}, {8.5, 8.5}},

    allowed_effects = {"consumption", "speed", "productivity", "pollution", "quality"},
    allowed_module_categories = {"speed", "productivity", "efficiency", "quality"},
    crafting_categories = {"big-smelting"},
    source_inventory_size = 1,
    result_inventory_size = 1,
    crafting_speed = settings.startup["TheBigFurnace-Fork-CraftingSpeed"].value,
    energy_source = 
    {
        type = "electric",
        usage_priority = "secondary-input",
        emissions_per_minute = { pollution = 0.005 }
    },
    vehicle_impact_sound = {
        filename = "__base__/sound/car-metal-impact.ogg",
        volume = 0.65
    },
    energy_usage = "3600kW",
    module_slots = 10,
    quality_indicator_scale = 1,

    working_sound = {
        sound = {
            filename = "__base__/sound/electric-furnace.ogg",
            volume = 0.7
        },
        apparent_volume = 1.5
    },
    graphics_set = {
        animation = {
            layers = {
                {
                    filename = "__TheBigFurnace-Fork__/graphics/entity/electric-furnace.png",
                    priority = "high",
                    width = 544,
                    height = 498,
                    frame_count = 1,
                    shift = util.by_pixel(0, 0),
                    scale = 1.2
                },
                {
                    filename = "__TheBigFurnace-Fork__/graphics/entity/electric-furnace-shadow.png",
                    priority = "high",
                    width = 517,
                    height = 389,
                    frame_count = 1,
                    draw_as_shadow = true,
                    shift = util.by_pixel(16, 0),
                    scale = 1.2
                }
            }
        },
        working_visualisations = {
            {
                animation = {
                    filename = "__TheBigFurnace-Fork__/graphics/entity/electric-furnace-heater.png",
                    priority = "high",
                    width = 137,
                    height = 127,
                    frame_count = 12,
                    animation_speed = 0.5,
                    shift = util.by_pixel(8, 150),
                    scale = 1.2
                },
                light = {
                    intensity = 0.4,
                    size = 6,
                    shift = {0.0, 1.0},
                    color = {r = 1.0, g = 1.0, b = 1.0}
                }
            },
            {
                animation = {
                    filename = "__TheBigFurnace-Fork__/graphics/entity/electric-furnace-propeller-1.png",
                    priority = "high",
                    width = 85,
                    height = 57,
                    frame_count = 4,
                    animation_speed = 0.5,
                    shift = util.by_pixel(-115, -132),
                    scale = 1.2
                }
            },
            {
                animation = {
                    filename = "__TheBigFurnace-Fork__/graphics/entity/electric-furnace-propeller-2.png",
                    priority = "high",
                    width = 53,
                    height = 34,
                    frame_count = 4,
                    animation_speed = 0.5,
                    shift = util.by_pixel(18,-238),
                    scale = 1.2
                }
            }
        }
    },
    render_layer = "object"
},