Melt butter in a large, heavy pot over medium-high heat. Add carrots, onion, garlic and ginger. Cook 5 minutes. Add chicken broth, tomatoes, and lemon rind; bring to a boil. Reduce heat, cover partially, and simmer until carrots are very tender, about 25 minutes. Cool slightly. Use blender to puree soup in batches. Return soup to pot to heat through. Season with salt.